Alice Tdr Tpc

CERN/ LHCC 2000–001ALICE TDR 77 January2000

A L I C E

Technical Design Report

of the

Time Projection Chamber

Cover designby CERNDesktopPublishingOffice

AliRoot EventDisplayof TPC

Printedat CERNDecember1999

ISBN 92-9083-155-3

i

ALICE Collaboration

Alessandria, Italy, Facolta di Scienzedell’Universita:G. Dellacasa,L. Ramello,E. ScalasandM. Sitta.

Aligarh, India, PhysicsDepartment,Aligarh Muslim University:N. Ahmad,S.Ahmad,T. Ahmad,W. Bari, M. IrfanandM. Zafar.

Athens, Greece,NuclearandParticlePhysicsDivision,Universityof Athens:A.L.S. Angelis1

, G. MavromanolakisandA.D. Panagiotou.

Athens, Greece, Instituteof NuclearPhysics,NRCDemokritos:K. Kalfas.

Bari, Italy, Dipartimentodi Fisicadell’Universita andSezioneINFN:R. Caliandro,D. Cozza,G. De Cataldo,D. Di Bari, D. Elia, R.A. Fini, B. Ghidini, V. Lenti, V. Manzari,E. Nappi1

, F. NavachandF. Posa.

Bari, Italy, PolitecnicoandSezioneINFN:F. Corsi,D. DeVenuto,R. Dinapoli,G. Lisco,C. MarzoccaandE. Monno.

Beijing, China, ChinaInstituteof Atomic Energy:X. Li, S.Lu, Z. Lu, B. Sa,J.Yuan,J.Zhou,S.ZhouandX. Zhu.

Bergen, Norway, Departmentof Physics,Universityof Bergen:E. Andersen,K. Fanebust,H. Helstrup,A. Klovning,O.A. Maeland,O.H. Odland,D. Rohrich,R. RongvedandA.S. Vestboe.

Bhubaneswar, India, Instituteof Physics:A.K. Dubey, D.P. Mahapatra,B. MohantyandS.C.Phatak.

Birmingham, United Kingdom, Schoolof PhysicsandSpaceResearch,Universityof Birmingham:I.J. Bloodworth,D. Evans,G.T. Jones,P. Jovanovic, J.B.Kinson,A. Kirk, O. VillalobosBaillie andM.F. Votruba.

Bologna, Italy, University/INFN:F. Anselmo,P. Antonioli, G. Bari, M. Basile,L. Bellagamba,D. Boscherini,A. Bruni, G. Bruni,G. CaraRomeo,E. Cerron-Zeballos,F. Cindolo,N. Coppola,M. Corradi,S.De Pasquale,D. Falchieri,A. Gabrielli,E. Gandolfi,P. Giusti,D. Hatzifotiadou,N.Y. Kim, G. Laurenti,M.L. Luvisetto,A. Margotti, M. Masetti,R. Nania,F. Palmonari,A. Pesci,F. Pierella,A. Polini, G. Sartorelli,A. Semak,G. Valenti,M.C.S.Williams andA. Zichichi.

Bratislava, Slovakia, Facultyof MathematicsandPhysics,ComeniusUniversity:J.Bracinık, V. Cerny, J.Ftacnik, V. Hlinka, R. Janik,R. Lietava,M. Pikna,J.Pisut, N. Pisutova,P. Rosinsky, B. Sitar, P. Strmen, I. SzarkaandM. Zagiba.

Bucharest, Romania, NationalInstitutefor PhysicsandNuclearEngineering:A. Andronic,V. Catanescu,M. Ciobanu,M. Duma,C.I. Legrand,D. Moisa,M. Petrovici, V. SimionandG. Stoicea.

ii

Budapest, Hungary, KFKI ResearchInstitutefor ParticleandNuclearPhysics,HungarianAcademyof Sciences:E. Denes,B. Eged,Z. Fodor, G. Harangozo,Z. Meggyesi,G. Palla,G. Rubin,J.Sulyan,J.Sziklai,B.N. VissyandJ.Zimanyi.

Cagliari, Italy, Dipartimentodi Fisicadell’Universita andSezioneINFN:C. Cicalo,A. De Falco,M.P. Macciotta-Serpi,A. Masoni,A. Pataki,G. Puddu,P. Randaccio,S.Serci,E. Siddi andG. Usai.

Calcutta, India, SahaInstituteof NuclearPhysics:P. Bhattacharya,S.Bose,SukalyanChattopadhyay, N. Majumdar, S.Mukhopadhyay, A. Sanyal,S.Sarkar, P. Sen,S.K. Sen,B.C. SinhaandT. Sinha.

Calcutta, India, VariableEnergy CyclotronCentre:SubhasisChattopadhyay, M.R. DuttaMajumdar, M.S.Ganti,T.K. Nayak,S.Pal, R.N. Singaraju,BikashSinha,M.D. TrivediandY.P. Viyogi.

Catania, Italy, Dipartimentodi Fisicadell’Universita andSezioneINFN:A. Badala,R. Barbera,M. Gulino,S. Ingrassia,A. Insolia,L. Lo Nigro, D. Lo Presti,A. Palmeri,G.S.Pappalardo,L. Pappalardo,C. Petta,N. Randazzo,S.Reito,F. Riggi, G.V. RussoandS.Vanadia.

CERN, Switzerland, EuropeanLaboratoryfor ParticlePhysics:J.Bachler, J.A.Belikov2

, T. BenAhmed,V. Berejnoi3

, R. Brun,M. Campbell,W. Carena,

F. Carminati,S.Chapeland,P. Chochula4, V. Colin deVerdiere,J.CruzdeSousaBarbosa,

M. Davenport,J.deGroot,A. Di Mauro,R. Divia,C. Eisenberg, C. Engster, J.EspiritoSanto,R. EsteveBosch,F. Formenti,E. Futo5

, B. Goret,T. Grassi6

, C. Gregory, M. Hoch,P.G. Innocenti,W. Klempt,

A. Kluge,J.C.Legrand,L. Leistam,B. Lenkeit, C. Lourenco, P. Martinengo,T. Meyer, A. Morsch,M. Mota,L. Musa,B. Perrin,F. Piuz,E. Quercigh,A. Rivetti, K. Safarık, J.-C.Santiard,K. Schossmaier, J.Schukraft,E. Schyns,W. Snoeys,P. Sonderegger, M. Spegel,D. Swoboda,P. Szymanski,G. Tabary, P. VandeVyvre, A. Vascotto,S.Wenig,P. WertelaersandJ.Zalipska.

Chandigarh, India, PhysicsDepartment,PanjabUniversity:M.M. Aggarwal, A.K. Bhatia,V.S.BhatiaandG. Sood.

Clermont-Ferrand, France, LaboratoiredePhysiqueCorpusculaire(LPC), IN2P3-CNRSandUniversite BlaisePascal:IN2P3: A. Baldit, V. Barret,N. Bastid,G. Blanchard,J.Castor, T. Chambon,P. Crochet,F. Daudon,A. Devaux,P. Dupieux,B. Espagnon,P. Force,A. Genoux-Lubain,L. Lamoine,F. Manso,P. Rosnet,L. Royer, P. SaturniniandG. Savinel.

Coimbra, Portugal, DepartamentodeFısica,FaculdadedeCienciaseTecnologia:R. FerreiraMarques,P. Fonte,J.PinhaoandA. Policarpo.

Columbus, U.S.A., Departmentof Physics,OhioStateUniversity:H.L. Caines,H.M. Dyke,T.J.Humanic,M. Lisa,B.S.Nilsen,G. Paic andE. Sugarbaker.

Copenhagen,Denmark, Niels Bohr Institute:I. Bearden,H. Bøggild,P. Christiansen,J.J.Gaardhøje,O. Hansen,A. Holm, B.S.NielsenandD. Ouerdane.

Cracow, Poland, HenrykNiewodniczanskiInstituteof NuclearPhysics,High Energy PhysicsDepartment:J.Bartke,E. Gładysz-Dziadus,E. Gornicki, M. Kowalski,A. Rybicki, P. Stefanski andZ. Wlodarczyk7

.

iii

Darmstadt, Germany, Gesellschaftfur Schwerionenforschung(GSI):R. Averbeck,E. Badura,C. Blume,P. Braun-Munzinger, H.W. Daues,A. Deusser, A. Devismes,C. Finck,P. Foka,U. Frankenfeld,C. Garabatos,G. Hering,M. Ivanov4

, J.Luhning,P. Malzacher,

C. Markert,A. Mischke,D. Mi skowiec,W.F.J.Muller, F. Rademakers,H. Sako, A. Sandoval, H. Sann,H.R. Schmidt,S.Sedykh,A. Sharma1

, H. Stelzer, R. VeenhofandD. Vranic.

Darmstadt, Germany, Institut fur Kernphysik,TechnischeUniversitat :A. Forster, H. OeschlerandF. Uhlig.

Frankfurt, Germany, Institut fur Kernphysik,JohannWolfgangGoethe-Universitat:C. Adler, W. Amend,J.Berger, J.Berschin,A. Billmeier, P. Buncic,D. Flierl, M. Gazdzicki,J.Hehner,S.Lange,R. Renfordt,H. Rheinfels-Immans,C. Roland,G. Roland,R. Stock,H. StrobeleandC. Struck.

Gatchina, Russia, St.Petersburg NuclearPhysicsInstitute:K. Egorov, B. Komkov, V. Kozlov, N. Miftakhov, V. Nikouline,V. Samsonov, O. Tarasenkova,V. Vishnevskii, S.Volkov andA. Vorobiev.

Heidelberg, Germany, Kirchhoff Institutefor Physics:R. Achenbach,O. Braun,M. Keller, F.O.Lesser, V. Lindenstruth,R. Schneider, M. Schulz,T. SteinbeckandL. Voerg.

Heidelberg, Germany, PhysikalischesInstitut,Ruprecht-KarlsUniversitat:H. Appelshauser, S.Damjanovic, T. Dietel,S.I.Esumi,K. Filimonov, P. Glassel,N. Herrmann,A. Marın, V. Petracek,J.Rak,A. Reischl,M.J.Richter, E. Schafer, W. Schmitz,W. Seipp,J.Slivova,H.C. Soltveit, J.Stachel,H. Tilsner, J.P. Wessels,T. Wienold,B. WindelbandandS.Yurevich.

Ioannina, Greece,Universityof Ioannina,Departmentof Physics:X. AslanoglouandN.G. Nicolis.

Jaipur, India, PhysicsDepartment,Universityof Rajasthan:A. Bharti,S.K. Gupta,R. RaniwalaandS.Raniwala.

Jammu, India, PhysicsDepartment,JammuUniversity:S.K. Badyal,A. Bhasin,A. Gupta,V.K. Gupta,S.Mahajan,L.K. Mangotra,B.V.K.S.Potukuchi,N.K. RaoandS.S.Sambyal.

JINR, Russia, JointInstitutefor NuclearResearch:P.G. Akichine,V.A. Arefiev, V.I. Astakhov, A.A. Baldine,A.M. Baldine,V.D. Bartenev, B.V. Batiounia,I.V. Boguslavsky, Z.V. Borissovskaia,P. Bylinkine, A.V. Chabounov, G.S.Chabratova, I.A. Chichov,V. Danilov, V.I. Datskov, V.K. Dodokhov, L.G. Efimov, A.G. Fedounov, O.A. Golubitsky,B.N. Guouskov, O.I. Iouldachev, V.G. Kadychevsky, I.E. Karpunina,E.K. Koshurnikov,A.D. Kovalenko, A. Lioubimtsev, V.L. Lioubochits,V.I. Lobanov, G.I. Lykasov, E.A. Matiouchevski,K.V. Mikhailov, I. Minaev, P.V. Nomokonov, I.V. Pouzynin,I. Roufanov, I.A. Shelaev, A.V. Sidorov,M.K. Suleimanov, G.P. Tsvineva andA.S. Vodopianov.

V. Kuznetsov8

andV. Shestakov8.

Ts.Baatar9, B. Khurelbaatar9

andR. Togoo9

.

K.G. Akhobadze10, A.K. Djavrishvili10

, T. Grigalashvili10

, E.S.Ioramashvili10

, A.V. Kharadze10

,

L. Khizanishvili10, T.V. Khuskivadze10

, L.V. Shalamberidze10

andN. Shubitidze10

.

iv

N. Grigalashvili11, M. Nioradze11

, M. Tabidze11

andY. Tevzadze11

.

D. Felea12, A. Gheata12

, M. Gheata12

, M. Haiduc12

, D. Hasegan12

, R. Marginean12

, R.I. Nanciu12

andS.I.Zgura12

.

Jyvaskyla, Finland, Departmentof Physics,Universityof Jyvaskyla andHelsinki Instituteof Physics:J.Aysto,M. Bondila,M. Komogorov, V. Lyapin,V. RuuskanenandW. Trzaska.

Khark ov, Ukraine, NationalScientificCentre‘Kharkov Instituteof PhysicsandTechnology’:G.L. Bochek,V.F. Boldyshev, A.N. Dovbnya,V.I. Kulibaba,N.I. Maslov, S.V. Naumov, S.M.Potin,I.M. ProkhoretsandA.F. Starodubtsev.

Khark ov, Ukraine, ScientificandTechnologicalResearchInstituteof InstrumentEngineering:V.N. Borshchov, S.K. Kiprich, O.M. Listratenko, G. Protsay, A.N. Reznik,A.N. Ryabukhin andV.E. Starkov.

Kiev, Ukraine, Departmentof High Energy DensityPhysics,Bogolyubov Institutefor TheoreticalPhysics,NationalAcademyof Sciencesof Ukraine:T. Hryn’ova,D.E.Kharzeev, O.P. Pavlenko, A. Velytsky andG. Zinovjev.

Kosice, Slovakia, Instituteof ExperimentalPhysics,Slovak Academyof SciencesandFacultyofScienceP.J.Safarik University:J.Ban,J.Fedorisin,M. Hnatic, A. Jusko, I. Kralik, A. Kravcakova, F. Krivan, I. Kulkova,M. Luptak,G. Martinska,B. Pastircak1

, L. Sandor, J.Urban,S.Vokal andJ.Vrl akova.

Lausanne, Switzerland, IntegratedSystemLaboratory(ISL), EcolePolytechniqueFederaledeLausanne(EPFL) :A. Aizza,F.A. Cherigui,M. Mattavelli andD. Mlynek.

Legnaro, Italy, LaboratoriNazionalidi Legnaro:A. Bologna,M. Lombardi,R.A. Ricci andL. Vannucci.

Lisbon, Portugal, DepartamentodeFısica,InstitutoSuperiorTecnico:P. Branco,R. Carvalho,J.SeixasandR. Vilela Mendes.

Lund, Sweden,Divisionof CosmicandSubatomicPhysics,Universityof Lund:L. Carlen,S.I.A. Garpman,H.-A. Gustafsson,P. Nilsson,A. Oskarsson,L. Osterman,I. Otterlund,D. SilvermyrandE.A. Stenlund.

Lyon, France, Institut dePhysiqueNucleairedeLyon (IPNL), IN2P3-CNRSandUniversite ClaudeBernardLyon-I:M.Y. Chartoire,M. Chevallier, B. Cheynis,L. Ducroux,E. Gangler, M. Goyot, J.Y. Grossiord,R. Guernane,A. Guichard,D. Guinet,G. Jacquet,P. LautesseandS.Tissot.

Marb urg, Germany, FachbereichPhysik,PhilippsUniversitat:V. Friese,C. HohneandF. Puhlhofer.

Mexico City, Mexico, CentrodeInvestigacion y deEstudiosAvanzados:R. HernandezMontoya,G. HerreraCorralandL.M. Montano.

Moscow, Russia, Institutefor NuclearResearch,Academyof Science:K.A. Chileev, M.B. Goloubeva,F.F. Gouber, T.L. Karavitcheva, A.B. Kourepin,A.I. Maevskaia,V.I. Razine,A.I. RechetineandN.S.Topilskaia.

v

Moscow, Russia, Institutefor TheoreticalandExperimentalPhysics:A.N. Akindinov, V. Golovine,A.B. Kaidalov, M.M. Kats,I.T. Kiselev, S.M.Kisselev, E. Lioublev,M. Martemianov, A.N. Martemiyanov, P.A. Polozov, V.S.Serov, A.V. Smirnitski,M.M. Tchoumakov,I.A. Vetlitski, K.G. Volochine,L.S.Vorobiev andB.V. Zagreev.

Moscow, Russia, RussianResearchCenter‘K urchatov Institute’:V. Antonenko, S.Beliaev, I. Doubovik, S.Fokine,M. Ippolitov, K. Karadjev, A.L. Lebedev, V. Lebedev,V.I. Manko, T. Moukhanova, A. Nianine,S.Nikolaev, S.Nikouline,O. Patarakine,D. Peressounko,I. Sibiriak,A. Vasiliev, A. Vinogradov andM. Volkov.

Moscow, Russia, Moscow EngineeringPhysicsInstitute:V.A. Grigoriev, V.A. KaplineandV.A. Loguinov.

Munster, Germany, Institut fur Kernphysik,WestfalischeWilhelmsUniversitat:D. Bucher, R. Glasow, N. Heine,T. Peitzmann,K. Reygers,R. Santo,H. Schlagheck,W. VerhoevenandM. Wahn.

Nantes, France, LaboratoiredePhysiqueSubatomiqueetdesTechnologiesAssociees(SUBATECH),EcoledesMinesdeNantes,IN2P3-CNRSandUniversite deNantes:L. Aphecetche,A. Boucham,S.Bouvier, J.Castillo,L. Conin,J.P. Cussonneau,H. Delagrange,D. D’Enterria,M. Dialinas,C. Drancourt,B. Erazmus,G. Guilloux, H.H. Gutbrod,M.S.Labalme,P. Lautridou,F. Lefevre,M. Le Guay, L. Luquin,L. Martin, G. Martinez,V. Metivier, M.J. Mora,W. Pinganaud,G. Puil, O. Ravel, F. Retiere,C.S.Roy, D. Roy, Y. SchutzandA. Tournaire.

NIKHEF , The Netherlands, NationalInstitutefor NuclearandHigh Energy Physics:M. Botje13

, A. Buijs14

, J.J.F. Buskop13

, A.P. De Haas14

, P.K.A. De Witt Huberts13 14

,

R. Kamermans13 14, P.G.Kuijer13 14

, D. Muigg14

, G. Nooren13

, C.J.Oskamp14

, A. VanDenBrink14

andN. VanEijndhoven14

.

Novosibirsk, Russia, Budker Institutefor NuclearPhysics:A.R. Frolov andI.N. Pestov.

Oak Ridge, U.S.A., InstrumentationandControlsDivision,OakRidgeNationalLaboratory:T. Awes,C.L. Britton, W.L. Bryan,J.W. Walker andA.L. Wintenberg.

Orsay, France, InstitutdePhysiqueNucleaire(IPNO), IN2P3-CNRSandUniversite deParis-Sud:L. Bimbot,P.F. Courtat,R. Douet,P. Edelbruck,D. Jouan,Y. Le Bornec,M. Mac Cormick,J.Peyre,J.PouthasandN. Willis.

Oslo, Norway, Departmentof Physics,Universityof Oslo:A.K. Holme,G. Løvhøiden,B. Skaali,T.S.TveterandD. Wormald.

Padua, Italy, Dipartimentodi Fisicadell’Universita andSezioneINFN:F. Antinori, N. Carrer, M. Morando,A. Pepato,F. Scarlassara,G. Segato,F. SoramelandR. Turrisi.

Prague, CzechRepublic, Instituteof Physics,Academyof Science:A. Beitlerova,J.Mares,E. Mihokova,M. Nikl, K. Pıska,K. Polak andP. Zavada.

Protvino, Russia, Institutefor High Energy Physics:A.M. Blik, M. Bogolyubsky, G. Britvitch, S.Erine,G.V. Khaoustov, I.V. Kharlov, V. Lichine,M. Lobanov, N. Minaev, S.A. Sadovski, V.D. Samoilenko, P.A. Semenov, V.I. Suzdalev andV. Tikhonov.

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Rez u Prahy, CzechRepublic, Academyof Sciencesof CzechRepublic,NuclearPhysicsInstitute:V. Hanzal,J.Hosek,I. Hrivnacova1

, S.Kouchpil,V. Kouchpil,A. Kugler, M. Sumbera,P. Tlusty,

V. WagnerandD. Zakoucky.

Rome, Italy, Dipartimentodi Fisicadell’Universita ‘La Sapienza’andSezioneINFN:S.Di Liberto,M.A. Mazzoni,F. Meddi,D. ProsperiandG. Rosa.

Saclay, France, Centred’EtudesNucleaires,DAPNIA:P. Ageron,A. Baldisseri,H. Borel,D. Cacaut,I. Chevrot, P. De Girolamo,J.Gosset,L. Gosset,P. Hardy, D. Jourde,J.C.Lugol andF.M. Staley.

Salerno, Italy, Dipartimentodi ScienzeFisiche‘E.R.Caianiello’dell’Universita andINFN:L. Cifarelli, B. Cozzoni,G. Grella,M. Guida,J.Quartieri,G. Romano,A. Seganti,D. VicinanzaandT. Virgili.

Sarov, Russia, RussianFederalNuclearCenter(VNIIEF):V. Basmanov, D. Budnikov, V. Ianowski, R. Ilkaev, L. Ilkaeva,A. Ivanov, A. Khlebnikov,E. Kolokolnikov, S.Nazarenko, V. Punin,S.Poutevskoi, I. Selin,I. Vinogradov, S.Zhelezov andA. Zhitnik.

Shanghai, China, ShanghaiInstituteof Ceramics(SICCAS):Q. Deng,P. Li, J.Liao andD. Yan.

St. Petersburg, Russia, Institutefor Physicsof St.Petersburg StateUniversity, Mendeleev Institutefor MetrologyandMesonScientificAssociation:L.Y. Abramova,V.S.Alexandrov, P. Bolokhov, A.A. Bolonine,M.A. Braun,V.M. Dobulevitch,G.A. Feofilov, S.Guerassimov, S.N.Igolkine,M.I. Ioudkine,A.A. Kolojvari, V. Kondratiev,I.A. Novikov, S.V. Potapov, O.I. Stolyarov, A.M. Switchev, T.A. Toulina,F.A. Tsimbal,F.F. Valiev,V.V. VetchernineandL.I. Vinogradov.

Strasbourg, France, InstitutdeRecherchesSubatomiques(IReS),IN2P3-CNRSandUniversite LouisPasteur:L. Arnold, J.Baudot,D. Bonnet,J.P. Coffin, M. Germain,C. Gojak,B. Hippolyte,C. Kuhn,J.Lutz andA. Tarchini.

Trieste, Italy, Dipartimentodi Fisicadell’Universita andSezioneINFN:V. Bonvicini, L. Bosisio,P. Camerini,E. Fragiacomo,A. Gregorio,N. Grion,G. Margagliotti,C. Piemonte,A. Rachevski, R. Rui andA. Vacchi.

Turin, Italy, Dipartimentidi Fisicadell’Universita andINFN:G. Alberici, B. Alessandro,R. Arnaldi, P. Barberis,S.Beole,E. Botta,P.G.Cerello,E. Chiavassa,P. Cortese,E. Crescio,F. Daudo,N. De Marco,A. Ferretti,L. Gaido,M. Gallio, G. Giraudo,P. Giubellino,A. Marzari-Chiesa,M. Masera,G. Mazza,P. Mereu,B. Minetti, M. Monteno,O. Morra,A. Musso,D. Nouais,C. Oppedisano,A. Piccotti,G. Piragino,L. Riccati,E. Scomparin,F. Tosello,E. Vercellin,A. WerbrouckandR. Wheadon.

Warsaw, Poland, SoltanInstitutefor NuclearStudies:D. Czerwinski,A. Deloff, K. Karpio,S.Kozak,M. Kozlowski, L. Lukaszek,H. Malinowski,T. Siemiarczuk,G. Stefanek,L. TykarskiandG. Wilk.

Warsaw, Poland, Universityof Technology, Instituteof Physics:J.Grabski,P. Leszczynski,T.J.Pawlak, W.S.Peryt,J.PlutaandM. Przewlocki.

vii

Wuhan, China, Instituteof ParticlePhysics,HuazhongNormalUniversity:X. Cai,S.Q.Feng,Y. Hu, W. Li, F. Liu, F.M. Liu, H. Liu, L.S.Liu, Y. Liu, W.Y. Qian,X.R. Wang,S.Q.Wu, T. Wu, C.C.Xu, C.B. Yang,Z.B. Yin, D.C. ZhouandD.M. Zhou.

Yerevan, Armenia, YerevanPhysicsInstitute:M. Atayan,V. Danielyan,A. Grigorian,S.Grigoryan,H. Gulkanyan,V. Kakoyan,Yu. Margaryan,S.Mehrabyan,L. Parlakyan,R. ShahoyanandH. Vardanyan.

Zagreb, Croatia, RuderBoskovic Institute:T. Anticic, K. KadijaandT.Susa.

Applying to join ALICE.1

Also at CERN,Geneva,Switzerland.2

On leave from JINR,Dubna,Russia.3

On leave from IHEP, Protvino,Russia.4

On leave from ComeniusUniversity, Bratislava,Slovakia.5

On leave from BudapestUniversity, Hungary.6

On leave from Dipartimentodi Fisicadell’Universita andSezioneINFN, Padua,Italy.7

Instituteof Physics,PedagogicalUniversity, Kielce,Poland.8

ResearchCentrefor AppliedNuclearPhysics(RCANP),Dubna,Russia.9

Instituteof PhysicsandTechnology, MongolianAcademyof Sciences,Ulaanbaatar, Mongolia.10Instituteof Physics,GeorgianAcademyof Sciences,Tbilisi, Georgia.

11

High Energy PhysicsInstitute,Tbilisi StateUniversity, Tbilisi, Georgia.12

Instituteof SpaceSciences,Bucharest,Romania.13

Foundationof FundamentalResearchof Matterin TheNetherlands.14

UtrechtUniversity, Utrecht,TheNetherlands.

viii

Acknowledgements

TheCollaborationwishesto thankall thetechnicalandadministrative staff involvedduringthepreparationof theTDR, in particular:S.Barras,D. Birker, P. Bonneau,M. Bosteels,R. Cook,M. Flammier, L. Levrat,F. Marcastel,S.Maridor, P. Mato,F. Morel, J.Navarria,G. Peon,A. Przybyla,D. Sanchez,S.Stappers,M. Wensveen,andM. Wilhelmsson.Wewouldalsolike to acknowledgethecontributionsmadeby thelateR. BrockmannandbyH.G. Fischerin theR&D stageof theTPCproject,in particularto theRCCprototypeandslowsimulator. Thehelpprovidedby theNA49 andCEREScollaborationsthroughouttheyearsis greatlyappreciated.

xi

Summary

This TechnicalDesignReportdescribestheALICE TimeProjectionChamber(TPC),which is themaindetectorfor trackingin thecentralbarrel.In thefollowing wesummarizethemaindesignconsiderationsandspecificationsfor theTPCandoutlinetheproposedtechnicalsolutions.

In Chapter1 we describebriefly the main physicsgoalsfor ALICE andthe resultingdesignspec-ificationsfor the TPC. A ‘traditional’ focusof physicswith the TPC will be hadronicphysics,wherein additionto efficient track reconstructionin the expectedhigh-multiplicity backgroundthe emphasiswill beon energy-lossresolutionandtwo-trackseparation.A new developmentis thata major partofthephysicsprogramwith theTPCwill be,in conjunctionwith theTransitionRadiationDetector(TRD)and Inner Tracking System(ITS), the measurementof high-pt electrons. This implies excellent ratecapability(we plan to inspect200centralPb–Pbcollisionspersecond)andvery goodenergy-lossandmomentumresolutionfor largemomenta.

In Chapter2 the resultingrequirementson precisionandtolerancesof themechanicalstructuresoftheTPCarediscussed.Here,ourapproachis to combinethebestreasonablyachievableprecisionfor thegasgain,electricdrift field, temperatureandgascompositionwith frequentandprecisecalibrationsusinglaserbeamsandradioactive krypton,andwith constantmonitoringof thetemperatureandcompositionof thedrift gas.

The TPC field cage,describedin Chapter3, is to provide a highly uniform electrostaticfield in acylindrical high-puritygasvolumeto transportprimarychargesover long distances(2.5m) towardsthereadoutend-plates.Two suchfield configurationsarechosen,back-to-backin a commongasvolume,with a commonhigh-voltage(up to 100 kV) electrodelocatedat the axial centreof the cylinder. Themechanicalstructureof theTPCfield cageis composedof six majorcomponents:the ‘outer andinnerfield-cagevessels’to form thesensitivedetectorvolume,the‘outerandinnercontainmentvessels’topro-videprotective detectorcontainment,andthetwo ‘end-plates’wherethereadoutchambersaremounted.Therequirementsonstructuralaccuracy andgastightnesscombinedwith smallmultiplescatteringimplythat compositematerialshave to beusedthroughout.An approximatelyhalf-sizedprototypewasbuiltandthoroughlytested.The resultsdemonstratethat the requiredperformancecanindeedbe achieved.As drift gaswe will usethemixture90%Ne,10%CO2, asis currentlyusedin theNA49 experiment.

In Chapter4 we presentour choicefor the readoutchambers.The ALICE TPC readoutchamberswill beconventionalmultiwire proportionalchamberswith cathodepadreadoutasusedin many TPCsbefore. This choicewas madeafter looking in somedetail, as describedin Chapter8, into variousdifferenttechnologiessuchasRing-CathodeChambers(RCC)or GasElectronMultipliers (GEM). Theazimuthalsegmentationof the readoutplanefollows that of the subsequentALICE detectors,leadingto 18 trapezoidalsectors,eachcovering20 degreesin azimuth.Theradialdecreaseof thetrackdensityleadsto changingtherequirementsfor thereadoutchamberdesignasafunctionof radius.Consequently,therewill betwo differenttypesof readoutchamber, the innerandouterchambers.Eachouterchamberis furthersubdividedinto two sectionswith differentpadsizes,leadingto a triple radialsegmentationofthereadoutplane,with 570000readoutpadsin all. Theelectrostaticsof thechambershasbeensimulatedin detailandthedesignof thepadandwire planeswasadjustedaccordingly. Thechambershave to runat a relatively large gasgain (about2 104), but simulationsandtestswith existing chambersindicatethatthis shouldposeno problems.

Thefront-endelectronicsandreadoutof theTPCarediscussedin Chapter5. Thereadoutchambersdeliver on their padsa currentsignalwith a fast rise time (lessthan1 ns), anda long tail due to themotionof thepositive ions.Thesignalamplitudehasa typicalvalueof 7 µA. Thedesignfor thereadoutelectronicsis basedon an approachin which part of the requiredtail cancellationis donein a digitalchip, thussimplifying theanalogchip considerably. Eachreadoutchannelhencecomprisesthreebasicunits: a charge sensitive preamplifier/shaperwith a gain of 12 mV/fC, a shapingtime of 200 ns and

an equivalentnoisecharge below 1000electrons;a 10-bit 10 MHz low-power ADC; an ASIC whichcontainsashorteningdigital filter for tail cancellation,baselinesubtractionandzero-suppressioncircuits,anda multiple-eventbuffer. Thepreamplifier/shaperwill beimplementedin 0.25µm technology.

The readoutof the TPC, asdiscussedin Chapter5, wasadaptedto the new role of the TPC asarelatively high ratedetectorfor dielectronmeasurements.In this scenariotheTPCdataaresentdirectlyinto thehostprocessorsof a Level-3 Trigger/DAQ processorfarm. The systemis scalablein termsofprocessingpower andbandwidthin order to meetthe computingrequirementsof the variousphysicsprogrammes.In a first stage,zero-suppressedraw dataare recordedwithout any further processing.Theintelligentreadoutsystem(Level-3) guaranteesthereadoutof unprocessedfull TPCeventsata rateof 10 Hz. Having understoodthe TPC responseto centralPb–Pbcollisions, the Level-3 systemwilltake over morefunctionalities. Almost losslessdatacompressionandselective readoutcanbe imple-mentedrelatively easily, sincethesetechniquesdo not needlargecomputingpower. Finally, by addingmorecomputingpower, onlinetrackfinding in thewholeTPCand,therefore,effectivedata-compressionmethodsandTPC-basedselective readoutschemesbecomefeasible. If thereshouldbe a needfor ahigherrateof unprocessedfull TPCevents,thebandwidthof thesystemcanbefurtherincreased.

In Chapter6 we summarizethematerialbudgetfor theTPC.Becauseof theconsistentefforts to useextremelysmall amountsof andonly low-Z materialsthe total thicknessof the TPC structureandgasdoesnotexceedabout3%of a radiationlength.

An evaluationof theperformanceof theproposedTPCin theLHC Pb–Pbcollision environmentispresentedin Chapter7. With amicroscopicsimulatorcontainingall relevantprocessesfor signalcreationin the TPC a detailedstudywasperformedto determinethe radial dependenceof the TPC occupancyfor centralPb–Pbcollisionsat themaximumexpectedparticledensity. Thethus-determinedmaximumoccupancy reachesabout40%in theinnermostsectionsof thereadoutchambersanddecreasesto about15% nearthe outer radius. Thesestudiesalso provide detailedinformation on the role of field- andspace-charge distortions,on theexpectedbackgroundsanddetectorload,andwereusedto validatethedesignchoicesfor thereadoutchambersandfront-endelectronicsmadeon thebasisof somewhatsim-plified simulationsandanalyticcalculationsreportedin Chapter4. Furthermore,themicroscopicsim-ulationsprovide the input for theclusterfinderandtrack-reconstruction program.Trackfinding in theALICE TPCis basedon theKalman-filteringapproachwhich providessimultaneoustrack recognitionandfitting andthepossibility to rejectincorrectspacepoints‘on thefly’, during theonly trackingpassover a track. Thetrackingsimulationsarestill underdevelopment,but theresultsreportedin Chapter7areencouraging:betterthan90%track-reconstruction efficiency evenfor thehighestmultiplicity densi-tiesanda resolutionin energy lossbelow 10%,asrequiredby thehadronandelectronprograms.Themomentumresolutionfor a 4 GeV/c electronis about8.5%for thehighestconceivablemultiplicities atamagneticfield of 0.2T. Takinginto accounttheadditionalimprovementsfrom usingtheITS andTRDin the trackfit (estimatedto be morethana factorof two), the required2.5%resolutionfor separationof theϒstatesshouldbeattainablefor runswith magneticfield of about0.4T. Firstestimatesalsoshowexcellentmatchingof theTPCtrackswith theITS andTRD.

Installation,slow control andsafetyarediscussedin Chapter9. As demonstratedthere,the instal-lation of theTPC is not easybut manageable.An elaborateslow control systemis plannedto monitormany parametersof the TPC suchascurrentsandvoltagesfor the high-voltagesystemof the readoutchambersandfor the low-voltagesystemof the front-endelectronics,aswell astemperatureandgaspurity. A detailedinvestigationrevealedno serioussafetyissuesfor theTPC.

Organizationalaspects,budgetsandschedulesarepresentedin Chapter10. TheTPCgroupnow com-prises13instituteswith considerableexperienceandmanpower. Thebudgetof approximately17MCHFis in line with expectationandpreviousestimatesandall time lines imply thatconstructionof theTPCshouldbefinishedby theautumnof 2003,in line with theoverall ALICE planning.

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Contents

Summary xi

1 Physicsobjectivesand designconsiderations 11.1 TheALICE experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Physicsrequirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2.1 Hadronicobservables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.2.2 Leptonicobservables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.3 TPCdesignconsiderations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Designobjectivesand mechanicalstructure 52.1 Precisionandtolerance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.2 Stability of themechanicalstructure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3 Field cageand gassystem 93.1 Fieldcage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.1.1 Generallayoutandchoiceof material . . . . . . . . . . . . . . . . . . . . . . . . . . 93.1.2 Assemblyandtesting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163.1.3 Voltagedivider . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223.1.4 Temperaturestabilizationandthermalscreen. . . . . . . . . . . . . . . . . . . . . . 273.1.5 Testsandprototypes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

3.2 Gassystem. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323.2.1 Specificoperationalrequirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . 323.2.2 Gaschoices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333.2.3 Designchoicesandlayout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

4 Readoutchambersand calibration 414.1 Readoutchambers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

4.1.1 Mechanicalstructureandsupport . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414.1.2 Wire planesandreadoutpads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 444.1.3 Electrostaticcalculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 584.1.4 Electronicsmountingandcooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . 694.1.5 Link betweenfield cageandreadoutchambers. . . . . . . . . . . . . . . . . . . . . 744.1.6 Testsandprototyping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

4.2 Gatingsystem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 804.2.1 Designconsiderations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 804.2.2 Gatingcircuit design. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 814.2.3 Performance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 814.2.4 Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

4.3 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 824.3.1 Lasersystem. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 824.3.2 Electronicscalibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 864.3.3 Krypton calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

5 Front-end electronicsand readout 915.1 Front-endelectronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

5.1.1 Introductionandoverview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

xiv

5.1.2 Interconnectionto thereadoutpadplane . . . . . . . . . . . . . . . . . . . . . . . . 945.1.3 Front-endbasiccomponents. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 965.1.4 Front-EndCard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1105.1.5 Readoutbus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1115.1.6 Readoutcontrolandservices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1165.1.7 Front-endelectronicreadoutefficiency . . . . . . . . . . . . . . . . . . . . . . . . . 1185.1.8 Low-voltagepower suppliesanddistribution . . . . . . . . . . . . . . . . . . . . . . 1205.1.9 Systemimplementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

5.2 Readout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1225.2.1 Physicsrequirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1225.2.2 Dataratereduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1245.2.3 Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1315.2.4 Interfaces. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1365.2.5 DataacquisitionandLevel-3software. . . . . . . . . . . . . . . . . . . . . . . . . . 140

6 Material budget 1436.1 Estimateof radiationlengthin η space . . . . . . . . . . . . . . . . . . . . . . . . . . . 1436.2 Estimateof radiationlengthin ϕ space . . . . . . . . . . . . . . . . . . . . . . . . . . . 145

7 Detectorperformance 1477.1 Requirementsanddetectorparameters . . . . . . . . . . . . . . . . . . . . . . . . . . . 1477.2 Simulationof TPCresponse. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147

7.2.1 Microscopicsimulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1477.2.2 Backgroundanddetectorload . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154

7.3 Trackreconstruction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1557.3.1 Trackingenvironment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1557.3.2 Clusterfinding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1627.3.3 Trackfinding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163

7.4 Trackingperformance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1657.4.1 Trackingefficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1667.4.2 Two-trackefficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1677.4.3 Momentumresolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1687.4.4 dE/dx resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168

7.5 Trackmatching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1697.5.1 Connectionto ITS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1697.5.2 Connectionto TRD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170

8 R&D for alternative readoutchambers 1738.1 Ring-CathodeChamber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173

8.1.1 Preamplifier/Shaper. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1748.1.2 TAB bonding. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1748.1.3 Padresponsefunction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1768.1.4 Gating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1778.1.5 Isochrony . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1788.1.6 Lead-beamtests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1798.1.7 Conclusionson RCC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180

8.2 GEM-basedreadoutchambers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1808.2.1 Principleof operationof GEMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1808.2.2 Basicpropertiesof GEMsrelevantfor TPCreadoutchambers. . . . . . . . . . . . . 1818.2.3 Conclusionson GEMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186

9 Installation, slow control and safety 1879.1 Implementationandinfrastructure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187

9.1.1 ALICE experimentalarea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1879.1.2 Implementationof theTPCdetector. . . . . . . . . . . . . . . . . . . . . . . . . . . 1889.1.3 Access,maintenanceandservices. . . . . . . . . . . . . . . . . . . . . . . . . . . . 1909.1.4 Assemblyandinstallationschedule. . . . . . . . . . . . . . . . . . . . . . . . . . . 192

9.2 Slow control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1949.2.1 Hardware. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1959.2.2 Communication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1959.2.3 Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195

9.3 Safetyandqualitymanagement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1989.3.1 Mechanical. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1989.3.2 Gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1989.3.3 Radiationprotection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1989.3.4 RF shielding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1999.3.5 Electricalsystemprotection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1999.3.6 Laser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2009.3.7 Safetyaspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200

10 Organization 201

References 205

1

1 Physicsobjectivesand designconsiderations

1.1 The ALICE experiment

ALICE (A Large Ion Collider Experiment)[1] is an experimentat the Large HadronCollider (LHC)optimizedfor thestudyof heavy-ion collisions,atacentre-of-massenergy 5 5 TeV. Theprimeaimoftheexperimentis to probe,in detail,nonperturbative aspectsof QCD suchasdeconfinementandchiral-symmetryrestoration.Furthermore,of interestto theexperimentarethenewly emerging physicstopicsrelatedto thestudyof QCD atvery highfield strengthsasdetailedin Ref. [2].

Thestrategy of theALICE experimentto studythe behaviour of matterat high densitiesandtem-peraturesat nearzerobaryochemicalpotentialis to combinea nearlyexclusive measurementof particleproductionin thecentralregionwith spectroscopy of quarkoniastatesatcentralandintermediaterapidi-tiesandcharacterizationof theeventcentrality. It thereforecombinesthreemajorcomponents:

Thecentralbarrel,containedin thelargeL3 magnetandcomposedof detectorsdevotedmainly tothestudyof hadronicsignals[1] anddielectrons[3]. It coversthepseudorapidityrange 0 9 η 0 9 over thefull azimuth.

Theforwardmuonspectrometer, dedicatedto thestudyof muonpairs[4] fromthedecayof quarko-nia in theinterval 2 5 η 4 0.

Theforwarddetectors,η 4, for adeterminationof thephotonmultiplicity andameasurementofthechargedparticlemultiplicity, alsoto beusedasa fasttriggeron thecentralityof thecollision.

Theset-upof theALICE experiment,with emphasison thecentralbarrel,is shown in ColourFig.I. TheTime ProjectionChamber(TPC),surroundstheInnerTrackingSystem(ITS) which is optimizedfor the determinationof the primary andsecondaryverticesandprecisiontrackingof low-momentumparticles.On theoutsidetheTransitionRadiationDetector(TRD) is designedfor electronidentification.The outermostTime-Of-Flight (TOF) arrayprovidespion, kaonandprotonidentification. In addition,thereare two single-armdetectors:the PhotonSpectrometer(PHOS)andan arrayof RICH countersoptimizedfor High-MomentumParticleIDentification(HMPID).

1.2 Physicsrequirements

Thephysicsobjectivesof theALICE centralbarrelhave beendetailedin Ref. [1]. However, therecentadditionof theTRD hasexpandedthephysicsobjectivesof theexperimentasdemonstratedin Ref. [3].As aconsequence,theperformanceandcorrespondingdesigncriteriahadto bereassessedandoptimised,takingalsointo accounttherequirementsfor dielectronstudies.

TheALICE TPCis themaintrackingdetectorof thecentralbarrelandtogetherwith theITS, TRD,and TOF hasto provide charged particle momentummeasurement,particle identificationand vertexdeterminationwith sufficientmomentumresolution,two trackseparationanddE/dx resolutionfor studiesof hadronicandleptonicsignalsin theregion pt 10 GeV/c andpseudorapiditiesη 0 9. In additionwe plan, with the centralbarreldetectors,to selectlow crosssectionsignalsand rareprocessesand,therefore,to generatea fastonline‘Level-3’ trigger.

All theserequirementsneedto be fulfilled at the designluminosity for Pb–Pbcollisions at LHCwhich correspondsto an interactionrateof 8 kHz of which 1 kHz arecentralcollisionswith impactparameterb 5 fm. The producedparticlemultiplicities aredifficult to predictandfor centralPb–Pb

2 1 Physicsobjectivesanddesignconsiderations

collisionsareexpectedto bein therangeof 2000–8000chargedparticlesperrapidityunit atmidrapidity.For the designof the detectorthe maximalexpectedmultiplicity of dNch/dy 8000 wasusedwhichresultsin 20000 chargedprimary andsecondarytracksin the TPC. At the designpresentedherethisparticlemultiplicity resultsin occupancies(definedastheratio of thenumberof readoutpadsandtimebinsabove thresholdto all bins in padandtime space)of theorderof 40%at the innermostradiusand15%at theoutermostradius.

Thechallengeto theTPCdesigngroupwasto ensurethat thephysicsdictatedrequirementscanberealizedin thisenvironment.

1.2.1 Hadronic observables

The TPC is the main tool to investigatehadronicobservablesin Pb–Pbcollisions. Hadronicmeasure-mentsgive informationon the flavour compositionof the fireball via the spectroscopy of strangeandmulti-strangehadrons,on the space–timeextent of the fireball at freeze-outvia the investigationofsingle-particleandtwo-particlespectraandcorrelations,andon event-by-event fluctuationsof thefire-ball.

Hadroniccorrelationobservablesplacethehighestdemandonrelativemomentumandtwo-trackres-olution. In two-pion Bose–Einsteincorrelationanalysis(‘Hanbury-Brown–Twiss’ analysis,HBT) oneconsiderscorrelationfunctionsin all componentsof the4-momentumdifference,with specialemphasison thedomainsδq 0. Themomentumwidthsof thesecorrelationfunctionsaresensitive to thegeo-metricalspace–timesourceextent of theexpandinghadronicfireball. Extrapolatingfrom typical sizesof about8 fm observedin HBT analysisof SPSPb–Pbcollisions,weexpecttheeffective sourcesizestoincreasein proportionto thecuberootof therapiditydensitydNch/dy whichgrowsfrom 400chargedpar-ticlesat theSPSto about6000–8000at theLHC. Thus,measurementshave to besensitive to sizesof upto 25 fm or δq 8 MeV c. Notethatthis relative momentumaccuracy is neededmostlyfor transversemomentabelow the averagevalueof pt, i.e. approximately500 MeV/c for pions. For the correlationfunctionsin theso-calledsidewardandlongitudinaldirections(δqside andδqlong) theserequirementsarefulfilled, as is demonstratedin Section7.4. More critical is the ‘outward’ correlationin δqout wheresourcesizesof 25 fm will probablyonly bemeasurablein runningwith ahigher-than-standardmagneticfield.

Anotherimportantrequirementon theTPC is sufficient acceptancein rapidity and pt for thestudyof space–timefluctuationsof the decomposingfireball at the level of individual events. For a detailedanalysisof kaon spectraand the kaon/pionratio on an event-by-event basis,one typically needs250analysedkaons,againsupportingtheneedfor largeacceptanceandgoodparticleidentification.

Specificrequirementson theTPCfrom hadronphysicsarethefollowing.

Two-track resolution: The two-track resolutionhasto be suchthat HBT measurementswitha resolutionin relative momentumof a few ( 5) MeV/c canbe performed. This may requirerunningat highermagneticfields.

Resolution in dE/dx: For hadronidentificationa dE/dx resolutionof 8% is desirable,followingtheexperienceof NA49. Dependingon thefinal particlemultiplicity this canjust bereachedwiththecurrentdesign.

Track matching capability to ITS and TOF: For the measurementof D-mesonsvia hadronicdecaychannels,of multi-strangebaryons,andof HBT correlations,efficient matchingwith theITS is very important. Dependingon the pt rangeconsidered,thematchingefficiency shouldbe85%–95%.

1.3 TPCdesignconsiderations 3

1.2.2 Leptonic observables

TheTransitionRadiationDetector(TRD) [3] will provide, in conjunctionwith datafrom theTPC andITS detectors,sufficient electronidentificationto measure,in thedielectronchannel,theproductionoflight andheavy vector-mesonresonancesfor Pb–PbcollisionsattheLHC, aswell asto studythedileptoncontinuum. In addition, the electronidentificationprovided by the TPC and TRD at relatively largetransversemomenta(pt 1 GeV/c) canbeused,in conjunctionwith theimpact-parameterdeterminationof electrontracksin theITS, to determinetheoverall amountof opencharmandopenbeautyproducedin thecollision. With asimilar techniqueonecanalsoseparatedirectlyproducedJ ψ mesonsfrom thoseresultingfrom B-decay. The latter could potentiallymaskthe expectedJ/ψ suppressiondueto quark-gluon plasmaformationandis, therefore,of crucial importancefor suchmeasurements.Furthermore,sincethe TRD is a fast tracker, it can be usedas an efficient trigger for high transversemomentumelectrons.This canbeusedto considerablyenhanceyields for ϒ detection,for themeasurementof thehigh-masspartof thedileptoncontinuum,andfor J ψ detectionathigh transversemomentum.

Specificrequirementson theTPCfrom electronphysicsareasfollows.

Tracking efficiency: Sincewe aremainly interestedin electronpairsthe trackingefficiency fortrackswith pt 1 GeV/c shouldbelargerthan90%.

Momentum resolution: Themomentumresolutionfor electronswith momentaof about4 GeV/cshouldbe betterthan2.5%. This is necessaryto keepthe massresolutionfor ϒ mesonsbelow100MeV, so that themembersof theϒ family canberesolved. As discussedin Section7.4 thisresolutioncanbe achieved, but only in conjunctionwith the ITS andwhenrunningat magneticfieldsaround0.4T.

Resolutionin dE/dx: For electronidentificationtheTPChasto provideadE/dx resolutionof bet-ter than10%in thehigh-multiplicity environmentof a Pb–Pbcollision. This will, in conjunctionwith electronidentificationfrom theTRD, leadto a pion rejectionof 103 at 90%electroneffi-ciency for electronmomentalargerthan1 GeV/c. Fromthesimulationsmadefor theTRD TP [3]this is sufficient for all dielectronphysicsplannedwith theALICE detector. Currentsimulations,reportedin Section7.4,imply thatsuchresolutionscanbeobtainedwith thepresentdesign.

Rate capability: For the inspectionand tracking of electroncandidatesidentified in the TRDthe TPC shouldbe operatedat centralcollision ratesof up to 200 Hz. While thereis not muchexperiencewith theoperationof largeTPCsattheserates,thecurrentloadonthereadoutchambersis notexcessive asdiscussedin Section7.2.2.

Detailedsimulationsfor theelectronphysicspotentialof theITS–TPC–TRDcombinationwerepre-sentedin Ref. [3] andwill notberepeatedhere.

1.3 TPC designconsiderations

Thegeneralconsiderationslistedabove have led to a designof theTPCwhich is ‘conventional’ in theoverall structure,but with many innovative aspectsin detail. Centralaspectsof the designare listedbelow.

Acceptance:Theoverallacceptanceof η 0 9matchesthatof theITS,TRD,andTOFdetectors.This acceptanceis importantfor event-by-event studiesof fluctuationsin hadronicobservablessuchastheK/π ratio, from which onecandeterminethestrangenesscontentof thefireball. Forelectronphysicsthe full acceptanceis absolutelycrucial to collect significantstatisticsfor high-massand/orhigh-pt electronpairs[3]. To cover this acceptancetheTPCis of cylindrical designwith aninnerradiusof about80 cm,anouterradiusof about250cm,andanoverall lengthin thebeamdirectionof 500cm.

4 1 Physicsobjectivesanddesignconsiderations

Material budget: The materialbudgetof the TPC hasto be kept as low aspossibleto ensureminimalmultiplescatteringandlow secondary-particleproduction.Thisrequiresspecialattentionto materialsusedin theconstructionof thefield cageandenforcestheuseof a light countinggas,90% Ne, 10% CO2. The overall thicknessof the TPC waskept to lessthan3% of a radiationlength.Detailsandsummariesaregivenin Chapter6.

Field cage: For very generalrequirementsof minimal materialnear90 relative to the beamdirection,thefield cageis basedon a designwith a centralelectrodeat high voltage.Becauseofthe gasmixture usedin the TPC andthe neededhigh ratecapability the field cagefor the TPChasto run at voltagegradientsof 400V/cm, implying voltagesin excessof 100kV at thecentralelectrode.Thishasled to extensive prototyping.Theresultingdesignis discussedin Section3.1.

Readout chambers: The readoutchamberscover the two end-capsof the TPC cylinder. Theoverall areato beinstrumentedis 32.5m2. Thechamberswill beconventionalmultiwire propor-tionalchamberswith cathodepadreadout.To keeptheoccupancy aslow aspossibleandto ensurethenecessarydE/dx andpositionresolution,therewill beabout570000readoutpadswith threedifferentsizesvaryingfrom 0.3cm2 neartheinnerradiusto 0.9cm2 neartheouterradius.Detailsof thedesignstrategy arediscussedin Section4.1.

Electronics: At about570000 channelsthe front-endelectronicsfor the TPC hasto be highlyintegrated.It will consistof threebasicunits for eachchannel:a low-impedancecharge-sensitivepreamplifier/shaper, a commercial10-bit ADC with up to 10 MHz frequency range,andanASICwith a digital filter for tail cancellation,with base-linesubtractionandzero-suppressioncircuitry,anda multiple event buffer, all to be implementedin CMOS technology. Designconsiderationsandexpectedcapabilitiesarediscussedin Section5.1.

Intelligent TPC readout: After zero-suppressionanddataencodingtheeventsizeof datafromthe TPC for a typical centralPb–Pbcollision will be about60 MBytes. Consideringthat oneneedsabout40 Hz data-takingcapability from the TPC for dielectronphysics(in fact, 200 Hzarecurrentlyconsidered,seeChapter5) this makesit mandatoryto investigatepossibilitiesfor an‘intelligent’ readout.Weareplanningto build, to thisextent,aLevel-3processorfarm,whichwilloperateon the raw datashippedvia optical links to the ALICE countinghouse.This will allowalmostno-lossdatacompression,selective readoutfor electroncandidatesidentifiedby theTRD,aswell asonline trackfinding andeventuallytrackingof thewholeTPC.The farmwill bebuiltasmuchaspossiblefrom commerciallyavailablecomponents.Detailsof the intelligent readoutincludingstrategiesfor its implementationarediscussedin Section5.2.

5

2 Designobjectivesand mechanicalstructur e

2.1 Precisionand tolerance

Experimentalconditionsat theLHC setstringentdesignandperformancecriteria for theTPC in orderto addressthefull rangeof physicsaccessibleat theLHC andplausiblyachieve our objectivesfor mo-mentumanddE/dx resolution,aswell asfor patternrecognitionin a highly congestedenvironment.Tofulfil thesegoals,theTPC,beinga largebut conceptuallysimpledetector, mustbebuilt with very highprecision,if theindividual trackinformationis to beretainedover long drift distancesor times.Nonlin-earsystematiceffectsleadto unrecoverabletrackdistortionsanderrorsin theenergy-lossmeasurements,andmustthereforebe reducedto a minimumat theoutsetof thedetectorconstruction.Thus,theulti-matelyachievableprecisionin trackreconstructionandenergy-lossmeasurementdependson theabilityto provide highstability anduniformity within thesensitive volumeof theTPCfor:

thegasgainor wire amplification, thedrift field, thetemperature, thedrift gaspurity.

TheTPCis designedto minimizeeffectsfrom misalignmentandmechanicaldeformationof all partsthatdefinethedrift field, i.e. thecentralelectrode,thepotentialdegradernetwork andtheend-plates.Carefulchoiceof materialswill furthermorereduceenvironmentalinfluenceson gastemperatureandpurity. Togive cleardesigndirectionsasto theprecisionto which theTPCmustbebuilt, we mustfirst assessthemaximumtolerancesthatcanbeallowedfor theabove four parameters.

Toleranceson gasgain.

For a fixed wire geometry(and drift gasmixture) the gasgain is entirely determinedby the voltageappliedto the anodewires. As shown in Section4.1.3.4,to achieve a gasgain of 2 104 an anodevoltageof 1.7 kV is required. This voltageis sufficiently high to causeelectrostaticdeflectionof theanodewirestowardsthereadoutplane,resultingin awire sagittaof 5 µm for theshortest(300mm) and65 µm for the longest(900mm) wires. This leadsto variationsin gasgainalongtheanodewire of 3%and5%, respectively. Theeffect canbealleviatedby calibratingthesignalresponsewith the injectionof radioactive 83Kr into the drift gas(seealsoSection4.3.3). This methodwassuccessfullyapplied,amongstotherdetectors,to the NA49 TPCs[1] andallowed overall gain variationsin electronicsandwire amplificationof theorderof 10%to becorrectedfor with aprecisionof 0.5%.Thus,themaximumvariationsof 5%in gasgainexpectedfrom thewire sagin theTPCareeasilymappedoutwith theabovecalibrationmethod.

Toleranceson drift field uniformity

Radialfield componentsin thedrift field changethetrajectoryof electronsfrom anideally straightpath(in z) towardsthereadoutplaneof theTPCandthushave aneffect on thespace-pointresolution.Thesefield nonuniformitiesshouldbewell below theintrinsicallyachievableresolutionof theTPC.Underidealconditionsthespace-pointresolution,dependingonthedrift lengthandtrackorientation,variesbetween300 and2000µm in rϕ, and600and2000µm in z. It hasbeenshown that radial field componentsoforder10 4 comparedto thefield componentin z leadto deviationsof theelectrontrajectory, however,

6 2 Designobjectivesandmechanicalstructure

150200250300350400450500550600650700750800850900950

0.20.40.60.8

11.21.41.61.8

22.22.42.62.8

33.23.43.63.8

E [V/cm]

Cha

nge

in v dr

ift [‰

/K]

Figure2.1: Relativechangeof theelectrondrift velocityfor atemperaturegradientof 1 Cfor 90%Ne,10%CO2,asa functionof thedrift field.

not exceeding200 µm in r and150 µm in rϕ, respectively. Hence,the mechanicalstructureandfielddefiningnetwork of theTPCaredesignedto keepradialfield nonuniformitiesto 10 4.

Temperature effects

If the drift velocity of electronsin the TPC is not saturated,i.e. dependingon drift field, gaspressureandgastemperature,externalinfluenceson thedrift gasmustbe reducedto a minimum. In particular,temperaturevariationscauselocal fluctuationsin density, andthusdirectly affect thevelocity of driftingelectrons.For reasonsoutlinedlater, we have chosena 90%Ne,10%CO2 mixtureandanelectricfieldof 400 V/cm for optimum operationof the ALICE TPC. Theseoperatingparameters,however, leadto a nonsaturateddrift velocity. Our studiesandexperiencewith the NA49 [1] experimentshow thatthe long drift pathof electronsimposesa tight control on temperaturegradientsto staybelow 0.1 Cwithin the sensitive drift volumeof the TPC, in particular, betweenits inner andouterradius. As theTPC is exposedto potentiallylarge heatsourcesin its immediateneighborhood,it is very challengingto guaranteetemperaturegradientsbelow 0.5 C, althoughtechnicalsolutionson thermalisolationoftheTPClook promising.While it is possibleto correct,with thelasercalibrationsystem,for long-termdistortionsarisingfrom timerelatedtemperatureeffects,localtemperaturedifferencesin thedrift volumecannotbemappedandhenceleadto unrecoverableerrorsin thetrackmeasurement.Therefore,weaimatatemperaturestabilityof 0.1 C, becauseevenundertheseconditionstherelativechangein drift velocitywith temperatureis 0.34%perdegreein 90%Ne,10%CO2 (seeFig. 2.1),anddistortionsof theparticletrajectorycanbeashigh as850µm for thefull drift distance.Althoughthis valueis comparableto thespace-pointresolutionin z, this typeof distortioncausesasystematicerrorin dip angle.

Limits on gasimpurities

Sincethetotal electronyield in thechosen90%Ne,10%CO2 mixtureis ratherlow, electroncapturebygasimpuritiesis a matterof primeconcern.Pastexperiencewith this drift gasin theNA49 experimenthasproventhattheelectronattachmentcoefficient isequalto 1%permetreof drift perppmof oxygen[1].Weconcludethat5 ppmof oxygenand10 ppmof watercontaminationareacceptablein termsof signallossat thereadoutfront-end.

2.2 Stabilityof themechanicalstructure 7

2.2 Stability of the mechanicalstructur e

The TPC is designedto be the major particle tracking systemfor ALICE, placedbetweenthe innertrackingsystem(ITS) [2] andthetransitionradiationdetector(TRD) [3]. Jointlywith theITS andTRD,theTPCmustplay theroleof ahigh-precisiontrackingsystemof its own andyetbecompatiblewith thephysicsobjectivesof its neighbours.

Hence,theTPCmustbeof very low massto minimizeinterferencewith its partnerdetectorsystems,andof high stability to satisfytheprecisionandreliability requirementsfor a stand-alonetrackingsys-tem.Constructingthedetectorfrom modularcylindrical elements,usingcompositematerialthroughout,providesanoptimumin termsof low mass,high rigidity andsafety. Thesystemis designedto providestructuralintegrity andto compensatefor effectsarisingfrom differentcoefficientsof expansion.In par-ticular, theoutercylindrical vessels,togetherwith theend-platesconstitutethemainstructuralelementof the whole detector, guaranteeingits high stability in z and rϕ. The inner structureof the TPC, to-getherwith theouterone,providesthe requiredparallelismof theendplateswith thecentralelectrodemountedinsidetheTPC.Theinnervesselis designedto positionandhold in placeboththeITS andthemultiplicity counterswith highprecision.

Thereadoutchambersof theTPCaremountedonthreepointsi.e.kinematicallyindependentof eachotherandof theend-platesupportstructure.Knowing their centreof gravity, they arepositionedwithintheend-plateframesuchthatno bendingmomentsareexertedon theend-platestructure,preservingitsplanarity. Thus,by not overconstrainingthereadoutassemblywith respectto thesupportstructure,theend-platescanbebuilt light andtheindividualdetectorswill notbesubjectto internaldeformations.Thedetectorscanalsobeoptimallyalignedin this fashionto build anoverall co-planardetectionsystem.

The centralelectrodewill be constructedfrom compositematerialaswell. This is to maintainitsflatnessdespitethe forcesdueto groundinclination (1.39%or 0 8 ) of the experimentalareaandaunidirectionalgasflow within thefield cagevolume.

After transportto thespaceframetheentiredetectorwill beplacedon four adjustablesupportpoints(Fig. 2.2) thataremountedon theend-plateswherethebulk of thedetectormassis concentrated.Hy-draulic jackson thesesupportpointsallow isostatic,i.e. kinematicallyindependent,adjustmentof theTPCto align it with theparticlebeam.Dampingelementscanbeplacedbetweenthesupportpointsandthespaceframerails in orderto isolatethedetectorfrom externalstructuralvibrationsshouldthey exist.

Figure 2.2: Isostaticplacementof theTPCwithin thespaceframe.

9

3 Field cageand gassystem

3.1 Field cage

3.1.1 General layout and choiceof material

Thebasicdesignphilosophyof theTPCfield cageis to provide a highly uniform electrostaticfield in acylindrical high-puritygasvolumeto transportprimarychargesover long distances(2.5m) towardsthereadoutend-plates.For reasonsof symmetryin colliding beamarrangements,two suchfield configura-tions arechosen,back-to-backin a commongasvolume,with a commonhigh-voltage(HV) electrodelocatedat theaxialcentreof thecylinder (seeFig. 3.1).ThecentralHV electrodeandtwo oppositeaxial

Figure 3.1: Conceptualview of theTPCfield cage.

potentialdegradersprovideuniformdrift fieldsof up to 400V/cm. Thedrift field is chosenasa functionof theintrinsic propertiesof thedrift gasaffectingthedrift velocity andthediffusionof primaryioniza-tion electronsin thatgas.Thus,giventhemaximumdrift pathof 2.5m, theHV at thecentralelectrodewill beaslargeas100kV. Themaximumover-pressureallowedfor thefield cagewill be5 mbar.

The actualfield cagevolumeis surroundedby an insulatinggasenvelope(containment)asshownin Fig. 3.2. Containmentof thedrift volumeis essentialfor personnelandoperationalsafetyandalsofor minimizing the amountof materialtraversedby particles. Its functionsaredescribedin detail inSection3.1.1.1andSection3.1.1.2.

ThebeamtubeandtheInnerTrackingSystem(ITS) requirethatboththefield cageandthecontain-mentvolumeareconstructedfrom two concentriccylinderseachandsealedby anannulardisc,calledtheend-plate,on eithersideof thecylindrical structure.Thus,themechanicalstructureof theTPCfieldcageis composedof six majorcomponents(seeColourFig. II): the‘outerandinnerfield cagevessels’toform thesensitivedetectorvolume,the‘outerandinnercontainmentvessels’to provideprotectivedetec-tor containment,andthetwo ‘end-plates’wherereadoutchambersaremountedto amplify andregister

10 3 Field cage andgassystem

Figure 3.2: Lateralview of theTPCshowing thesensitivedetectorandinsulationvolumes.

the primary charge of particletracks. This modularstructureallows eachcomponentto be optimizedindividually in termsof costandperformance.Theconstructionof thefield cageandconsequentlythechoiceof materialaredrivenby thefollowing constraints:

highstructuralintegrity againstgravitationalandthermalloads;

very low permeabilityto atmosphericgascomponentsconsideredharmfulto thedrift gas(O2, N2

andH2O);

negligible vapourpressureof contaminantsemanatingfrom materialexposedto thedrift volume;

adequatesurfacesmoothnessto protectagainstHV discharges;

low-densityandlow-Z materialto reducemultiple scatteringandconversionprocesses.

This hasled to thechoiceof compositematerialsfor all four cylinders. Compositesandwichstructurestodayprovidethehigheststability/massratioandarecommonlyusedin theaerospaceindustryprovidingthecompetenceandtoolingalsofor themanufactureof theTPCcylinders.Thecylinderskinsaremainlycomposedof (seealsoFig. 3.3):

aninnerhoneycomb-like structureof Aramidefibre(Nomex), rectangularin shape,andwith acellsizeof 4.8mm,coveredon eithersideby:

two layersof Kevlar prepreg, i.e. wovenAramidefibersembeddedin anepoxyresin,and

gas-tightfoils of Tedlar, typeTWH 20BS3.

In order to obtainuniform andhomogeneousquality of the detector, the curing of resinsusedfor thedifferent componentsmust be carriedout in an autoclave. The basicfeaturesof the TPC field cagematerialcomponentsarelistedin Table3.1.

3.1 Field cage 11

Figure3.3: Cross-sectionof thecompositematerialusedfor theTPCfield cage.

Table3.1: List of materialsusedin theTPCfield cageandtheirmechanicalandelectricalproperties.

Density Modulus/ Surface GasMaterial tensilestrength resistivity permeability

[g/cm3] [GPa] [Ω ] [cm3/(100in2)(d)(atm)(mil)]

Kevlar 49 1.45 120 n.a. n.a.Glass-fibre 2.54 69 n.a. n.a.Tedlar 1.71 0.062 6 1015 0.25(N2), 3.2(O2), 11.1(CO2)Mylar 1.39 4.5 5 1014 n.a.Nomex 0.03 0.055 n.a. n.a.(3.62%filling) (compression)Glue 1.25 n.a. n.a. n.a.Macrolon 1.2– 1.44 n.a. 1014 1015 n.a.Aluminium 2.7 n.a. n.a. n.a.

3.1.1.1 Outer containment vessel

The outer containmentvessel(Fig. 3.4) is the outermostand largestof the four TPC cylinders thatcomprisethe entire field cageassembly. It is 5.1 m long and hasa diameterof 5.56 m. It hasfivefunctions:

1. provide mechanicalstabilityandprecisionfor theentireassembly;

2. provide containmentof thevery high voltagesappliedto thefield-cagenetwork;

3. provideadditionalprotectionof thedrift gasagainstleakageof atmosphericgasesandvapoursintothegasvolume;

4. serve asanexteriorheatshieldagainsttemperaturevariations/gradientsin thedrift volume;

5. reduceradiofrequency interferencewith thesensitive front-endreadoutsystem.

The two flangesaremadeof a compactstructureof prepreg with the following composition:solidlaminateepoxyresin(125 C) with a glass-fibrefabricof minimum50%fibre contentby volume. It isenvisagedto cover thesurfacesin contactwith theO-ringswith 50µm of Tedlarto improvegastightness


Figure3.4: Lateralview of theentirefield-cageassembly.

of theseseals.Sealingis achievedwith O-ringsmadeof fluorinebasedelastomers,suchasViton, with ahigh shorevalue(for example,70 shore).

For thecylindrical wallsof theoutercontainmentvesselthefollowing compositionis foreseen:

1 layerof aluminiumfoil, 50 µm thick;

1 layerof Tedlar, typeTWH 20BS3,50 µm thick;

2 layersof prepreg (weaving layout 77–81,60% fibre and40% resinby volume,epoxyresinat125 C), 0.3mmthick each;

1 honeycomb-like structureNomex (height 30 mm,29kg/m3);

2 layersof prepreg (asabove);

1 layerof Tedlar(asabove);

1 layerof aluminiumfoil (asabove).

Thealuminiumfoils needto be glued longitudinallyonto thesurfaces,i.e. alongthecylinder axis.This is to ensurecontinuouselectricalconductivity alongtheentirelengthof thecylindrical shell.There-after, thefinishedlayerswill beelectricallyconnectedtogetheratbothendsof thecylinder. Thisprovidesprotectionagainstelectricalshockandalsoactsaselectrostaticshieldingagainstspuriousradiofrequencynoise. Furthermore,during operationof the detectorthe inner conductive surfaceof this vesselis ex-posedto thepotentialof thecentralHV electrode(100kV), mountedinsidethefield cagevessel,acrossa 150 mm thick, CO2 filled, gap. This surfacemust thereforebe smoothto minimize the risk of HVbreakdowns. Theoutervesselweighsapproximately420kg.

3.1 Field cage 13

3.1.1.2 Inner containment vessel

Theinnercontainmentvesselis theinnermostandhencesmallestof thefour TPCcylinders(Fig.3.4). Itsfunctionsareotherwiseidenticalto thoseof theoutercontainmentvessel.Thoughconsiderablysmallerthanits counterpartit consistsof threeparts:

two conicalend-pieces,calledtheleft andright conicaldrum,whichintegratetheconicalfront-endabsorberof themuonarm.They areidenticalandof thesamecompositionastheoutervessel;

an ultra-light cylindrical centraldrum at small radiuswithin the acceptanceregion of the TPC( η 0 9), in order to put materialas closeas possibleto the vertex, which minimizestheinfluenceof multiplescatteringonmomentumresolution.Theinnerdiameterof the‘centraldrum’is suchthatit allows theinstallationandmountingof theITS.

Eachof thesethreedrumswill beproducedin anautoclave. In additionto its outsideflange,eachconicaldrum will alsohave a light positioningflange,to hold andpositionthe centraldrum andthe ITS. Theattachmentof thetwo conicaldrumsto thecentraloneis to becarriedoutata laterstagethroughbondingat roomtemperature.Theexactcompositionof thethreedrumsis asfollows:

Conicaldrums:

– 1 layerof aluminiumfoil, 50 µm thick;

– 1 layerof Tedlar, typeTWH 20BS3,50 µm thick;

– 2 layersof prepreg (weaving layout77–81,60%fibreand40%resinby volume,epoxyresincuringat125 C), 0.3mmthick each;

– 1 honeycomb-like structureNomex (height 20 mm,29kg/m3);

– 2 layersof prepreg (asabove);

– 1 layerof Tedlar(asabove);

– 1 layerof aluminium(asabove);

– positioningflangesof prepreg (solid laminateepoxy resin at 125 C, glassfibre fabric ofminimum50%fibre by volume).

Centraldrum:

– 1 layerof aluminium,50 µm thick;

– 1 layerof Tedlar, typeTWH 20BS3,50 µm thick;

– 2 layersof prepreg (weaving layout120,50%fibreand50%resinby volume,epoxyresinat125 C); 0.1mm thick each;

– 1 honeycomb-like structureNomex (height 5 mm,29 kg/m3);

– 2 layersof prepreg (asabove);

– 1 layerof Tedlar(asabove);

– 1 layerof aluminium(asabove).

Similar to the outercontainmentvessel,the inner andouteraluminiumlayersof eachof the threecylinders will be electrically connectedtogetherat the endsof eachcylinder. InsulationagainstHVbreakdowns in the centraldrum is assuredby smoothsurfacesoppositethe field cagewall anda CO2

gapof 150 mm, taperingin the conicalsections.The handlingandtransportingof the delicateinnervesselwill be securedwith a supporttubeasshown in Fig. 3.5. The inner containmentvesselweighsapproximately85 kg.


Figure3.5: Theinnercontainmentvesselwith its transporttube.

3.1.1.3 Inner and outer field cagevessels

The inner field cagevessel,togetherwith the outerone,comprisesthedrift or sensitive volumeof theTPC (Fig. 3.4 on page12). Thus, the compositionand fabricationof thesevesselsunderliestringentquality andselectioncriteriato ensurethat:

residualoxygenandwaterlevelsaremaintainedat 5 and 10 ppm,respectively;

aminimumof pollutantsis releasedinto thegas;

theinsulatingwall materialis of highdielectricstrength;

high parallelism( 100 µm ) betweenthe centralelectrodeand the readoutchamberscan beachieved;

multiple scatteringande γ conversionarereducedto aminimum.

Thesecriteriaareagainbestmetwith compositestructuresfabricatedin autoclaving cycles.Thetwoflangesof eachvesselwill be madeof prepreg with the following composition:Solid laminateepoxyresin(125 C) with aglass-fibrefabricof minimum50%fibrecontentby volume.

For thecylindrical partsthefollowing compositionwaschosen:

1 layerof Tedlar, typeTWH 20BS3,50 µm thick;

2 layersof prepreg (weaving layout 120, 50% fibre and 50% resin by volume, epoxy resin at125 C); 0.1mmthick perlayer;

1 honeycomb-like structureNomex (height 20 mm,29kg/m3);

2 layersof prepreg (asabove);

1 layerof Tedlar(asabove).

As wewill demonstratein Section3.1.3,wehaveadoptedascheme[1] in whichthevoltage-dividingnetwork of thefield cageis physicallydecoupledfrom thefield-cagewalls (Fig. 3.6). Thus,theprime

3.1 Field cage 15

Figure 3.6: View of thefield cagevesselswith thepotentialdegraderseparatedfrom thecylinderwalls.

Figure 3.7: Schemeof thepotentialfollow-upstripson thefield cagevessel.

functionof thefield-cagevesselsis to containandmaintaina puregasvolume,andarrangetheelectricfield separately.

Nonetheless,to avoid theaccumulationof staticsurfacecharges,thehighly insulatingwalls of thefield-cagevesselsmustbeequippedwith afew potentialstripswhichare,contraryto thepotentialdivider,directlygluedto theinnerandoutersurfacesof thevessels.Thesearenarrow aluminiumstrips(0.05mmthick and13 0 2 mm wide) appliedequidistantly(270mm) alongthevesselwalls. They aregluedtothe surfacewith epoxyresinsthat releaseno significantvapoursto the gas[2]. Thus,every 270 mm,concentricaluminiumrings,connectedto eachotheracrossthefield cagewalls, gradually‘follow’ thepotentialof the actualvoltagedivider asshown in Fig. 3.7. This ensuresa gradualpotentialdecreaseacrosstheinsulationgapwith thecontainmentvesselstowardstheend-platesheldat ground.Theouterandinnerfield-cagevesselsweighapproximately370and80 kg respectively.


Figure 3.8: Designof theend-plateandtheservicesupportwheel.

3.1.1.4 End-plates

The end-plateswill be madeof an aluminiumalloy (Fig. 3.8). They area weldedstructure,machinedafter welding, in order to obtain the requiredmechanicalprecisionof order 100 µm. For optimumsealingwith thecylinder flangesandalsowith thereadoutchambers,theend-platestructuremustbeofhigh rigidity andsurfacequality.

In view of theconsiderableweightto besupportedonbothendsof theTPCfield cagewehavechosento mountthereadoutchamberswithout thefront-endelectronicson theend-platesandto distribute thebulk load given by the electronicsand the associatedcablingandserviceson specialsupportwheelsadjacentto theend-plates(Fig. 3.8).

This schemeof separatingloadsconsiderablyreducesstressandthusdeformationsof thepreciselymachinedend-platesthatneedto bewell alignedwith respectto thecentralelectrodeasshown in Sec-tion 4.1.5.4.It alsoeasesthemountingandpositioningof thereadoutchambersin theend-plateframe.

3.1.2 Assemblyand testing

Themodular, low-massstructureof theTPCrequiresthattheindividual cylindersandtheend-platesareassembledin averticalposition.In orderto reducetherisk of damagingtheverydelicatevessels,specialtooling andhandlingstructuresmustbe usedthroughoutthe assemblyprocedure. It is thus foreseento reusethe movable assemblyframe of the DELPHI Barrel RICH detector, allowing the necessarymovementsand rotation of the bulky componentsof the TPC with high precision. In particular, theassemblyof the electricalcomponentsinside the field cagerequiresa cleanareaof at least250 m2.In the following sectionswe describethe exact assemblysequenceof the TPC until its installationinthespaceframe.Prior to theassemblyof thesystem,every individual partof theTPCfield cagewill bethoroughlytestedandcertified.Also, in thecourseof themountingprocedure,it is usefulto testpartiallyassembledunitsbeforeproceedingwith final assembly. This is partof our quality-controlmeasurestoassurereliability of thesystemat theoutsetof theconstructionandassembly.

3.1 Field cage 17

3.1.2.1 Assemblyof the inner and outer containmentvolumes

The first phaseof assemblingthe TPC is to build the two containmentvolumesseparatelyfrom theirindividual parts.Thesepartsare

theinnercontainmentvesseltogetherwith theinnerfield-cagevessel,and

theoutercontainmentvesseltogetherwith theouterfield-cagevessel.

For theinnercontainmentvolume,first theinnercontainmentcylinder is placedverticallyonaprecisionmountingtable(Fig.3.9).Theslightly largerinnerfield-cagecylinder is thenconcentricallyslid over the

Figure 3.9: Mountingof theinnercontainmentvolume.

othervesselandboltedto it. This completestheassemblyof the innercontainmentvolumewhich willsubsequentlyundergo leakandHV testingprior to furtherassembly.

The procedurefor mountingthe outervesselsis slightly different in that first the outerfield-cagecylinder is preciselypositionedbeforethe outercontainmentcylinder is placedaroundit (Fig. 3.10).Also theoutercontainmentvolumewill betestedbeforecontinuingthegeneralassemblyof theTPC.

3.1.2.2 Mounting of the support bracketsfor the potential degrader rods

Before the two containmentvolumesare combinedto form the actualdrift volume, provisions mustbe madefor mountingthe potentialdegraderrodswhich supportthe Mylar strips. The supportrodsthemselves will be installedat a later stage. To do this, both containmentvolumesare moved intohorizontalpositionandheldonarotatingmandrelfor easeof manipulation(Fig. 3.11).A high-precisionruler marksandpositionsthemountingbracketsof the rodsbeforethey aregluedon theoutersurfaceof the inner containmentvolumeor the inner surfaceof the outercontainmentvolume. This fixesthepositionof thedegraderrodsthatareto bemountedin a laterstep.


Figure3.10: Mountingof theoutercontainmentvolume.

a b

Figure 3.11: Preparationof inner(a)andouter(b) potentialdegrader.

3.1.2.3 Assemblyof the drift volume

To assemblethedrift volume,thetwo containmentvolumesneedto befixedvertically on oneend-platethatis placedflat onaprecisionmountingtable(Fig. 3.12a).Thesecondend-platewill only bemountedafteronehalf of theelectricalnetwork of thepotentialdegraderhasbeeninstalledinsidethedrift volumeasdescribedin thenext section.

3.1.2.4 Mounting of the potential degraderand the central electrode

With oneend-platemountedto thecylindersof thedrift volume,theinstallationof thepotentialdegradercancommence.A detaileddescriptionof thepotentialdefiningnetwork is given is Section3.1.3. Theassemblyis kept in a vertical position. Enteringthe drift volumefrom below the end-plate,the innerandoutersupportrodsarenow mountedon their brackets,althoughfor only half the full lengthof thecylinder. Thereafter, the Mylar stripsarestretchedaroundthe rodsandfixed into position with highprecisiontooling ( 50µm) (Fig. 3.12b).

A simplevoltagetestof theresistorchainchecksfor shortsbetweenstripsat regularintervalsduringthewinding process.

3.1 Field cage 19

a b

Figure 3.12: (a) Mounting of the two containmentvolumeson oneend-plateand(b) the lower sectionof thepotentialdegradernetwork.

Figure 3.13: Mountingof thecentralelectrode.

Having mountedthe potentialdegradingnetwork in the lower part of the TPC drift volume, thecentralHV electrodecanbe loweredthroughthe top openingof the assemblyandfixed into its finalposition(Fig. 3.13).Theprecisionof its positionis determinedby theend-pointsof therods,which arealsomachinedwith high precision.

OncethecentralHV electrodeis in place,afirst HV testof thesystemcanbeperformedin air. If noelectricalproblemsareencountered,theentireassemblywill beturnedaroundby 180degrees,aftertheotherend-platehasbeenmountedon top of thefield cagevolume. In thenew positiontheotherhalf ofthepotentialdegradernetwork will beinstalledandtestedsimilarly to thefirst half. Uponcompletion,theTPCfield cagewill bemovedinto horizontalpositionto preparethemountingof thereadoutchambers.

3.1.2.5 Mounting of the readoutchambersinto the end-plates

Beforethe readoutchambersarefixedto theend-platestructure,a detailedsurvey of theend-plateandthemountingprovisionsfor thechambersmustbemade.Photogrammetryhasbeenchosento measure,in particular, the positionof the threemountingpoints. Accordingto the outcomeof this survey, themountingbracketswill beadjustedprior to theinstallationof thereadoutchambers.This is describedinmoredetail in Section4.1.5.6.


Figure 3.14: Thefully assembledfield cagewithout readoutchambers.

Thefully assembledfield cagewithout thereadoutchambersis shown in Fig. 3.14.

3.1.2.6 Dedicatedleak tests

Gastightnessof all vesselsof theTPCis of primeimportancefor theproperfunctioningof thedetectorwhenin operation.Themaximumpermissibleleakratesfor atmosphericoxygenaregiven in Ref. [3].Thus,asa measureof quality assurance,four different leak testsmustbe carriedout to determinetheresidualoxygenandwater level in a test gas(for exampleargon) due to leaksandoutgassingin thevessels.Thesetestsmustbecarriedout prior to delivery at CERN.Theleakratesin thedifferentcom-partmentsof theTPCfield-cagesystem,i.e. in theoutercontainment(CO2), theinnercontainment(CO2)andthedrift gasvolume,areexperimentallydeterminedby flushingtherecipientwith a testgasof de-fined oxygencontent. In anequilibriumstate,the residualoxygencontentof the testgas,measuredattheoutletof therecipient,determinesthetotal leakratein therecipientincludingits sealing.As testgaswe envisageusingargonwith amaximumof 1–4ppmof oxygen.Thelevel of oxygenwill bemeasuredwith a high-precisionoxygenmeter(precisionof 0 1 ppmin therangeof 50 ppm). Thegasflow intothevesselundertestdeterminestheoxygenlevel, afteranequilibriumstatehasbeenreachedfor agivenleakrate.Furthermore,boththeflow rateandthegasvolumedefinethetimeconstantin theexponentialfunctionthatdescribesthechangeof oxygencontentwith time. It shouldbeemphasizedthatthetolera-ble oxygenlevel in theouterandinnerCO2 volumesduringoperationof theTPCcouldbesignificantlyhigherthanthat of the actualdrift gas. The assumptionson flows madein thesecalculationsaresuchthatanequilibriumstateof 10ppmresidualoxygenis to bereachedwith atestgascontainingnooxygenitself.

Inner and outer containment volumes

It is assumedthat therecipientundertestis exposedto atmosphere.Themaximumover-pressureof thetestgas,with respectto atmosphericpressure,will be 5 mbar. Furthermore,we assumethat the frontfacesof thecylindrical volumeareperfectlyleak tight. In casethecylinders,forming thecontainmentvolumes,arebuilt from severalsegmentsgluedtogether, theglue joints betweensegmentsareassumedto beasgastight asthesegmentsurfacesthemselves.

3.1 Field cage 21

Giventhespecificleakratesof thesealingmaterial,i.e. theTedlarfoils andViton O-rings[3], andthesizeof thesurfacesin contactwith thecontainmentvolume,onecanexpecta total leakratefor oxygenof 6 cm3/h for theinnerand 25cm3/h for theoutercontainmentvolume.Hence,if aresidualoxygencontentof 10 ppmmustbereachedin thesevolumes,a gasflow of 0 6 m3/h and 2 5 m3/h mustbeprovidedfor theinnerandouterCO2 containers,respectively.

End-plates

For theleakratemeasurementof thedrift volume,asdescribedin thenext section,we mustfirst deter-mine the leak rateof the two end-plateseals.A simplemethodto checkthe tightnessof theend-platesealsis to mountthemback-to-backagainsteachother, usingthe two O-ringsasforeseenfor thefinalmounting(Fig. 3.15). To simulatethesealingpropertiesaroundthereadoutchambers,which arethem-

Figure3.15: Arrangementof thetwo end-plates(back-to-back)with Mylar sealsto replacethereadoutchambers.

selvesnot subjectto this testing,we cover theend-plateopeningswith aluminizedTedlaror Mylar foilsmountedvia O-ringson the end-plateframestructure(Fig. 3.15). To correctlyestimatethe leak rateof theend-plateensemble,oneneedsto computethetotal surfacecoveredwith foils (32.5m2) andthelengthof all O-ringseals( 370m). Fromthiswederiveatotal leakratefor oxygenof 22cm3/h. Hence,for 10 ppmof residualoxygenin thisvolume,aflow rateof 2.2m3/h is necessary.

Drift volume without readoutchambers

This leak testwill beperformedby closingthedrift volumeon eithersidewith theend-platescarryingfoils thatreplacethereadoutchambersasdescribedin theprevioussection(Fig.3.16).Theleaktightnessof theend-flangeis thendeterminedby theleakrateof the foil surface,thesealingof thefoils with theend-plateandthesealingof theend-platewith thefield-cageflange.Therearetwo O-ringsoneachside.It is assumedthat,in this test,boththeouterandinnercontainmentvolumesareflushedwith CO2 sothatthediffusionof oxygenthroughthesurfaceof theinnerandouterfield-cagecylinderscanbeneglected.Thus,thetotal leakratefor oxygenis estimatedto be20cm3/h, requiringaflow of 2 m3/h for 10ppmofresidualoxygen.

Drift volume with readoutchambers

In this leaktesttheend-platesarefully equippedwith readoutchambersinsteadof thefoils used,beforeanddescribedin thepreviousparagraph.Sincetheleakrateof thechambersis lower by a factorof 3thanthat of the foils, the total leak rateto be expectedfor oxygenin this test is only 18 cm3/h, which


Figure 3.16: Testof thedrift volumewithout readoutchambers(replacedby Mylar/Tedlarfoils).

translatesinto agasflow of 1.8cm3/h for 10ppmoxygenresidualgas.For this testthefinal ALICE TPCgassystemcouldbeusedwhichprovidesgasflowsof 15 m3/h.

Specialpotential degradersupport rods

Someof thestrip supportingrodsof thepotentialdegradingnetwork aremadehollow to usethemforspecialfunctionsdescribedin Section3.1.3.2.Amongthem,the four resistorrodswith their internallymountedvoltagedividing resistorchainneedto beflushedwith a liquid coolantto removethewasteheatproducedby the resistors(60 W). For theotherrods,in particulartheHV supplyrod feedingtheveryHV of 100kV to thecentralelectrode,CO2 gaswill beusedasanadditionalsafetymeasureagainstHVbreakdowns andoxygenintrusioninto thedrift volume. Thesespecialrodsareextensively leak testedprior to their installationin thefield cage.Thus,we donotexpectany significantleaksin theserods.

3.1.3 Voltagedivider

3.1.3.1 Designconsiderations

An importantdesignelementof theTPCis to physicallyseparatethepotentialdefiningcircuitry of thefieldcagefromthewallsof thefield-cagecylinders.Thisschemeisnotobviousapriori asit compromisesphysicalacceptancefor improving field uniformity in drift space.It wassuccessfullyappliedin theNA49TPCs[1] andshowed that the improvementin field quality warrantsthe trade-off in spacedeliberatelygiven up for this purpose. The surfacesof the TPC field-cagewalls aremadeof a highly insulatingmaterial(Tedlar)andarethusproneto charge accumulation.Thesestaticchargeswould interferewiththeprecisefield definingnetwork of thefield cageif theannularpotentialstripswerein physicalcontactwith thefield-cagewalls. This leadsto far-reachingdistortionsof thedrift field andhencesignificantlyreducesthe acceptanceof the detectorin radial space. Therefore,following the NA49 example,theentirepotentialdefiningnetwork of theTPCis arrangedon 18 supportrodsmountedequidistantlyover360degrees,31mmawayfrom thecylinderwalls. This is shown in Fig.3.6onpage15anddonefor boththe innerandouterfield-cagecylinderson eithersideof theTPCcentralplane.Thefollowing sectionswill demonstratehow our requirementson field uniformity, asoutlinedin Chapter2, aresatisfiedwiththechosenarrangementof theALICE TPCvoltagedivider network.

3.1 Field cage 23

A centralelectrodeat high voltageandthe readoutend-platesdefinethe drift field, foreseento be400V/cm. This requiresthecentralelectrodeto beat100kV, thedrift lengthbeing2.5m oneitherside.To ensuretheuniformity of thedrift field in thesensitivevolumethecentralHV electrodeandthereadoutplanemustbeparallelto within 100µm. Therequiredprecisionis achievedby adjustingtheindividualmountingprovisionsfor thereadoutchamberswith respectto thecentralelectrode(Section4.1.5.6).Aschematicof the potentialdegradersystemis shown in Fig. 3.17. The very high voltageof 100 kV

Figure 3.17: Schematicof theTPCpotentialdegradersystem.

on the centralelectrodeis screenedfrom the groundpotentialby a CO2-filled gap(150 mm) betweenthefield andcontainmentvessels.Therefore,in thedesignof thesystem,two aspectshave beengivenspecialattention:thedistanceof thevery high voltagefrom thegroundedcontainmentcylinder, andthedistortionof thesensitivefield insidetheTPC.Bothof thesehavebeenthoroughlytestedwith simulationsaswell aswith experiment(seealsoSection3.1.5.1). The matchingof the readoutplaneto the driftfield of theTPC is alsocrucial to thedesignanddetailedanalysisof this problemhasbeenperformedaccordingly. In the following we describethe designandoptimizationof eachcomponentof the TPCpotentialdegrader.

3.1.3.2 Systemcomponents

Central electrode

In line with theoverall TPC design,the centralelectrodeshouldbe aslight aspossibleandyet, at thecentreof theTPCfield cage,definea‘solid’ referencefor thetwo adjacentdrift fields.Low-massdesignsfor a membraneof nearly20 m2 surfacerequiretheuseof compositematerial,if onewantsto supporttheactualelectrodematerialparallelto thereadoutplaneandpreserve its alignmentto within 250µmwheninstalled.Thedesigncurrentlybeinginvestigatedfor thecentralelectrodeis onewhichemploys aNomex honeycombsheet,6 mmthick andcoveredwith thin layersof carbonfibreandaluminizedMylar(Fig. 3.18). This resultsin a self-supportinglow-masselectrodenecessitatingno further reinforcementor additionalsupport.Thecentralelectrodewill beinsertedinto thefield cage,in verticalposition,via apiston-like insertiontool (shown in detailin Section3.1.2).


Figure 3.18: Thecentralelectrodeof theTPCfield cage.

Furthermore,the incorrectmatchingof the end-platepotentialwith the field cagecylinder canbecausedby misalignmentof componentsor by wrong potential. This canalsoleadto distortionsof theorder of 250 µm if the end-plateis not parallel to the equipotentialsto within 10 4. This issuewillbe addressedin more detail in Section4.1.3 wherealso the effect of end-platedeformationwill beinvestigated.

Potential degradernetwork

Following conceptuallytheNA49 schemethepotentialdegradingcircuitry is suspendedon18rodsin an18-foldpolygonalsymmetryaroundtheinnerandouterfield-cagewallsasshown in Fig. 3.19a.A close-upview of oneof theserodsis shown in Fig. 3.19b. ThissegmentationmatchesthemodularstructureofthereadoutchambersandtheTransitionRadiationDetector(TRD) in our efforts to jointly optimizethe

a b

Figure 3.19: (a) Arrangementof theMylar stripson rodsinsidetheTPCfield cageand(b) a close-upview of asupportrod for theTPCpotentialdegraderstrips.

3.1 Field cage 25

10-6

10-5

10-4

10-3

10-2

2

4

68

103

2

4

68

104

2

4

(E-Enom)/Enom

No.

of s

ampl

es

3-4 cm 2-3 cm 1-2 cm 0-1 cm

a b

Figure 3.20: (a) Equipotentiallines in the vicinity of the stripsand(b) histogramsof the relative deviationsoftheelectricfield from thenominaldrift field in slicesof 1 cmthicknessparallelto theplaneof thestrips.Thefourcontourlines indicatethe zonesin drift spacein which residualfield inhomogeneitiesareto be expected.From2 cminwardsthedrift spacetheseinhomogeneitieshavedroppedto 10 4.

acceptanceof bothdetectionsystemsin azimuthalspace.Thereare166potentialstepsalongonez-direction,madeby strips,over a distanceof 2.5m. Thus,

the pitch of thesestrips, i.e. their width plus the gapbetweenstrips, is 15 mm. This valuehasbeenoptimizedwith analyticcalculations[4] andthroughfinite elementanalysis.Eachstrip is placedwitha voltagecorrespondingto its centreposition in z from the centralelectrodesincethe distortionsareminimum at the centresof the strip and the gap, as seenin Fig. 3.20a. This resultsin a drift-fieldnonuniformityof 10 4 , i.e. with a radial-fieldcomponentEr 10 4 relative to thelongitudinalfield Ez

insidethesensitive volume.Thestripsareof 25 µm thick aluminizedMylar.Precisepositioningof the strips also plays an importantrole, sincea displacementwill result in

field distortions.This is ensuredby a positioningtool which putstheMylar stripsinto placeto within 50 µm.

Takinga close-uplook at theequipotentialsin a cornerof theintersectionbetweenthecentralelec-trodeandthefield cagestrips,wecomputethedrift field alongaradialline atdifferentdistancesfrom thecentralelectrodeat severalz (Fig. 3.20b). We show heretheworstpossibleregion of distortionsin thewholeTPCsensitive volume.Theaveragedistortionsin theslicenearestto thestripsareof theorderof20%.Between1 cm and2 cm thedistortionsarealreadydown to 0.3%,andbetween2 cm and3 cm theaveragedeviationsare4 10 5. Thefigureshows thefield distortionsin thedrift volumeresultingfromthefinite stripwidth andgapaswell asthepresenceof thegroundplanein theoutercontainmentvessel.Theappropriatevoltageto theMylar stripsis suppliedby a voltagedivider resistorchain.Thevalueoftheresistorsof 1 MΩ betweenstripswaschosensuchthatthetotal chaincurrent( 600µA) will notbechangedby any reversecurrentsprovoked by positive ions in thedrift volume. Currentchangesin theresistorchainwill alterthevoltagesettingsontheMylar stripsandthusleadto changesof thedrift field.However, themaximumpositive ion densityin thedrift volumeis only 2 10 18 C. The resistorsaremountedin a sequenceof parallelpairsof 2 MΩ eachinsidefour of the 72 rods(the resistorrods)onwhich theMylar stripsareheld.A sketchof theresistorchainandtheresistorrod is shown in Fig. 3.21.


Figure3.21: View of theresistorrodwith thevoltagedividermountedinside.

Mounting the resistorsinside the rod hasseveral advantages.First, it protectsthe resistorsand theirjunctionsto thestripsfrom mechanicaldamageduringinstallation.Second,partof theheatdissipatedatmaximumvoltagesettingalongtherod (60 W) canbecarriedaway to prevent local heatingof thedriftgassurroundingtheserods.Thus,theconstantflow of aninsulatingliquid throughthis rod,which is toprovide additionalelectricalprotectionto the resistorchain,will alsoserve to transportthebulk of thisheatawayby forcedconvection(seeSection3.2.3.5).

Theresistorsneedto beof goodtolerance.Metal-oxideresistorsarein generalmoreinsensitive totemperaturechangesthancarbonresistors.If resistorsfail or break,resultingin shortcircuits or openconnections,field distortionswill occurthatmaywell reachinto thesensitivevolume.Figure3.22showsplotsof equipotentialcontoursfor thecasewhentwo stripsareconnectedby ashortedresistor. Theriskof having an opencircuit betweentwo stripsis substantiallyreducedby the fact that two resistorsaremountedin parallelfor eachvoltagestep.

Figure 3.22: Effectof a resistorshortbetweentwo neighbouringstripson theequipotential.

3.1 Field cage 27

HV supply system

The voltageof 100kV will be suppliedby a commercialrack-mountedpower supplyanddeliveredtothefield cagevia a coaxialcableratedat 125kV maximum.Thefar endof this cableis insertedinto aspecialhollow HV supplyrod. This rod, beingoneof thestrip supportingrodson theouterfield cagevolume,makeselectricalcontactwith the centralelectrodeandis, like the resistorrods,flushedwithCO2 for additionalelectricalprotection. To avoid voltagemismatcheswithin the electricalfield-cagesystem,only thiscablefeeds,via thecentralelectrode,boththeinnerandouterpotentialdegraderchainson eithersideof theTPC(Fig. 3.23). A secondreserve power systemincludingthefeedcablewill alsobe installednext to theTPC to replacethemain systemin caseof failure. This intervention,however,requiresaccessto theTPC.

Figure 3.23: Layoutof theHV feedsystemin theTPCfield cage.

3.1.4 Temperature stabilization and thermal screen

3.1.4.1 Operating temperature of the TPC and thermal insulation

The gasesto be usedin the TPC area consequenceof carefully chosenoverall designandoperationparametersthatshallguaranteeoptimumperformanceof theTPCataminimumrisk of failure.However,evenat electricdrift fieldsashigh as400V/cm, electrondrift velocitiesin thesegasesarenot saturatedandarethussubjectto variationsin temperature.As stipulatedin Section2.1,temperaturevariationsof afractionof adegreearealreadydetrimentalto theprecisetrackmeasurements.Thus,oneof theprincipaldesignobjectives for the field cageis to protectthe TPC drift volumefrom the influenceof any heatsourcesthatcouldcauselocalandtime-relatedtemperaturechangesof thedrift gasby morethan0.1 C.

If oneassumesthat all detectorsinsidethe L3 magnetareinternally cooledto maintaina constantglobal temperatureenvironmentof say20 C no net heatflow is exertedon the TPC, and only localor time-relatedtemperaturevariationshave an influence. Spuriouslocal heatingcanbe producedbyexposedpower-transmittingcomponents,suchascables,the field-cageresistorchainandnearbyelec-tronics, while time-relatedtemperatureexcursionsoccur when neighbouringdetectorsare subjecttopower and/orcoolingfailureswhichgraduallychangetheirbulk temperatureuntil normaloperationandthermalequilibriumarerestored.For theouterTPCsurface,for example,onecanassumetheeventofanunforeseenheattransferof theorderof 30 W/m2 betweentheTRD andtheTPCacrossanair gapof160mm. Furthermore,heatsourcesthatarein directcontactwith theTPCcannotbeentirelyshieldedfrom the TPC drift gas. Theseare for examplethe TPC front-endelectronicsandthe voltagedividerresistorchainswhereevery effort needsto be madeto remove wasteheat,andcool themto the sametemperatureastheTPCdrift gas.


3.1.4.2 Systemoptionsand choices

For thoseheatsourcesthat are not in direct contactwith the TPC, active thermalscreensoffer bestinsulationand regulationpropertiesat a minimum of physicalspaceandmaterialtaken. Contrarytopurelypassive insulationschemes,thermalpanelsallow selectedlocalheatingandcooling,thusenablinga flat temperature‘profile’ to be maintainedon andaroundthe skin of thedetector. The possibility ofcontrolledglobal cooling will alsoavoid the risk of thermalrun-away of the system.In orderto keepthe amountof passive materialnearthe inner radiusof the TPC at a minimum, no thermalscreeningbetweenthe ITS and the central inner containmentvesselof the TPC is as yet foreseen. However,the conical sectionsof the inner containmentvessel,which areexposedto power and servicecablesof the ITS, shouldbe protectedby a screen. At present,we estimatethat thermalpanelsenvelopingthe outer surfaceof the TPC and the conical sectionsof the inner containmentvesselcan provide atemperaturestabilizationof 0.5 C. Furthersmoothingof residualtemperaturegradientsis given bythe CO2 containmentvolumeandthe 20 mm and30 mm thick honeycombskinsof the TPC vessels.Thetemperatureprofileof thedrift volumewill becomputedfrom dataprovidedby temperatureprobeslocatedoutsideandinsidethedetector. Thelattershallbeplacedinsidethesupportrodsof thefield cage(Section3.1.3.2),neartheend-plates.

3.1.4.3 Heat/coolingpanels

First studiesindicatethat, in termsof particleinterference,the lightestpanelsaremadeof aluminium.They offer bettermechanicalandheat-transferpropertiesthancomparablescreensmadeof intrinsicallylightermaterialssuchaspolycarbonates.Two 200µm thick aluminiumfoils, suitablyprofiled,aresand-wichedto provide1–1.5mmthick flow channelssuchthatinternaltemperaturegradientswithin thepanelitself arenearlyeliminated.A schematicof sucha panelis shown in Fig. 3.24. For theamountof heatto be removed andconsequentlythe massof coolantto be transferred,theseflow channelsneedto beadequatelydesignedin orderto avoid buckling of thematerial.For example,in a panelof 1.25 1 m2

size,designedfor a waterflow of 60 l/h, theseflow channelsareat least15 mm wide. As far as theoutersurfaceof the TPC is concerned,the thermalscreenis bestplacedon the space-framestructureinsidetheTPC–TRDair gap. A suitableandeconomicsolutionwould be to constructthepanelssuchthatthey matchthe18-foldsectorgeometryof theTPCandTRD in ϕ with four additionalsegmentationspersectoralongtheTPCz-axis.For theinnerscreen,thesamesegmentationaroundeachconicaldrum,thoughwith no subdivision in z, would besufficient. In this case,thepanelswould bemountedon theTPCcontainmentvessel.

Figure 3.24: A thermalscreenpanelwith parallelzigzagchannels.

3.1 Field cage 29

Figure 3.25: Schematicsof thewatercirculationsystemfor theouterTPCthermalscreen.

3.1.4.4 Closed-circuit water coolingsystem

The proposedgranularityof the thermalscreenincreasesthe complexity andmassof the systemand,in particular, bearsthedangerof fluid leakage.Theamountof distribution manifoldsandpipe routingthat arenecessaryfor the entiresystemraisethe questionof usingan under-pressure(calledleakless)cooling circuit to avoid damageof the detectorsfrom lossof fluid. Figure3.25shows sucha coolingsystemconsistingof a pump, locatedin the experimentalcavern, and individual flow regulatorsandheatersfor eachseparatepanel.Thus,thetemperatureequilibriumof thesystemis controlledby separatetemperaturemeasurementson eachpaneland,in caseof differences,thefluid to eachindividual sectionis selectively heateduntil acommontemperatureprofile is achieved.

3.1.5 Testsand prototypes

To put theproposedideason compositematerialsto a thoroughtest,a 2.7 m long and1.1 m diameterprototypevesselwasmanufacturedby industry. It wasbuilt from a 20 mm thick honeycombsandwich(Nomex) structurecoveredwith layersof glassfibre andTedlarprepregs. TheprototypeTPCfield cageis shown in Fig. 3.26. It wasconstructedfrom two cylinders,each1.35m long, andeachmadefrom

Figure3.26: TheprototypeTPCfield cage.


two axialhalf-shellsof thesamelength,gluedtogetheraftertheautoclaving process.Thisgluejoint wassealedwith anadditionalTedlarfoil to avoid leakage.It shouldbenotedthatassemblyof thelargeTPCcylinders from prefabricatedsegmentsis to be expected,asmany manufacturersdo not operatelargeautoclavesto curethesecylindersin onepiece.

Following its delivery in early 1999,this field-cagecylinder hasundergonevariousmechanicalin-spectionsandtests,includinggastightness.After inspectionandcertification,theprototypefield cagewasinstalledin a slightly largeraluminiumvessel(Fig. 3.26)providing safecontainmentof theelectri-cal network andavoiding leakageof oxygeninto thedrift volume.Similar to thefield-cagearrangementfor theALICE TPC,a CO2 insulationgapof 150mm betweencylinder walls assuredHV operationof

100kV. Theprototypevesselwassuccessfullyinstalledwith no detectableleaks.To verify theelectrostaticperformanceof thefield cage,the prototypevesselwasequippedwith a

similar field-definingnetwork to that foreseenfor thefinal TPC.The testprogrammeconsistedof twophases:

verificationof the purely electrostaticbehaviour of the device including safetymargins of 20%beyondnominalvalues;and

performanceof thedeviceasarealprototypeTPCwith apadreadoutchambermountedononeofits end-plates.

Therefore,in thefield cageprototype,a thin aluminiumplatewasmountedhalfway alongits axistoform thecentralHV electrode(seeFig. 3.26).Five concentricaluminiumringsperhalf-sideweretapedequidistantlyontheinnerandoutersurfacesof thehoneycombstructureandinterconnectedvia gas-tightfeedthroughs(seeFig. 3.7 on page15). Theserings werealsoconnectedto the voltagedivider chain,directly oppositethenearestresistorof thechain.Thisavoidstheaccumulationof chargeson thehighlyresistive surfaceof the field cageandalsothe build-up of high potentialsneartheend-platestypicallyheldatground.

In all, 91 stripsconfiguringthe actualdrift field along the axis of the cylinder weremadeof alu-minizedMylar, 12.7mm wide, andplacedat 15 mm pitch, i.e. with a gapof 2.3 mm betweenadjacentstrips. They aresuspendedby six insulatingsupportrodsof Macrolonattachedto the inner field-cageshellandmountedon thin Stesalitestand-offs. Oneof theserodshasa largerdiameterto accommodatethevoltage-dividing resistorchainon its outsidesurface,andtheHV supplycableinsideof it. The farendof this rod wastied to thecentralelectrodeandthesecondHV distribution rod in theoppositedriftvolume.Thenominaldrift field of 400V/cm impliesamaximumpotentialof 55 kV for adrift lengthof 1 30 m.

3.1.5.1 Electrostatic tests

After testing all individual componentsof the HV distribution network, the entire field cage, fullyequippedwith 91 potential-definingMylar stripsin eachhalf-volume,wastestedwith argon andneongasmixtures.To monitorthecurrentin theresistorchainanddetectHV breakdowns,a 1.8kΩ externalresistorwasmountedbetweenthe lastpotentialstrip andground.Variationsof thevoltagedropacrossthis resistorindicateHV instabilitieswithin thevoltage-dividing network, thusgiving thepossibilityofmanualinterventionbeforedamagecanoccur.

Theresultsaresummarizedin Table3.2. They demonstratetheenvisagedstability of thefield cageandits suspendedfield-definingnetwork upto fieldsof 600V/cm. Thelastrow of thistableshowsresultsof a specialHV testmadein a separateset-updesignedto investigatethe limiting potentialbetweenneighbouringMylar strips.With animprovedgeometryof thehooksandtheattachmentof thestripswecouldreachvoltagesof 8 kV betweenneighbouringstrips,i.e.approximately10timesthenominalvaluefor afield of 400V/cm.

3.1 Field cage 31

Table3.2: List of electrostatictestsperformedwith theTPCfield-cageprototype.

Voltage EquivalentSystemon test applied field Testedin Outcome

[kV] [kV/cm]

Baresandwichshell 70 n.a. Ambientair OK, nomeasuredcurrentHV supplyrods 80 n.a. Dry air OK, stablecurrent

throughresistorchainFC + containment 120 8 CO2 OK, nomeasured(150mmgap) currentfor 1 hourFC + voltagedivider 82.5 0.6 Ar–CO2 OK, stablecurrentfor 1 day

[80–20]FC + voltagedivider 82.5 0.6 Ar–CO2 OK, afterstabilization

[90–10] for few hoursFC + voltagedivider 82.5 0.6 Ne–CO2 OK, stablecurrentfor 1 day

[80–20]FC + voltagedivider 55 0.4 Ne–CO2 OK, stablecurrent

[90–10]FC + voltagedivider 82.5 0.6 Ne–CO2 2 µA darkcurrent

[90–10]Mylar stripsand 8 34.8 Ar–CO2 stable,nobreakdownsnew hooks [80–20]

Figure 3.27: Arrangementof thereadoutchamberwith theprototypefield cage.

3.1.5.2 Cosmic-ray tests

A conclusive benchtestof thechosenTPCfield-cageconceptis to measureparticletracksover nearlytheentireacceptancerangeof thefield cage.Straighttracksproducedby cosmicraysaresuitabletoolsto searchfor nonlineareffects indicatingelectrostaticdefectsin the field configuration. In particular,edgeeffectsnearthefield-cageboundariesareof primeinterest,but alsothosenearthereadoutchamberinterfacewith thefield cage.Thetransitionfrom thefield region to theamplificationandreadoutregionbecomesparticularlydifficult for theenvisagedhighdrift fieldsof 400V/cm. Thus,matchingof thedriftfield with thefield of the readoutchambersat a minimumlossof efficiency andprecisionwastheaimof thesetests.The chamberchosenfor readoutwasa wire chamberof theNA35 TPC [5] (Fig. 3.27).


70

60

50

40

30

20

10

0 0 1– 1– 2 1.5 0.5– 0.5– 1.5 2

residual (mm)

cou

nts

Figure 3.28: Distribution of theresidualsfrom a straight-linefit on cosmic-raytracksmeasuredin theprototypeTPC.

Its active wire-padareawas large enoughto accepttracksfrom the centralregion of the TPC to andbeyondtheedgesof thefield cage.Thus,asystematicstudyof thefield qualityover theentirefield-cageacceptancecouldbemade.

Initially, high-energy cosmic rays were selectedwith a simple scintillator array interleaved with10 cm of lead(alsoshown in Fig. 3.27). The NA35 moduleandits associatedelectronicsallowed usto read448 padsper cosmictrigger andrecordthe eventsfor dataanalysis.A track-fitting algorithm,appliedto selecteddatasamples,reconstructedcosmictracksto betterthan260 µm deviation from astraightline (Fig. 3.28). Thesepreliminaryresultsindicatethat thechosenprincipleof configuringtheelectricfield insidetheTPCis viablewith regardto thephysicsperformancestipulatedin Section2.1.

3.2 Gassystem

3.2.1 Specificoperational requirements

Thelong(2.5m) drift distanceof theALICE TPCandtheuseof CO2 in theoperatinggasmixturemakeit mandatoryto limit theconcentrationof oxygenandwatervapourin thedrift volumeto a minimum.This is oneof themainrequirementsfor thegassystemandthedetectoritself. Theaccuracy andstabilityof thegasmixture,alongwith its pressureregulation,is alsoimportant,aswell asthecorrectdesignofthecontainmentvessels.

Oxygenmoleculesin CO2 arevery proneto electronattachment.In thecaseof Ne–CO2 mixtures,theelectronattachmentis 1% perppmoxygenandmetreof drift. Oxygencontaminationcomesmainlyfrom sealingjoints, which roughly scaleswith the total surfaceof the detector. The large volume-to-surfaceratio of our TPC and the purity achieved in other TPC gasvolumes(seeTable3.3) makes itfeasibleto limit theoxygencontentto near1 ppm,providedthesealingconcept,thepurificationof thegas,andits flow aredesignedaccordingly. Physicsrequirementssettheabsolutemaximumto 5 ppm.

A CO2 gasvolume envelopsthe drift volume for reasonsof HV insulation,thus providing extrasealingagainstpotentialleaksalongtheinnerandoutercylinder walls. In addition,severalhollow rods,locatedinsidethedrift volume,will alsobeflushedwith CO2 for coolingreasons.Theseinsulationandservicevolumesareflushedwith asingle-passgassystem.

3.2 Gassystem 33

Table 3.3: Oxygenlevels attainedin existing TPCs. The maximumlimit for the caseof the ALICE TPC isquoted.

Volume/surface[m] O2 [ppm] Ref.

DELPHI 0.34 1–2 [6]ALEPH 0.55 20 [7]NA49 VTPC 0.26 3–4 [1]NA49 MTPC 0.47 1–2 [1]CERES 0.28 5–7 [8]ALICE 0.70 5

3.2.2 Gaschoices

Thewell-known 90%Ne, 10%CO2 [9] is thebaselinegasmixturechosenfor operatingthedetectorata reasonabledrift field andstill ensuringattractive chargetransportpropertieswith anonflammablegas.For reasonsof primaryyield andstability of thereadoutchambersat relatively high gains,theadditionof anextra, warmquenchersuchasCF4 is beingstudied.An extra 10%CF4 in thegasmixturewouldincreasetheprimaryelectronyield by 10%with no increasein diffusionor electricfield. It would alsoincreasethemaximumattainablegainof thedetectorsincetherewould be morequencheravailabletoabsorbphotonsproducedin the avalanches.The possibleelectronattachmentof the fluorocarbonandthechemicalcompatibilityof thissubstancewith thematerialsof theTPCremainto bedetermined.Theadditionof argonwouldcontributeto theprimaryyield aswell, at theexpenseof anincreasein multiplescatteringand,moreimportantly, in spacechargein thedrift volumedueto theslowerpositive ions.Thiseffect wouldalsoincreasethecorrenspondingfield distortionsby a factorof 5 comparedto neon.

Thegasusedfor insulatingthehigh voltageagainsttheexterior is CO2, for practicalreasons:it is alow-costgas,it hasgooddielectricstrength,andit is alsooneof thecomponentsof theTPCoperatinggasmixture.

3.2.3 Designchoicesand layout

Thelargedetectorvolumeandtheuseof a high-costgasmixturemake aclosed-loopcirculationsystemcompulsory. The systemproposedwill consistof functional modulesthat are designedand built asstandardunits for all LHC gassystems.Table3.4 indicatesthe locationof thesemodules.Themixingandpurificationunits, aswell asan optionalgasrecovery plant,arelocatedin the gasbuilding on thesurface(SGX2). Thecirculationrackwill be locatedon theshieldingplug in thepit PX24. However,

Table3.4: Functionalmodulesof theTPCgassystemandtheir locations.

Funcionalmodules Location

Primarygassupplies SGX2BuildingMixer SGX2BuildingInsidecirculationloopDistribution Pit PX24Purifier SGX2BuildingPumpandpressureregulation Pit PX24Nerecovery (optional) SGX2Building


Figure3.29: Overview of thegasdistributionsystem

the componentsizesandrangesareadaptedto meetthe specificrequirementsof the TPC system.Anoverview of thegasdistribution systemcanbeseenin Fig. 3.29.Thebasicfunctionof thegassystemisto mix thecomponentsin theappropriateproportionsandcirculatethegasthroughtheTPCdrift volumeat a pressureof 1 mbarabove atmosphericpressure.Somebasicparametersof theTPCgassystemaregivenin Table3.5.

Table3.5: Basicparametersof theTPCgassystem.

Drift Outerinsulation Innerinsulation

Volume 95 m3 12.62m3 2.77m3

gasmixture Ne CO2 CO2 CO2

Workingoverpressure 1 mbar 1 mbar 1 mbarFilling rate 4 m3/hCirculationflow rate 19.4m3/h 2.5m3/h 0.64m3/hFreshgasinjection(Vol) 1% 100% 100%Operationalperiod 8 months 12months 12months

3.2.3.1 Mixing unit

An LHC gasmixing unit, schematicallyshown in Fig. 3.30,will beusedto mix thecomponentsin theappropriateproportions. The flows of componentgasesaremeteredby mass-flow controllers,whichhave an absolutestability of 0.3%over oneyear, anda mediumterm stability of 0.1%in steady-state

3.2 Gassystem 35

Figure3.30: Gasmixerunit locatedin thesurfacegasbuilding

conditions.Flowsaremonitoredby aprocess-controlcomputer, whichcontinuallycalculatesthemixturepercentagessuppliedto thesystem.At aregenerationrateof 99%theexpectedfresh-gasflow atoperatingconditionsis of the order of 200 l/h. Filling of the detectorwill be donewithout recirculationwith20 timeshigherinlet flows usinga secondsetof mass-flow controllers.This device changewill allow afull TPCvolumeexchangewith freshgasin oneday. The6–8volumechangesneededfor achieving thegasoperatingconditionleadsto astart-upperiodof oneweek.

3.2.3.2 Gascirculation system

A flow diagramof the circulationsystemcanbe seenin Fig. 3.31. The gasmixture is circulatedin aclosedloop with ananticipatedregenerationrateof 99%. This will allow high gasflows in thedetector,asshown in Table3.5, with a moderatefresh-gasinjection. Returngasfrom the drift volumemustbecompressedto approximately100mbarabove atmosphericpressureto pumpit backto thesurfacegasbuilding whereit will berecycledthroughthepurifier. Thegaspressureinsidethefield cageis regulatedto 1 mbarabove atmosphericpressureby a control valve mountedin parallel to the compressor. TheHV centralelectrodeof thefield cagehasa gaspassagetransparency of 15%,resultingin a negligibleimpedanceto theflow. Thegasinputpressureis reducedto afew mbardownstreamof theflow regulatorin the inlet pipeon theplug. This regulatorallows manualflow adjustmentsfrom theaccessiblezone,combinedwith remotemonitoringof thetotal gasflow. Effective over- or under-pressureprotectionforthedelicatefield-cagestructureis achievedin two steps:

Threepressuretransducers,independentof theregulation,measurethepressureson theinlet aftertheflow controller, at thefield cageandat the input of thecompressor;thus,thecontrol systemcanpermanentlycheckthesevaluesandstopthegasflow if necessary.

For ultimateprotection,asafetybubbleris installedin theracksituatedin thecavernverynearthedetector.

Duringoperation,changesin atmosphericpressureareautomaticallycompensatedfor by thepressureregulationsystem.If thegasflow is stopped,aback-upsystemis availableat theoutletof theTPC.Thiswill eitheraddgasfrom a pre-mixed bottleor relieve gasto atmospherethrougha bubbleradjustedto1 mbar. In this manner, rapid changesof atmosphericpressurein the event of a power failure canbefollowedwithout modifying thegasmixtureinsidetheTPC.

Filling and purging of the operatinggasis donein single-passmodeby switching the three-wayvalve situatedin theSGX2building to directthegasto anexhaustor to a recovery plantif required.Gas


Figure 3.31: Basiccomponentlayoutof theTPCgascirculationloop. Thepressurein thefield cageis regulatedto +1 mbarrelative to atmosphericpressureby thecontrolvalve in parallelto thecompressor. Theinput pressuresignalfor thecontrolleris takendirectly from theTPC.Thetotal circulationflow is adjustedby themanualflowregulatorin theinput line.

3.2 Gassystem 37

replacementfor filling canbe optimizedby injecting the lighter Ne CO2 mixture (density0.99kg/m3)at theuppersideof theTPCwhile extractingtheCO2 (density1.94kg/m3) from thelower side.Duringshutdown periods,thedetectorwill beflushedwith CO2 in oneof thetwo following ways:

Closed-loopcirculationasduring operation.This requires,however, electricity for runningandcontrollingthegassystem,aswell asminimal operatorsurveillance.

Manualpurge modewhereCO2 is suppliedfrom the plug, flushedthroughthe detectorandex-haustedthrougha bubbler. The control systemcanmonitor flows andpressures,but this is notmandatory.

In caseof a longershutdown period,thefield cagecanbefilled with CO2, andtheback-upinjectionandrelief systemcanjust maintain1 mbarover-pressurein the detectorby injecting or ventinga minimalamountof gas.

3.2.3.3 Purification system

Closed-loopgascirculationsystemsrequiregaspurificationin thereturnline from thedetectorsin orderto achieve high recycling rates(of the orderof 99%) andlow fresh-gasinput. The main impuritiesofconcernwhichaccumulatein thesystemareoxygenandwatervapour, enteringvia thejointsof thevesselwalls [3]. TPCdetectorsareverysensitive to thesecomponents,whichmustremainbelow 5 and10ppm,respectively. Thepurifier systemremovesthesecontaminantsfrom thegasmixture. A schematiclayoutof thepurifierunit canbeseenin Fig. 3.32.

Purificationwill bedonewith cartridgesfilled with two cleaningagents:a molecularsieve (3A) toremove watervapour, andactivatedcopperasa reducingagentfor oxygen.A configurationin parallelallows oneto run the gasthroughonepurificationcylinder whilst the otheroneis being regenerated.

Figure 3.32: Schematiclayoutof thepurifierunit.


Both agentsin thesamecylinder canberegeneratedat thesametime by heatingthecolumnsto 220 Cin an93%Ar, 7%H2 mixture.

Nitrogencannotbe removed by the purificationsystem,and as a consequencethe amountof N2

penetratinginto thecircuit will limit theachievableregenerationrate. This in turn determineswhetheror not a gasrecovery plant is needed.Purging ratesbelow 1% couldbeaffordablein termsof gascost.Studiesof theamountof nitrogenasa functionof purgeflow will becarriedout in aprototypefacility.

3.2.3.4 Calibration and monitoring

As discussedin Section4.3.3,X-rays from decaysof 83Kr will be usedfor calibratingthe gain of allreadoutchannelsof theTPC.A bypassvesselcontainingtheradioactive sourcewill be foreseenin thegassystem,after the mixing unit. Continuousmonitoringof thegasgainandthedrift velocity, whicharesensitive to pressure,temperature,gas-composition,andwater-vapourfluctuations,is alsonecessary.A setof smalldrift andproportionaldetectorswill beimplementedinto theslow controlsystemfor thispurpose. The detectorswill monitor the gaspropertiesat variouslocationsin the gascircuit. Directmonitoringof oxygenandwatervapourwill alsotake placebeforeandafter thepurifier unit. Detectedlevelsabovesetvalueswill generatealarmsignals,whichmaystopthegascirculationif necessary. Otherparametersrelevantto theoperationof thegassystem,suchasgasflows,pressuresatdifferentlocations,andCO2 content,will alsobemonitoredthroughtheslow controlsystem.

3.2.3.5 Ancillary gasvolumes

Theinnerandoutercontainmentvolumeswhichprotectthefield cagewill befilled with CO2, in ordertoprovide sufficient electricalinsulationto maintainthe100kV high tensionbetweenthetwo surfaces.Inaddition,it is foreseento flushcertainhollow supportrodswith CO2. Theserodshave specialfunctionssuchasfeedingtheHV to thecentralelectrodeandprovide temperaturemeasurements.A flow -diagramof the systemcanbe seenin Fig. 3.33. It will be a single-passsystemusingmechanicalpressureandflow regulationdevices,andwill maintainaconstantpressurein thevolumesof 1 mbar. Outletgasfromthesevolumeswill beexhausteddirectly to atmospherevia bubblers.

Figure 3.33: Schematicsof theCO2 purgesystem.

3.2.3.6 Distribution pipework

All tubesandfittings within thedrift-gassystemwill bemadeof stainlesssteel.Thetubeswill bebutt-weldedtogetherto reducethepossibilityof contaminationandleaksto aminimum.Existinggaspipesatthepit will bereusedasfar aspossible.Table3.6shows anoverall view of themainpiping parameters.At theshielding-plugend,thetubeswill bemodifiedto link up with thenew positionof thedistributionrack. In theexperimentalcavern(UX25) they will beextendedinto theL3 solenoidmagnetandupto the

Table3.6: Main pipingparameters.

Number Pipeinner Pipe Normal Gas Reynolds Pressureof diameter length flow velocity number drop

pipes [mm] [m] [m3/h] [m/s] [mbar]

Drift volume

SGX-plug 2 73 90 20 2.58 16000 2.1Plug-manifold 2 73 100 20 2.58 16000 2.3Manifold-TPC 8 33 20 5 3.15 8800 1.8

CO2 volume

Outer 2 20 120 2.5 1.62 6600 0.5Inner 2 20 120 2.5 0.39 1600 0.04

TPC.Input pipesto theTPCwill bethermallyinsulated,thusadding15 mm to thediametersquotedinTable3.6. Inlet andoutletpipesto thedrift andcontainmentvolumeswill beappropriatelymanifoldednearthedetector. Whenthedetectoris in theextractedposition,it will be linked to thegassystembysupplementarymanifoldslocatedoutsideof theL3 magnetto maintainthegascirculation. UnlesstheTPCneedsto beopened,noair shouldenterthedrift volumeduringany kind of manoeuvre.

41

4 Readoutchambersand calibration

4.1 Readoutchambers

It is the taskof theALICE TPCto provide, for thecentraldetector, themainchargedparticletrackingcapabilityandmomentumdetermination,combinedwith particleidentificationvia themeasurementofthe specificenergy lossdE/dx. Large-scaleTPCshave beenemployed andproven to work in colliderexperimentsbefore[1–3] , but noneof themhadto copewith theparticledensitiesandfluxesanticipatedfor theALICE experiment.For thedesignof the readoutchambers,this leadsto requirementsthat gobeyondanoptimizationin termsof momentumanddE/dx resolution.In particular, thequestionsof ratecapabilityandtwo-trackseparationin ahigh trackdensityenvironmenthave to beconsidered.

TheALICE TPCreadoutchamberswill beconventionalmultiwire proportionalchamberswith cath-odepadreadoutasusedin many TPCsbefore.DifferenttechnologiessuchasRing CathodeChambers(RCCs)or GasElectronMultipliers (GEMs)[4] havebeenconsideredandtestedin variousR&D projects(seeChapter8 for an overview), but noneof themhave beenproven to meettheALICE TPCrequire-ments.

In thefollowing, wedescribetheconceptfor thereadoutchambersanddemonstratehow thetechnicalsolutionis adaptedto therequirements.

4.1.1 Mechanical structure and support

4.1.1.1 Sizeand segmentationof the readoutplane

Theoverall designof thereadoutplaneis chosento optimize,in thehigh multiplicity environmentof acentralPb–Pbcollision, themomentumanddE/dx resolution,providing full azimuthalcoverage.Thisleadsto the requirementto maximizethe total active area,leaving a minimal deadareain radial andazimuthaldirection. However, for practicalreasonstherehasto bea subdivision of the readoutplanesinto individual modularreadoutchambers.The readoutchamberswill bemountedinto thesectorcut-outsof theend-plates(Fig. 4.1).

Figure4.1: Segmentationof thereadoutplane.

42 4 Readoutchambers andcalibration

Figure 4.2: Cross-sectionthroughaninnerreadoutchamber. Shown arethealuminiumframewith thestiffeningribs, the O-ring groove andthreadsfor the three-pointsuspension,the padplanePCB,andthe auxiliary strongbackplate.

The azimuthalsegmentationof the readoutplanefollows that of subsequentALICE detectors[5],leadingto 18 trapezoidalsectors,eachcovering20 in azimuth.

Theradialdependenceof thetrackdensityleadsto changingtherequirementsfor thereadoutcham-berdesignasa functionof radius.Consequently, therewill betwo differenttypesof readoutchambers,henceforthcalledthe inner andouter chambers,leadingto a radial segmentationof the readoutplane.This decisionis supportedby the easeof assembling,handlingand integrating two smallertypesofreadoutchamberscomparedto onelargeone,coveringthefull radialextensionof theTPC.

The deadspacebetweenneighbouringreadoutchamberscanbe minimizedby a specialmountingtechnique(describedin Section4.1.5)by which thereadoutchambersareattachedto theend-platefrominsidethedrift volume.

The inner andouter readoutchambersare radially aligned,againmatchingthe acceptanceof thefollowing detectors.Theactive radial lengthvariesfrom 84.1cm to 132.1cm (134.6cm to 246.6cm)for theinner(outer)readoutchambers.Thedeadspacebetweentwo adjacentchambersin theazimuthaldirectionis 27 mm. This includesthewidth of thewire framesof 12 mm on eachchamber(seebelow)anda gapof 3 mmbetweentwo chambers.Thetotalactive areaof theALICE TPCis 32.5m2.

4.1.1.2 Mechanical structure of the readoutchambers

Fig. 4.2 shows a cross-sectionof a TPC readoutchamber. A readoutchamberconsistsof threemaincomponents:thepadplanemadeof a multilayerprintedcircuit board(PCB),anadditionalstrongbackplatemadeof fibreglass-epoxy, anda trapezoidalaluminiumframe. To minimize mechanicalstresstothereadoutchambersandtheend-plate,a three-pointmountingandastressfreesealto theend-platewaschosen,asdescribedin Section4.1.5.

Thealuminiumframeprovidestheoverall mechanicalstability of the readoutchambers.It is rein-forcedby stiffening ribs to prevent deformationfrom gravitational forcesandwire tension.The resultof a finite elementcalculationof theouterchambersis shown in Fig. 4.3. Deformationsarelargestinz-direction, but not exceeding25 µm. They arehencesmallerthan the expectedwire sagcausedbyelectrostaticforces(seeSection4.1.3).

4.1 Readoutchambers 43

Figure 4.3: Finite elementcalculationof thedeformationof theouterchambercausedby thewire tension.Themaximumdeformationis 25µm.

Figure 4.4: Overall dimensionsof theALICE TPCreadoutchambers(in mm).


Figure 4.5: Servicesupportwheelin front of theTPC.

Thepadplaneconsistsof amultilayerPCB,with thereadoutpadpatternexposedto thedrift volume.Thesignalsinducedon thepadsarereadout on the rearsideof thepadplane(seeSection4.1.4). Thethicknessof thepadplanePCBis 2 mm. Thegeometricalaccuracy of thepadstructureis 50 µm, whichcanbe achieved usingglassmasksfor the PCB etchingprocess[6]. The multilayering techniqueforthepadplanePCBallows thethrough-platedholesto bedisplacedin thedifferentlayersto avoid leaks.The strong back plate(3 mm thickness)is gluedon top of the padplanePCBs,for addedmechanicalstiffnessof the sandwichstructure.This also improvesgastightness,sincethe strongbackcoversallthe through-platedholes,leaving only cut-outsfor the transferpointsto the front-endelectronics(seeSection4.1.4).

Theassemblyandmechanicaltolerancesof theALICE TPCreadoutchambers,stayingwell below50 µm, arein line with similar designsusedin otherTPCs[1,2,6,7]. Theweightof oneinner (outer)readoutchamberis 12kg (32kg). Theoveralldimensionsof thereadoutchambersareshown in Fig. 4.4.

4.1.1.3 Service support wheel

To relieve the readoutchambersandtheend-platefrom thestressof gravitational forcescausedby thereadoutelectronicsandservices,a so-calledservicesupportwheel is mountedin front of eachof theTPCend-plates(seeFig. 4.5andSection4.1.4).It is madeof aluminiumI-beams,designedto withstandastressof 200kg/m2. Theservicesupportwheelhasamassof 300kg andis suspendedonthesamerailsystemthat alsosupportsthe TPC.A ‘soft’ connectionis madebetweenthe end-plateandthe servicesupportwheel to avoid any additionalmechanicalstresson the end-plate. The designof the servicesupportwheelallows individual readoutchambersto beexchangedin situ.

4.1.2 Wir eplanesand readoutpads

ChargedparticlestraversingtheTPCvolumeionizethegasalongtheir path,liberatingelectrons.Theseelectronsdrift towardstheend-platesof theTPC.Theprimaryelectronsby themselvesdo not inducea


sufficiently large signal in the cathodesegments(readoutpads). The necessarysignalamplificationisprovidedby avalanchecreationin thevicinity of theanodewires.Thetwo-dimensionalsegmentationofthecathodeplaneprovidesthemeasurementof many individualspacepointsperparticletrackin ther-ϕ-plane.Theadditionalmeasurementof thedrift timeof eachof thepadsignalsallowsathree-dimensionalreconstructionof theparticletrack[8].

4.1.2.1 Wire planes

TheALICE TPCreadoutchambersemploy aschemeof wire planescommonlyusedin aTPC,i.e.agridof anodewiresabovethepadplane,acathodewire grid, andagatinggrid. All wiresrun in theazimuthaldirection. Sincethedesignconstraintsaredifferentfor the innerandouterchambers(seebelow), theirwire geometryis different, as shown in Fig. 4.6. The gapbetweenthe anodewire grid and the padplaneis 3 mm for theouterchambers,andonly 2 mm for the innerchambers.Thesameis truefor thedistancebetweentheanodewire grid andthecathodewire grid. Thegatinggrid is located3 mm abovethecathodewire grid in both typesof chambers.The anodewires andgatinggrid wires arestaggeredwith respectto thecathodewires. Henceforthwe abbreviate thewire geometryof theinnerchamberby(2-2),andthatof theouterchamberby (3-3).

Inner Chamber

3mm

3mm

3mm

1.25mm

2.5mm

2.5mm

2mm

2mm

3mm

1.25mm

Gating Grid

2.5mm

Cathode Wire Grid2.5mm

Anode Wire Grid

Pad Plane

Outer Chamber

Figure 4.6: Wire geometryof theALICE TPCouter(left) andinner(right) readoutchambers.

Anode wir egrid

Becauseof thehighparticlemultiplicity andtherelatively largegasgainsrequiredfor thereadoutcham-bers(seebelow) a smallanodewire pitch waschosenfor theALICE TPCto minimizetheaccumulatedcharge per unit lengthof the anodewire andhencethe risk of rate-inducedgasgain variations. Thisled to thechoiceof a 2.5 mm pitch for theanodewires(seealsoSection4.1.3). Sincetheanodewiresarenot readout, thereareno field wires in betweenthemto prevent crosstalkbetweenadjacentanodewires.Furthermore,field wireswouldreducethesignalcouplingto thepads,asthey pick upasignificantfractionof thesignal[9]. Theabsenceof field wiresalsoconsiderablyreducesthemechanicalforcesonthewire frames.However, a chamberwithout field wiresrequiresa somewhathighervoltageto achievetherequiredgasgain,asshown in Section4.1.3.


Cathodewir e grid

The cathodewire grid separatesthe drift volume from the amplificationregion. A large amountofthe ions producedin the amplificationavalanchearecollectedat the cathodewires without causinganoticablereductionin electrontransmission.Thecathodewire pitch is 2.5mm.

Gating grid

The gatinggrid is locatedabove the cathodewire grid, with alternatewires connectedtogetherelec-trically. In the opengatemode,all the gatinggrid wires areheld at the samepotentialVG, admittingelectronsfrom thedrift volumeto enterthe amplificationregion. In theabsenceof a valid trigger, thegatinggrid is biasedwith a dipolarfield VG ∆V, which preventselectronsfrom thedrift volumefromenteringthe amplificationregion. This considerablyreducesthe integral charge depositon the anodewires.

In addition, the closedgatestopsions createdin the avalancheprocessesof previous eventsfromdrifting back into the drift volume. This is importantbecauseescapingions accumulatein the driftvolumeandcancauseseveredistortionsof thedrift field (seeSection7.3.1.2).Thegoalis thereforenotto increasethe ion charge densityabove that createdby primary ionization. The resultingrequirementis that the ion leakagefrom the amplificationregion hasto be lessthan10 4. To achieve an electrontransparency closeto 100%in theopenmodewhile trappingionsandelectronsin theclosedmode,theoffsetandbiaspotentialsof thegatinggrid haveto becarefullyadjusted(seeSection4.1.3).Ontheotherhand,any ionizationproducedby particlestraversingthegapbetweenthegatinggrid andpadplanewillunavoidablybeamplifiedat theanodewiresandthuscontributeto theintegralchargeaccumulation1. Tominimizethiseffect,thegapbetweenthegatingandcathodewire grid is only 3 mm,sufficient to traptheionswithin a typical gateopeningtime of 100µs (Section4.1.3). To keepthealternatingbiasvoltageslow, thepitchbetweenthegatinggrid wiresis 1.25mm.

Table4.1: Wire parameters.

Anodewires Cathodewires Gatinggrid wires

Material Au platedW Cu/Be Cu/BeDiameter 20 µm 75 µm 75 µmStretchingforce 0.45N 1.2N 1.2NLength(inner) 27– 44 cm 27 – 44 cm 27– 44 cmLength(outer) 45– 84 cm 45 – 84 cm 45– 84 cmTotalnumber(inner) 200 201 400Totalnumber(outer) 456 457 912

Wire diameter, length, material and stretching force

The wire length is given by the overall detectorlayout andvariesfrom 27 cm to 44 cm in the innerchambers,andfrom 45cm to 84cm in theouterchambers.

Owingto theirsuperiorstrength,gold-platedtungstenwiresarepreferableto copper-beryllium wires(analloy of 98%Cu and2% Be). However, tungstenhasa 4.1 timesshorterradiationlengthanda 2.6timeslargerZ thancopper. Tungstenis, therefore,usedonly for thethin anodewires,whilst thethickercathodeandgatinggrid wireswill bemadeof copper-beryllium.

1Thecontributionof directradiationto theintegral chargeaccumulationis of theorderof 10%,seeSection7.2.2.


At constantpotential,the gasgain increaseswith decreasinganodewire diameter. Thus,a smallanodewire diameteris preferred.However, electrostaticandgravitational forcescausetheanodewiresto sag,leadingto gasgainvariationsalongthewire. Theelectrostaticsagis approximatelyproportionalto thesquareof thelengthof thewire, andinverselyproportionalto thestretchingforce,whilst thegrav-itationalsagdependson thedensityof thewire material.Therefore,thewiresneedto bemechanicallystrongenoughto withstandtherequiredstretchingforces.Wehavechosenfor theanodewiresadiameterof 20 µm anda stretchingforceof 0.45N. Thecathodeandgatinggrid wireshave a diameterof 75 µmanda stretchingforceof 1.2N. Thewire parametersaresummarizedin Table4.1. For acomputationoftheresultingwire sagandgainvariationsseeSection4.1.3.

Wire frames

Thespacetakenby theattachmentof thewiresalongtheedgesof thereadoutchambersinevitably leadsto insensitive regions. Therefore,thewire framesshouldbebuilt ascompactaspossible.On theotherhand,they mustprovide sufficient room for propergluing andreliableelectricalcontactof the wires.Finally, thewire framesthemselvesandtheirattachmentto thealuminiumframeof thereadoutchamberhave to be mechanicallystrongenoughto not deformunderwire tension. Several different solutionshave beenprovento work for TPCreadoutchambersin thepast.

12mm

Strong Back

Pad Plane

Gating Grid

Anode Wire Grid

Cathode Wire Grid

2mm

3(5)mm

Al Frame

29mm

3mm 2mm

3mm

3(2)mm

3(2)mm

Figure 4.7: Wire fixation in theouterreadoutchamber(dimensionsfor theinnerreadoutchamberarein paran-theses).Theframesaremadeof fibreglass-epoxy.

The wire framesfor the ALICE TPC readoutchambersare shown in Fig. 4.7. The positionsoftheanodewire grid andthecathodewire grid will beadjustedwith respectto thepadplanesurfacetominimizegasgainvariations.Displacementsof thegatinggrid give riseto distortionsof thedrift field.


It will thereforebealignedwith respectto thealuminiumframeof thereadoutchambervia anexternalreference(granitetable). This way, thepositionof thegatinggrid canbe surveyed after integrationofthe readoutchambersinto the end-plate(seeSection4.1.5). Basedon experiencewith existing TPCexperimentsemploying similar readoutdesigns[1, 2,6,7], a precisionof 20 µm canbe achieved forpositioningthewires.

4.1.2.2 Optimization of the pad layout

Moving from theanodewire towardsthesurroundingelectrodes,positive ions,createdin anavalancheprocess,inducea positive signalon the padplane. A very precisemeasurementof the locationof theavalanchecanbeobtainedif theinducedsignalis distributedover severaladjacentreadoutpads(chargesharing),usinganappropriatecentre-of-gravity algorithm.Thepositionof theparticletrack in thedriftdirectioncanbe determinedby samplingthe time distribution of eachpadsignal. The resultingtwo-dimensionalpulseheightdistribution in pad-timespaceis calledacluster.

In the following, we presentthe optimizationof the readoutpad design. The emphasiswas puton thequestionof occupancy, theminimizationof which wasthemajoraspectduring theoptimizationprocedure.MomentumanddE/dx resolution,aswill beshown below, turnout to bewidely independentof the actualpadshape,as long as the total numberof readoutchannelsand the full coverageof thereadoutplanearepreserved.

Pad responsefunction

The pulseheighton a given padrepresentsthe (time-dependent)integral of the inducedcharge distri-bution over thepadarea(seeSection7.2). Therelative pulseheightdistribution of signalson adjacentpads,inducedby a point-like avalanche,is calledthepadresponsefunction(PRF).Whilst theshapeofthe inducedcharge densitydistribution dependsonly on the wire geometryandnot on the shapeandsizeof the readoutpads,thePRFdependson both,becausethepadgeometrydefinesthe limits of theintegrationof theinducedchargedistribution in two dimensions.In thefollowing, thepadlengthl is theextensionof asinglepadin theradialdirection,andthepadwidth w its extensionin azimuth(Fig. 4.8).

Anode Wires

Pad length l

Pad width w Anode Wires! ! ! ! ! ! ! ! ! ! ! ! ! ! !! ! ! ! ! ! ! ! ! ! ! ! ! ! !! ! ! ! ! ! ! ! ! ! ! ! ! ! !! ! ! ! ! ! ! ! ! ! ! ! ! ! !! ! ! ! ! ! ! ! ! ! ! ! ! ! !! ! ! ! ! ! ! ! ! ! ! ! ! ! !" " " " " " " " " " " " "" " " " " " " " " " " " "" " " " " " " " " " " " "" " " " " " " " " " " " "" " " " " " " " " " " " "" " " " " " " " " " " " "

Pad width w

Pad length l

f * wx

Figure 4.8: Definition of padwidth w andpadlength l for rectangularpads(left panel)anddisplacedchevronpadswith anoverlappingfactor fx # 1 (right panel).Also shown is thepositionof theanodewires.

ThePRFcanbecalculatedin thefollowing way (seealsoSection7.2):

PRF$ x% y&'SQ$ x()% y()& dS% (4.1)

whereS is thepadareaandQ$ x( % y( & is theinducedchargedistribution, which is determinedby thewiregeometry[10]. Theintegral in (4.1),however, dependson thesizeandtheshapeof thepads.In addition


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Figure 4.9: Left panel:PRFfor 6 + 10 mm2 and4 + 7, 5 mm2 padsandfor the (2-2) and(3-3) wire geometry.Rightpanel:Non-linearityxtrue-xrecasafunctionof theavalanchepositionxalongthewire. Solidlines: rectangularpads;dashedlines:displacedchevronpadswith fx # 1.

to rectangularpads,alternative padshapes,suchaschevrons[11], have alsobeenusedin TPCreadoutchambers[7]. Theconsiderationsfor theproperchoiceof thePRFarethefollowing:

Thesignalshouldbespreadover two or threeadjacentpadsto allow thecentreof thedistributionto be determinedwith a high resolution. If morethanfour padsreceive a signal,the resolutiongenerallysuffersfrom a low signal-to-noiseratio.

The pad responseshouldbe linear. Non-linearity is definedas the differencebetweenthe truepositionandthereconstructedpositionof theavalanchextrue-xrec.

The PRFhasbeencalculatedfor differentpadandwire geometriesby evaluatingEq. (4.1), usingtheparametrizationfrom Ref. [10] for theinducedchargedistribution Q$ x( % y( & . Theresultsareshown inFig.4.9for 6 10mm2 and4 7 5 mm2 padsandfor the(2-2)and(3-3)wire geometry. Thecalculationsaremadefor rectangularpads(solidlines)andfor displacedchevronpads[11] with anoverlappingfactorfx 1 (dashedlines).Thetwo aforementionedconditionsaremetif thepadwidth w is two timeslargerthanthegapbetweentheanodewiresandpadplane[12]. Thenon-linearityis smallerfor chevron pads,but still negligible for rectangularpads.On theotherhand,chevron padsgive riseto awiderPRF, whichis unwantedfor reasonsof occupancy (seebelow). We have thereforechosenrectangularpadsfor theALICE TPCreadoutchambers;with a gaussianfit to thecalculatedPRFswe obtainσPRF 2 mm forthe4 mm wide padsin the innerchamber(2-2) andσPRF 3 mm for the6 mm wide padsin theouterchamber(3-3).


Cluster size

As pointedout in Section7.2.1,the relative padpulseheightdistribution is not entirelydeterminedbythePRF, becausethedistribution of primaryelectronsarriving at theanodewirescannotbeconsideredaspoint-like. Therefore,thesizeandshapeof aclusteralsodependsonthediffusionin thegas,thetrackinclination,andE B-effectsin thevicinity of theanodewire [8]. Thelattereffect is small in thecaseof theALICE TPC,becausethemagneticfield is relatively weakandthe90%Ne,10%CO2 gasmixtureleadsto a smallωτ (seeSection7.1). Whilst thediffusioncontribution is givenby thedrift lengthandthediffusionconstantof thedrift gas,theinfluenceof thetrackinclinationdependsonthepadgeometry.Sincetheazimuthalwidth of thechargedistribution from aninclinedtrackrepresentsaprojectionof thetracksegmentover thepadlength,theclustersizecanin principlebeminimizedby choosingshortpads.

Similar argumentshold for the drift direction. The width of a pad signal generatedby a singleelectronavalancheis given by the shapingconstantof the front-endelectronics(190 ns FWHM, seeSection5.1.1). Again, theelectrondistribution from an ionizing particleis broadenedby diffusionandtrackinclination,thelatterdependingon thepadlength.

Becauseof the high particledensityanticipatedin the ALICE TPC, the numberof closelyspacedtrackswhich tend to produceoverlappingclustersin pad-timespaceis likely to be significant. ThetrackdensityF in a givenpad-timeregion is expressedin termsof occupancy, definedasthenumberofpad-timebinsabove a certainthresholddividedby thetotal numberof pad-timebins. Equivalently, theoccupancy O is givenby (seealsoSection7.3.1.1):

O 1 exp$- F . seff &- (4.2)

The single-trackclusterareaseff is the extent of an isolatedclusterin pad-timespace. It can becalculatedby countingthenumberof pad-timebinsabove thresholdandmultiplying it by thepadwidthandthe lengthof onetime bin (0.566cm at a drift velocity of 2.83cm/µs anda samplingfrequency of5 MHz). Notethat for low trackdensitiestheoccupancy dependslinearly on seff , whilst theoccupancystartsto saturateif thenumberof overlapsbecomessignificant(seeSection7.3.1.1).

For a givenoverall detectorsize,the trackdensitydependson thedetailsof theparticleproductioncross-sections.However, the single-trackclusterareaseff dependson the propertiesof the detector.Sincethe unfolding of overlappingclustersusuallycomesat the expenseof a loss in momentumanddE/dx resolution,andin thelimit of very high occupanciesfails completely, anoptimizationof thepadlayout in termsof occupancy becomesnecessary. Thegoal is to find a padconfigurationfor which theresultingaveragesingle-trackclusterareais minimal.

To reducethecontribution from diffusiona ‘cool’ drift gaswith smalldiffusionconstantsDT andDL

in thetransverseandlongitudinaldirectionwaschosen(90%Ne,10%CO2: DT DL 220µm0/ cm).This issueis discussedin Section7.1.

Forbudgetaryreasons,thepadsizecannotbechosenarbitrarilysmall.As aconsequence,theaveragepadareashouldnot besmallerthan60 mm2, leaving sufficient freedomin thechoiceof its aspectratio.Therefore,a variety of differentpadshapeswasinvestigatedin termsof their impacton the resultingaverageclustersize.

In afirst optimizationstep,padsof approximatelyconstantarea(60-70mm2) havebeeninvestigated.Theresultsof amicroscopicsimulation(seeSection7.2.1)areshown in Fig. 4.10.Theone-dimensionalclustersizecanbe expressedin termsof the numberof padsor time bins per clusterabove a certainthreshold. This is relevant becausea minimum numberof padsand time bins (typically 2-3 in eachdirection)is neededto provide apropercentre-of-gravity determination.This requirementis fulfilled byall padshapesunderconsideration.On theotherhand,if thenumberof padsabove thresholdexceedsfour, asis thecasefor padlengthsl 10 mm, thesignal-to-noiseratio deteriorates2. However, in viewof occupancy, theclustersizein physicaldimensionsis relevant,andthereforethenumberof pads(timebins)hasto bemultiplied by thepadwidth (spatiallengthof a time bin). On theleft panelin Fig. 4.10

2Thelatterargumentdoesnothold in timedirectionsincethesignalis not integratedover thelengthof a time bin.


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theclustersizein thepaddirectionis shown. Also shown is thewidth of thePRFusedin thecalculationfor the different pad shapes. The numberof padsper cluster that have at leastone time bin abovethresholdincreaseswhenthepadwidth decreases.However, if thenumberof padsis multiplied by theactualpadwidth, theclustersizein thepaddirectionincreaseswhenthepadwidth is increased.This isbecauseawideningof thepadshasto beaccountedfor by thechoiceof awiderPRF, whichconsequentlyleadsto larger clusters.In the middle panelthe sameis shown for the time direction. In this casethetransformationfrom timebinsto spatialextentis trivial, becauseeachtimebin correspondsto 0.566cm.It canbeseenthat theclustersizein thetime directiondecreaseswhenthepadsbecomeshorter, astheprojectionlengthfor inclinedtracksis reduced.It shouldbementionedthat theprojectioneffect in thepaddirection is generallysmallerthan that in the time direction,becausethe averagebendingof thetracksin themagneticfield is smallcomparedto theaveragepolarangle.

Theaveragesingle-trackclusterareaseff is shown in Fig. 4.10(right panel)for differentpadgeome-tries,demonstratingaclearminimumfor apadsizeof 6 10mm2.

Thenext optimizationsteptook into accountthefactthatthetrackdensityis notconstantthroughoutthe TPC,but is highestat small radii. In orderto improve the above resultsfurther, smallerpadsizesat small radii have beenconsidered.In addition,a narrower PRFleadsto an evensmallersingle-trackclusterarea.A PRFwidth of 2 mm canbeobtainedby choosingananodewire to padplanedistanceof2 mmin conjunctionwith apadwidth of 4 mm(seeFig. 4.9).

Theaveragesingle-trackclusterareaseff in theregion r 130cm is shown in Fig. 4.11for a setofsmallerpads.Fromthis,themostsignificantimprovementcanbeachievedwith apadsizeof 4 7 5 mm2

andananodewire to padplanedistanceof 2 mm,resultingin σPRF 2 mm.

On the otherhand,the requirementson padsizearelessstringentat the outerradii of the TPC astheoverall trackdensitydecreasesby 1 r2. 3 Therefore,a padsizeof 6 15 mm2 waschosenfor radiir 200cm.

The overall pad layout of the inner andouterchambersis shown in Fig. 4.12 andsummarizedinTable4.2. The innerchambershave a padsizeof 4 7 5 mm2. The total numberof padsin the innerchamberis 5732,distributedover64padrowsrunningparallelto thewires.To minimizeangulareffectscontributing to theclusterareaandthepositionresolution,thepadsareradiallyorientedaccordingto theaveragelocal trackangle.This resultsin a slight tilt with respectto thewire normalbeinga maximumof 10 at theedgesof thechamber.

Thenumberof padsin theouterchamberis 10110,with a padsizeof 6 10 mm2 for r 198 6 cm(64rows)and6 15mm2 for r 198 6 cm(32rows). In thischamberthepadsarealsoorientedradially.Notethattherearetwo differentpadsizesin theouterchambers,whereasthewire geometryis thesame.This is dueto thefactthatall padshave thesamepadwidth andthereforethesamePRFfor afixedwiregeometry, whilst all padlengthsareamultiple of theanodewire pitch of 2.5mm.

Thetotalnumberof padsin theALICE TPCis 570312(seeTable4.2).

Table4.2: Readoutpads.

Padsize[mm2] Numberof rows Numberof pads

Innerchamber(84 1 r 132 1 cm) 4 7 5 64 5732Outerchamber(134 6 r 198 6 cm) 6 10 64 6038Outerchamber(198 6 r 246 6 cm) 6 15 32 4072TPCtotal 160 570312

3Becauseof saturationeffects the decreasein occupancy is only 5 16 r at small radii and large occupancies(seeSec-tion 7.3.1.1).


2477

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Figure 4.12: Padlayoutof theALICE TPCreadoutchambers.Distancesfrom thebeamaxisarein mm.


4.1.2.3 Resolutionand gasgain

Thissectionaddressestheimpactof thechosenreadoutpadgeometryon theperformanceof theTPCintermsof momentumanddE/dx resolution,with specialattentionto thequestionof gasgain.

Position and momentumresolution

At agivenmagneticfieldB, achargedparticlewith transversemomentumpt isbentalongatracksegmentwith curvatureρ, accordingto:

pt 0 3 . Bρ (4.3)

Thenominalvalueof themagneticfield is B 0 2 T. However, a higherfield of B

0 4 T will beusedin specificphysicsrunsto optimizethemassresolutionof ϒstates.TheradiusR 1 ρ of a tracksegmentis determinedby fitting theindividual spacepointsalongthetrajectory. Theazimuthalpositionresolutionof asinglespacepoint rδϕ leadsto a transversemomentumerroraccordingto Ref. [13]:4

∆pt p2t rδϕ

0 3 . B . L2

720N 7 4

(4.4)

L is thetotal visible track length,N is thenumberof spacepointson thetrajectory. WhereasB andL aregivenby theoverall designof theALICE TPC,theresolutionrδϕ andN aredesignparametersofthe readoutchambers.Note thatEq. (4.4) assumesthat themomentumis determinedin theTPCaloneandthatthecontributionsfrom multiplescatteringandenergy lossarenot includedhere[14].

In thefollowing we list theeffectsthatinfluencetheazimuthalpositionresolutionrδϕ [8]:

theamountof ionizationcontributing to theindividual spacepoint measurements,in termsof thenumberof electronsne.

thediffusionbroadeningof theelectroncloudduringdrift, which is determinedby thetransversediffusionconstantDT.

exponentialfluctuationsin thegasamplificationof singleelectrons,whicheffectively enhancethedelocalizationcausedby diffusionby a factor / 2.

thenatureof secondaryelectronproduction(Landaufluctuations).This contribution entersonlywhenthetrackis inclinedwith respectto thepadorientation.

thesignal-to-noiseratio.

effectsof digitizationandthreshold.

Someof thesecontributions arerelatedto the padgeometry. The padlengthdeterminesthe totalnumberof electronsne, limiting thestatisticalsignificanceof thecentre-of-gravity of theelectroncloud.On the other hand,angularcontributions are enhancedwhen the pad length is increased.Insteadofan analyticevaluationof the resultingpositionresolution,a detailedmicroscopicsimulationhasbeenperformed.

4Theangularresolutionis determinedby thelongitudinalpositionresolutionδz andentersthetotal momentumresolutionaswell. However, this contribution is smallandwill notbediscussedhere.


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For a given front-endnoiselevel (here,we assumea noiselevel of 1000e), the overall signal-to-noiseratio5 dependsentirely on the gasgain. In generalone wishesto keepthe gasgain as low aspossibleand it is thereforeimportantto investigatewhen the position resolutionno longer improves.Resultsfrom simulationswith minimum ionizing particles(MIPs) areshown in Fig. 4.13for the threedifferentpadsizesusedin theALICE TPC.Whilst theimprovementin δz, theresolutionin drift direction,alreadysaturatesat relatively low gasgains,theazimuthalresolutionrδϕ still benefitsfrom gasgainsashigh as2D 5 E 104 to 3 E 104 in the innerchamber, and1D 5 E 104 to 2 E 104 in theouterchamber, bothcorrespondingto a signal-to-noiseratio of about30. Theazimuthalresolutionrδϕ is then1100µm intheinnerchambersand800-850 µm in theouterchambers.However, reducingthegasgainin theinnerchambersfrom 3 E 104 to 1D 5 E 104 resultsin a deteriorationof rδϕ of only 10%. In thedrift direction,theresolutionδz is 1250µm in theinnerchambersand1100µm in theouterchambers.

For reasonsof operationalsafety, thesegasgainsarestill ratherhigh. It is, therefore,worthwhilesearchingfor meansto decreasethewire amplificationwithouta lossin performance.For example:

F anincreasedsamplingfrequency enhancestheprobability for a givenpadto have moretime binsabove threshold,andthuscontribute to thecentre-of-gravity calculation.This allows thegasgainto bereducedsignificantly, asshown in Fig.4.13,albeitattheexpenseof anincreaseddatavolume.

F thesignalthat is inducedon thepadplanecorrespondsonly to a fractionof the total chargepro-ducedin the avalanche,the restbeinginducedon the surroundingwire planes. In addition, thefinite shapingtimeof thepreampcausesa lossin efficiency. Basedontheexperiencewith existingexperiments,we assumethat the combinationof both effectsreducesthe signalby a factorof 5(compareSection7). In additionto improving the readoutelectronics,the padcouplingcanbeincreasedby furtheroptimizationof thewire geometry. In particular, asymmetricwire geometriesoffer a largersignalcouplingto thepads,asdiscussedin Section4.1.3.

5The signal-to-noiseratio for a given clusteris definedasthe pulseheight in the maximumpad-timebin divided by thenoiselevel.


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Figure4.14 shows the averagepositionresolutionfor variouspadshapes,calculatedfor a signal-to-noiseratio of 30. In theazimuthaldirection,the resolutionvisibly improveswhenthepadlengthisincreased.This is aconsequenceof theincreasingamountof ionizationcontributing to themeasurement.In thedrift directionthesituationis different: theresolutionδz levelsoff at around1100µm, andevendeteriorateswhenusingthelongestpads.This is becausethegainin ionizationfrom longerpadsis offsetby a strongerinfluenceof angulareffects. Again, this behaviour reflectsthatangularcontributionsaremuchmorepronouncedin thedrift directionthanin theazimuthaldirection.

Eventhoughtheazimuthalpositionresolutionis bestfor thelongestpads,this is notnecessarilytruefor the transversemomentumresolution,becauseshorterpadsallow moresinglepoint measurementsalongthetrack(compareEq.(4.4)). This is demonstratedin Fig. 4.15,wherethetransversemomentumresolution∆pt G p2

t is plottedasa function of pt for differentpadsizes(B H 0D 2 T). In fact, for verysmall pt the resolutionis worst for the longestpads,becauselow pt trackshave the largestdeflectionandhencearemoresensitive to angulareffects.However, multiple scatteringdominatesthemomentumresolutionfor low pt tracks.At pt I 0D 5 GeV/c thecontribution of thesinglepoint resolutionsaturatesat∆pt G p2

t J 1%(GeV/c) K 1 for all padsizesunderconsideration,whilst at pt H 1D 0 GeV/c, thecontributionsof thepositionresolutionandmultiple scatteringareapproximatelycomparable.

dE/dx resolution

Similar to momentumresolution,dE/dx resolutionis alsonot expectedto dependdrasticallyon thepadlength(samplingthickness),as long asthe total lengthof the measuredtrack, i. e. the productof thepad length l and the numberof points N per track, is constant. Lehraus[15] proposedan empiricalrelationshipbetweendE/dx resolutionandN andl :

∆dE G dxdE G dx

H 13D 52D 35

LN M l N K 0O 37 D (4.5)


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Figure 4.16: dE/dx resolutionfor isolatedtracksin the ALICE TPC asa function of the pad length(samplethickness).

Allison andCobb [16] found a differentparametrization,which slightly favours shorterpadsandhencemoreindividual measurementsfor agiventracklengthatatmosphericpressure:

∆dE G dxdE G dx

H 0D 962D 35

N K 0O 46l K 0 O 32 D (4.6)

Our resultsfrom microscopicsimulationsarein goodagreementwith theseparametrizations,asdemon-stratedin Fig. 4.16.ThedE/dx resolutionfor a0.4GeV/c MIP, emittedatapolarangleof 90\ , is shownfor differentpadsizes.Assumingthatall pointson a trackarefoundanda truncationof theupper30%is applied,adE/dx resolutionof 5%for isolatedtrackscanbeachievedin theALICE TPC.


4.1.3 Electrostatic calculations

In this section,a detailedevaluationof theelectrostaticpropertiesof theALICE TPCreadoutchambersis presented.It addressesthequestionof optimizationof TPCreadoutchambersin moregeneralterms,however, showing that thebaselinedesignpresentedin Section4.1.2is well suitedto meettherequire-mentsof the ALICE TPC. All electrostaticsimulationshave beenperformedin the framework of theGARFIELD package[9].

4.1.3.1 Layout

Variouskindsof readoutchambershavebeenconsideredfor theALICE TPC,all of whichfeaturereadoutof a cathodeplanewith a two-dimensionalsegmentationinto individual readoutpads(seeChapter8 formoreinformation). In thefollowing we will assumeflat pads.Othershapessuchasring cathodeshavebeenconsideredbut arenotpartof thecurrentbaselinedesign.

Wehave,asa baselinesolution,optedfor amplificationon anodewires. It is customaryto sandwichtheanodewiresbetweenthepadplaneandacathodewire planewhichis heldat thesamepotentialasthepads.Suchacathodewire planeenhancesthegain,absorbssomeof theionsproducedin theavalanches,anddecouplestheadjustmentof thefield in theamplificationregion from thefield in thedrift volume.Itdoesnot causea lossin electronefficiency.

All TPC readoutschemescontaina gatewhich preventselectronsfrom enteringthe amplificationregionoutsideagatingtimewindow. Thegatinggrid has,dueto thepresenceof thecathodewires,littleinfluenceontheoptimizationof thereadoutpartproper. However, acarefuladjustmentof thegatinggridpotentialsis mandatoryto obtainasmoothtransitionbetweendrift volumeandamplificationregion,andto meettherequirementsin termsof electronandion transparency.

Wewill, throughout,expresspotentialsrelative to thepadpotential(i.e.ground).

4.1.3.2 Parameter space

Assumingamplificationis achievedon anodewires,thefollowing parametersdeterminethelayoutof aTPCreadoutchamber:

F Thegasusedin thechamberandthedrift field.

F Padshapeandsize.

F Wire diameter, wire length,wire material,andtension.

F Anodewire pitch,distanceto pads,andvoltage.

F Cathodewire pitch,distanceto pads,andvoltage.

F Gatinggrid wire pitch,distanceto pads,andvoltage.

F Presenceor absenceof field wiresbetweenindividual anodewires.

Thebaselinedesignto whichwereferbelow is describedin Section4.1.2.All thecalculationsweremadeassuming20µm diameterW anodewiresstretchedwith 0.45N, and75µm diameterCu/Becathodeandgatinggrid wiresstretchedwith 1.2N.

Thedrift linesof ionizationelectronsto theinnerandouterreadoutchamberareshown in Fig. 4.17.


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Figure4.17: Drift linesof electronsfrom atrackin theinner(left panel)andouter(right panel)readoutchamber.

4.1.3.3 Optimization goals

F Amplification : To ensurea signal-to-noiseratio of about20–30,the amplificationshouldbe oftheorderof 1D 5 E 104 to 2 E 104. (seeSection4.1.2.3).

F Isochrony: Theelectroncollectionisochrony shouldbesmallerthanthearrival timespreadcausedby thediffusionandtrackinclination.

F Signalcollection: In orderto haveahighsignal-to-noiseratiowith thegivenamplification,whichis desirablein orderto reducethe risk of wire ageingandrate-inducedgainvariations,aslarge apartaspossibleof theinducedchargeovera typicalpreamplifier/shaperintegrationtimeof 200nsshouldflow into thereadoutpads.

F Signal shape: The signal shouldnot displaysecondarypeakscausedby the arrival of ions onelectrodes.

F Gasgain homogeneity: Wire displacementstowardsandawayfrom thepadplane,mainlycausedby the electrostaticforce, result in gain inhomogeneities.Suchdeflectionsshouldbe limited sothatthegainis homogeneousata level b 5%.

F Gate transparency: Ionizationelectronsproducedin thedrift region shouldbeableto enterthereadoutchamberswhenthegateis open,but shouldbestoppedwhenthegateis closed.Thegateshouldnot let morethanoneion peravalancheenterthedrift region.

4.1.3.4 Choiceof parameters

Gasand drift field

Thegaschoiceis madeprimarily on thebasisof ionizationpropertiesanddiffusionin thedrift volume.The ALICE TPC drift gasis a mixture of 90% Ne and10% CO2. This gashas,for drift fields in therangeof 200–400V G cm,adrift velocityof 1.35–2.83cmG µs, andanapproximatelyequaltransverseandlongitudinaldiffusion constantof 210–220µm/c cm. The presenceof a 0.4 T magneticfield parallel


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Figure 4.18: Left: Gasgain vs. anodewire voltagein the ALICE TPC readoutchambers.The grey bandindicatesthe desiredamplification. The accuracy of the gain calculationsis of the orderof 20%, resultingin apotentialuncertaintyof h 30V. Right: Sagof theanodewiresasa functionof Vanode, for differentwire lengths.

to the electric field, reducesthe transversediffusion by only 4%. Nei ions have a mobility of 3.9–4.1cm2/Vs in thisgasmixturefor drift fieldsrangingfrom 200to 400V G cm [17,18].

Theuseof agasmixturewith higherprimaryionizationappearsattractive,becausethiswouldpoten-tially leadto a betterspatialresolutionat lower gasgains.Replacingneonby argonmakesthegas20%fasterat 400 V G cm, increasesthe diffusion by 10%, andmore thandoublesthe numberof ionizationelectronsper cm. However, the ion mobility in an argon-basedmixture is lessthan40% of that in aneon-basedmixture,which would considerablyaggravatethecharge accumulationin thedrift volume.Neonis alsopreferablebecauseof its longerradiationlength: 3D 5 E 104 cm comparedto 1D 2 E 104 cmfor argon, theuseof argon would thereforeleadto a deteriorationof themomentumresolutionduetomultiple scattering.

Thegasmixturedoesnot containorganiccomponents,which shouldmake thechamberlessproneto ageingthanmixturesthatcontain,for example,methaneor ethaneasaquencher[8].

Thedrift field is chosento be 400V G cm, in ordernot to exceeda drift time of 90 µs for electronsstartingfrom thecentralelectrode.This is well matchedwith thegateopeningtimeof 100µs. Diffusionspreadstheelectronslaterallyover 3.4mm andcausesanarrival time spreadof 120ns. Ionsneedup to160msto reachthecentralelectrode.

Pad shapeand size

Thepadshapeis chosento minimizetheoccupancy andto maximizethemomentumanddE/dx resolu-tion within a reasonablebudget(Section4.1.2).ALICE haschosenfor thepadsasizeof 4 E 7D 5 mm2 inthe innerchambers,and6 E 10 mm2 and6 E 15 mm2 in theouterchambers.Thepadshape,however,hasanegligible impacton theelectrostaticpropertiesof thereadoutchamber.

Anode wir evoltage

A givengasgaincanbeachievedby thechoiceof theproperanodewire voltage.Apart from theanodewire voltageanddiameter, thegainalsodependsonthegas,thepresenceor absenceof field wires,andon


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Figure 4.19: Left: Anodewire sagasa functionof anodewire to padplanedistancefor ananodewire voltageof 1450V. Right: Sensitivity of thegain to electrostaticsag,computedfor threedifferentanodewire to cathodedistances,calledgapin thefigure.Negativesagsaretowardsthepadplane.Theanodewire voltageis adjustedtoa gasgainof 2 j 104.

thedistancesbetweentheanodewiresandtheneighbouringelectrodes.However, theapplicableanodewire voltagereachesa limit whenelectrostaticdisplacementsof theanodewiresexceedthetolerances.

Thehighestpotentialsrequiredto achieve a givengainareneededin chamberswithout field wires,with a maximal distancebetweenanodewires and pads,and with minimal anodewire pitch. TheALICE TPC readoutchambers(Section4.1.2), which do not have field wires, are expectedto reacha gasgain of 2 E 104 at a voltageof 1450V for the inner chambers(2-2), and1720V for the outerchambers(3-3) (seeFig. 4.18,left panel).Theincreaseof theanodewire sagasa functionof theanodewire voltageandfor differentwire lengthsis shown in Fig. 4.18(right panel)for the(3-3) geometryoftheouterchamber(seealsonext paragraph).

Anode wir e to pad planedistance

The distancebetweenthe anodewires and the pad planeis constrainedby the desiredpad responsefunction(seeSection4.1.2).Thedistanceshouldalsobesufficiently small to achieve therequiredgain,yet largeenoughto ensurethat thegainvariationsdueto electrostaticsagremainwithin tolerance.Wewill, in thisparagraph,assumethatthedistancebetweentheanodewiresandthepadplaneis thesameasthedistancebetweentheanodewiresandthecathodewires. Theimplicationsof anasymmetriclayoutarediscussedin thenext paragraph.

Theanodewire sagasa functionof thedistancebetweenthepadplaneandanodewiresis shown inFig.4.19(left panel)for differentwire lengthsandtheoperationalvoltageof theinnerchamber(1450V).Thus,theexpectedanodewire sagin the innerchambervariesbetween10 µm and35 µm. In theouterchamber, the anodewire sagvariesbetween25 µm and70 µm (Fig. 4.18, right panel). The relativegainvariationasa functionof thewire sagfor differentanodewire to padplanedistancesis shown inFig. 4.19(right panel).

Theanodewire voltageis adjustedto agasgainof 2 E 104 in all cases.Theabove mentionedanodewire sagsfor theinnerandouterreadoutchambersresultin arelativegainvariation b 4%,hencestayingwell within therequirements.


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Figure 4.20: Thechargewhich flows to thepadsfor ionsheadingfor thedrift volume(0m ), integratedover thefirst 200ns.Resultsfor differentanodewire pitchesareshown.

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Figure 4.21: Fractionsof chargeflowing to thevariouselectrodesin theinner(left panel)andouter(right panel)readoutchamberasafunctionof theangleunderwhichtheionsleavetheanodewire. 0m is towardsthedrift region.

During the first 200 ns, ions producedin the avalancheinduceasmuchcharge in the anodewirewheretheavalanchetakesplaceasin all theotherelectrodescombined.Of thelatterpart,only a certainfraction is seenon thereadoutpads.This fractiondependson theanodewire to padplanedistanceandon theanodewire pitch, asdemonstratedin Fig. 4.20. In Fig. 4.21thefractionof chargeflowing to thevariouselectrodesasa functionof theangleunderwhich the ions leavestheanodewire is shown. Forthe innerchamber(Fig. 4.21,left panel),40%of thesignalis seenon thereadoutpads,while 30%areinducedon thecathodewiresand15%on theneighbouringanodes.Owing to the largeranodewire topadplanedistancein theouterchamber, thereadoutpadsseeonly 32%of thesignal,22%areinducedon thecathodewires,and22%on theneighbouringanodes(Fig. 4.21,right panel).6

6Notethat theangulardistribution of ions leaving theanodewire is expectedto be isotropiconly in thelimit of very highgasgains. For the gasgainsdiscussedhere,the angulardistribution of the incidentelectronsis preserved (seeFig. 4.17onpage59), resultingin a peakof theion distributionaround0s .


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Figure 4.22: Left: In thenominalconfiguration,theanodewiresarehalf-way betweenthepadandthecathodewire plane. When they aremoved towardsthe pads(negative asymmetryin the figure), the padscollect morecharge. Right: At thesametime thesagof theanodewires increases,asshown herefor theouterchamberswithananodewire voltageof 1700V.

Implications of an asymmetric layout

The charge collectedby the padsincreaseswhenthe distancebetweenthe padplaneandanodewiresdecreaseswith respectto thedistancebetweenthecathodewiresandanodewires(‘asymmetriclayout’).This increaseis substantial,asdemonstratedin Fig. 4.22(left panel).

Suchanasymmetrydoeshowever increasetheelectrostaticanodewire sag. In theouterchambers(Fig. 4.22,right panel),onecannotmove the longestwiresby morethan100µm beforereachinga sagof 100µm (correspondingto a gainvariationof 5%; compareFig. 4.19,right panel).This leadsto onlya 5%–10%increaseof thepadsignal. Althoughthewiresareshorterandtheanodevoltageis lower intheinnerchamber, leadingto smallersagsfor a givenasymmetry, thesituationis similar, becausein theinnerchamberthegasgainvariationsaremoresensitive to wire sag,asshown in Fig. 4.19(right panel).

Asymmetriclayoutsappearattractive, becausethey would potentiallyallow the readoutchambersto be operatedat lower gasgains. However, sincethe expectedgain variationsexceedthe tolerances,asymmetriclayoutsarenot consideredin thebaselinedesign.

Anode wir epitch

For a given distancebetweenthe anodewires andpadplane,the fraction of the signalwhich goestothe padscanbe increasedwhenincreasingthe anodewire pitch (Fig. 4.20). In addition, in chamberswithout field wires thegain increaseswith increasinganodewire pitch for a given anodewire voltage(Fig. 4.23,left panel).A large anodewire pitch alsoreducesthecollectionisochrony, but this effect issmallcomparedwith thediffusionspread(Fig. 4.23,right panel).

On the otherhand,a smalleranodewire pitch slightly reducesthewire sag(Fig. 4.24, left panel).Mutual repulsionof theanodewiresincreaseshowever, andcanpotentiallyleadto alternatingdisplace-mentsof theanodewires.However, alternatingdisplacementsare,dueto theasymmetryof thecathodes,only probablewith ananodewire pitch lessthan1 mm andananodewire potentiallarger than3000V.For a pitch larger than2 mm, themostprobableinstability is thatof ananodewire hitting thepadplaneafteranexternalmechanicalshock.Thezonenearthepadplanewhich is energeticallyfavouredover the


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Figure 4.23: Left: Variationof the voltagerequiredto achieve a gain of 2 j 104 with a given anode-cathodedistanceandanodewire pitch. Right: Thecollectionisochrony deteriorateswhenthepitch increases,but theRMSis lower thanthediffusionspreadfor all valuesof theanodewire pitchconsideredhere.

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Figure 4.24: Left: The anodewire sagis not a strongfunction of the anodewire pitch, computedherefor aanodewire potentialof 1400V. Right: Theenergy requiredto forcetheanodewire into a parabolicshapewith agivenpositionof themiddleof thewire (ordinate)hasa clearminimumfor anodewire potentialsup to 3000V.Theanodewire pitchandthedistancebetweenanodewiresandcathodesare3 mm in thiscalculation.

equilibriumpositionreachesa thicknessof 200µm whentheanodewire potentialis 3000V (Fig. 4.24,right panel). Theanodewires arethereforeintrinsically stablefor all anodewire potentialsandanodewire pitcheswhichareof practicalimportance.

On average,about60 ionizationelectronspermm2 reachthe readoutchambersin a centralPb–Pbevent (seeSection7.2.2). This translatesinto a rateof 6 E 105 e mmK 2 sK 1 during the readouttime


of 100µs. The estimatedgain variationscausedby theseratesarenegligible [8]. Measurementsindi-cate[19] thatat theseratesgainvariationsin multiwire proportionalchambersarebelow 3% but reachalready10%at tentimeshigherrates.Takingtheuncertaintiesinto account,a smalleranodewire pitchreducesthe risk of rate-inducedgain variations,sincethe total numberof avalanchesis spreadover alargernumberof anodewires.

As a compromisebetweentheaforementionedarguments,we havenchosenananodewire pitch of2.5mm.

Ageing is not a major issue[8] in the designof the ALICE TPC readoutchambersthanksto theuseof a gasmixturewithout organiccomponentsandanintegratedchargeof only 1.1mC peryearandpercm of anodewire for theinnermostanodewiresin Pb–Pbinteractions,assuminga multiplicationof2 E 104 andananodewire pitch of 2.5mm.

Field wir es

Field wiresenhancethefield on theanodewires,thusreducingthepotentialneededto achieve a givengain. They alsoreduce,for a given anodewire potential,the electrostaticsagof the anodewires. Incontrast,they attract,in certainconfigurations,someof the ions producedin the avalanche.Owing tothesubstantialfieldsin thevicinity of thefield wires,thearrival of theionson thesewirescangive riseto a secondarysignal. Furthermore,thesignalinducedon thereadoutpadis drasticallyreducedby thepresenceof field wires[9]. Wehave thereforechosenageometrywithout field wires.

4.1.3.5 Gate adjustments

In theopenstate,equalvoltagesareappliedto all gatinggrid wires,this voltageis calledoffsetvoltageVG. To closethegatinggrid, alternatinglower andhighervoltagesareappliedto thegatinggrid wires.The differencebetweenthe offset voltageandthe potentialof the gatinggrid wires when the gateisclosedis calledbiasvoltage ∆V.

Offset voltage

For aproperchoiceof VG, thefollowing conditionshave to bemet:

F In theopenstate,thegatinggrid hasto beentirelytransparentfor electronscomingfrom thedriftvolume.

F Theisochrony of electronsarriving at theanodewiresshouldbebetterthanthearrival timespreadcausedby diffusionandtrackinclination,i.e.120ns.

F Ionsstartingfrom theanodewiresshouldnot beableto reachthegatinggrid in lessthanthegateopeningtime, i.e.100µs.

Fig. 4.25(left panel)demonstratesthatthegatinggrid in theopenmodeis transparentfor electronsovera large rangeof offset voltages. The isochrony hasa broadminimum betweeny 500 V z VG z 0 V,stayingwell below thearrival timespreadof theelectrons(Fig. 4.25,right panel).

Thetime neededby Nei ionsto drift from theanodewiresto thegatinggrid in theinnerandouterreadoutchamberis shown in Fig. 4.26.Apart from VG, thedrift time dependson theanodewire voltageandthe chambergeometry(distancebetweenthe anodewires andcathodewires). At the operationalanodewire voltages,both chambertypesrequireVG I y 100 V to achieve a Nei drift time of at least100µs.


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Figure 4.25: Electrontransparency (left panel)and isochrony (right panel)asa function of the offset voltageVG. Thecalculationsaremadefor theouterchamberanddifferentanodewire voltages.Theresultsfor the innerchamberleadto thesameconclusions.

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Figure 4.26: Drift time of Ne ions from the anodewires to the gatinggrid asa functionof theoffsetvoltageVG. Thecalculationsaremadefor theinnerchamber(left panel)andouterchamber(right panel),andfor differentanodewire voltagesin stepsof 250V.

Bias voltage

To prevent electronsandions from passingthe gatinggrid, alternatingpotentialsVG | ∆V areappliedon thegatinggrid wires.Thebiasvoltage∆V dependson thegatinggrid wire spacingandthemagneticfield. Apart from this, it dependsstronglyon VG, andweakly on the anodewire voltage(Fig. 4.27).Applying abiasvoltageof ∆V 80V atVG H~y 100V, theelectrontransmissionthroughtheclosedgateis lessthan2%.


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Figure 4.27: Minimum biasvoltage∆V to be appliedto achieve an electrontransmissionsmallerthan2% asa function of the offset voltageVG. The calculationsareshown for the outerchamberanddifferentanodewirevoltages.Theresultsfor theinnerchamberlook similar.

4.1.3.6 Electrostatic matching of inner and outer chamber

Ideally, thedrift field is determinedby thepotentialof thecentralelectrodeandterminatedby thecathodewires of the readoutchamberswhich areon groundpotential. This, however, is not exactly true sincepartof theamplificationfield leaksthroughthecathodewire plane,penetratinginto thedrift volume,andresultingin a shift of theeffective groundplane. If this shift is similar for the inner andouterreadoutchambers,it canbecompensatedfor by properadjustmentof thelastresistorof thevoltagedividerchain(seeSection4.1.5.1).

In their nominalposition, the gatinggrid wires of the inner andouter readoutchambershave thesamedistanceto thecentralelectrode.However, innerandouterreadoutchambershave differentwiregeometryandanodewire voltages,andhencethemagnitudeof field leakinginto thedrift volumeis alsodifferent.This can,in principle,becompensatedfor by slightly shifting thez-positionsof theinnerandouterreadoutchamberswith respectto eachother.

Wedefinehereasvirtual groundthepositionof theplanewherethepotential,extrapolatedfrom theregionof constantfield insidethedrift volumeinto thereadoutchambers,is equalto zero.As mentionedabove,we do notexpectthisplaneto coincidewith thecathodewire plane.

The locationof the virtual groundplanerelative to the cathodewire grid, computedfor inner andouterreadoutchambersanda gatingoffsetvoltageVG Hy 100V, is shown in Fig. 4.28asa functionoftheanodewire voltage.

For both the innerandtheouterreadoutchambers,thevirtual groundplaneis locatedbetweenthecathodewires andthe gatinggrid, at a distanceof about0.9 mm from the cathodewires in the innerchamber, andat about0.7mm from thecathodewiresin theouterchamber. Thedifferenceis explainedby themorecompactstructureof theinnerchamber, which makestheanodewire field penetratedeeperinto theregion betweenthecathodewiresandgatinggrid. Both for theinnerandfor theouterchamber,thelocationof thevirtual groundplanevariesby lessthan250µm for anodewire voltagesrangingfrom1000V to 2000V.


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Figure4.28: Positionof thevirtual groundplanerelativeto thecathodewire planefor theinnerandouterreadoutchamberasa functionof theanodewire voltage.

FromFig. 4.28weconcludethattheinnerandouterreadoutchambersshouldbeshiftedwith respectto eachother by 180 µm in the z-direction to matchtheir virtual groundplanes. However, the driftfield non-uniformitiesresultingfrom theobservedmismatchof thevirtual groundplanesare b 10K 4 andhencewithin tolerances.

4.1.3.7 Operational parameters

In Table4.3we summarizetheoperationalparametersof theALICE TPCreadoutchambers,asderivedfrom electrostaticcalculations.

Table4.3: Operationalparametersof theALICE TPCreadoutchambers.

Innerchamber Outerchamber

Anodewire voltage 1450V 1720VGasgain 2 E 104 2 E 104

Signal-to-noise 20 30,40Anodewire sag(max) 35 µm 75 µmGasgainvariation(max) 4% 4%Gateoffsetvoltage y 100V y 100VGatebiasvoltage | 80 V | 80 VPadcoupling 40% 32%σPRF 2 mm 3 mm


4.1.4 Electronicsmounting and cooling

4.1.4.1 Distribution of fr ont-end electronics

EachFront-EndelectronicCard(FEC)containsall theanaloganddigital componentsnecessaryto readout 128padsignals(for a detaileddescriptionof theFECseeSection5.1.4). Thehigh channeldensityof up to 0.33channels/cm2 in theinnerchamber, andthelimited accessibilityfrom therearsiderequirea verycompactarrangementof theFECs,asshown in Fig. 4.29.

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Figure 4.29: Distributionof theFECson thereadoutchambers.

Owingto thedifferentpadsizesusedin thereadoutchambers(seeSection4.1.2)threetypesof FECsareneeded.They areidenticalin functionalitybut differentin size.The64padrowsin theinnerchamber(padsize4 E 7D 5 mm2) will bereadoutby FECsof dimension14 E 19cm2. Arrangedin two rows,atotalof 45 of theseFECsareneededfor theinnerchamber. Thefirst 64 padrows of theouterchamber(padsize6 E 10 mm2) arereadout by a total of 48 FECsof dimensions14 E 29 cm2. The last32 padrowsof theouterchamber(padsize6 E 15 mm2) areequippedwith onerow of 32 FECsof size14 E 40 cm2.An arrangementwith FECsof identicalsizewouldhavedrasticallyincreasedtheaveragetraceandcablelengthsbetweenthepadsandthepreamplifier/shapers.

Therewill beno electroniccomponentsbondedon thepadplanePCB.Theanalogpadsignalsare


Figure 4.30: Distribution of transferpointson the backsideof thepadplanePCB of the innerchamber. Alsoshown areinnerdimensionsof thealuminiumframe.

collectedon thepadplanePCBto a transferpoint in groupsof 16. Flexible cables(seeSection5.1.2)are solderedto thesetransferpoints. Multipin-connectorsat the other end of the flexible cablesarepluggedinto the FECs. In this way, the signalsof 16 neighbouringpadsareconnectedto the samepreamplifier/shaperchip. The multilayering techniquefor the padplanePCB and the flexible cablesprovidesgoodshieldingfrom externalnoisesourcesandbetweenneighbouringsignals,sincethe padsignalsarealwayssurroundedby groundedlines andsurfaces. In addition,a constantandcontinuousimpedanceseenby thesignalscanbeprovided.

Thedistribution of the transferpointsin the inner chamber, which hasthehighestchanneldensity,is shown in Fig. 4.30. Also shown arethe inner dimensionsof the aluminiumframe,which limit theaccessiblearea. The opensquaresarethe transferpointsof size9 E 22 mm2 wherethe collectedpadsignalsarrive at the surfaceof the pad planePCB and to which the flexible cablesare soldered. InFig. 4.30groupsof 16padsaremarkedin differentgrayscales,indicatingthattheir signalsarecollectedon thesametransferpoint. Fig. 4.31shows detailsof theconnectionbetweenthereadoutpadsandthetransferpointsondifferentlayersof thepadplanePCB.


êìëíëíîðïòñóîõôíöð÷øï

ùûúýü0þ ÿ þóü ú

ùûúýü0þ ÿ þü úýú

Figure 4.31: Connectionschemebetweenpadsandtransferpoints.

Thedistancebetweenthepadandthepreamplifier/shapercircuit variesbecauseof this connectingscheme.It hasa minimum of 60 mm for the majority of the pads,but canreachup to 120 mm in theworst case,for exampleat thecornersof the inner chambers.This variationis dictatedby mechanicalconstraintsof thereadoutchamberdesign.

A single pad hasa capacityof about4 pF. The presentschemeof mountingthe electronicsandcollectingthepadsignalsaddsacapacitive loadof 6 to 12pF to thepreamplifier/shaper. Thesenumbersareconsistentwith anoiselevel below 1000electrons.

Details of the mechanicalstructurefor positioning and mounting the FECs at the chamberaresketchedin Fig. 4.32. Supportbarswhich are attachedto the servicesupportwheelare mountedintheradialdirection,betweenadjacentFECcolumns.For thelongitudinalfixationof theFECs,two holesareforeseeninto which two dowel pinsarefed. Thesedowel pins areattachedto a brassanglewhichis screwed togetherwith thebrasscoolingpipe to theFEC.This providesa flexible but exactposition-ing of theFECsto theservicesupportwheel,without introducingany gravitationalforcesonthereadoutchambersor theend-plate.At theuppercornersof theFECsanadditionalsnap-onfixationagainstlateralmovementsis foreseen.This fixation is mountedon additionalcrossbarsto theservicesupportwheel.Thesewill bedimensionedin sucha way thatall themechanicalstresscomingfrom thedifferentcablesandconnectorsis takenaway from theFECsandtransferredcompletelyto theservicesupportwheel.


Figure 4.32: Mountingandcoolingof theFEC.

4.1.4.2 Cooling system

Eachof thetwo readoutplanesof theALICE TPCconsistsof 18 innerand18 outerreadoutchambers,equippedwith 4500FECs. EachFEC hasa power consumptionof up to 12 W. The TPC-FECcool-ing system,therefore,hasto be designedfor 55 kW. The operatingtemperatureof the TPC hasto bestabilizedto thelevel of | 0D 1 \ C. To solve this problemwe will usethesecondgenerationLeaklessliq-uid CoolingSystem(LCS2)which is currentlyusedin severalTPCdetectorsystemsat CERN(NA49,CERES/NA45) andat BNL (STAR), andhasbeenproposedin LHC experiments(ATLAS CalorimeterandtheCMS Pixel Detector)[20].

TheLCS2fulfills therequirementsfor anLHC detector:

F Sincethedetectoris locatedin a temporarilyinaccessiblearea,thefilling, bleeding,anddrainingoperationsarefully automated.

F The numberof active componentsinstalledin the inaccessibleareasarereducedto a minimumandareableto operatein magneticfieldsandunderradiationexposure.

F Therisk of leaksis reducedasfar aspossible.

F All parametersarecontrollablefrom thecontrolroom.

F As a coolingmediumdistilledwateror fluorocarbonscanbeused.

F The equipmentwill be selectedin collaborationwith future maintenanceandoperationserviceswith theST/CVgroupatCERN.

Theprincipleof operationof the thenew LCS2coolingsystemdevelopedby theSFsectionof theCERN/EST/SMgroupis sketchedin Fig.4.33.Thecoolingliquid, distilledwaterin ourcase,is heldin astoragetank(1) positionedatthelowestpointof thesystem.Theliquid is movedby acirculatorpump(2)into theheatexchangers(3). Thesearecooledby chilled waterfrom theCERNnetwork. Theflow of


4 4

44

5

3

1

2

P1 > atm.

P2 < atm.

P3 = P2 + ∆P

Figure 4.33: Thecoolingliquid is heldin a storagetank(1) below atmosphericpressureby a vacuumpump(5).The liquid is movedby a circulatorpump(2) into the heatexchanger(3), andthenflows to the heatsourcesbypressureregulators(4) which maintainanequalpressurein all sub-circuits.

thecooling liquid throughthecoolingbrasspipeswhich arein mechanicalcontactwith theFECs(seeFig. 4.32),is controlledby pneumaticregulationvalves(4). At theinput of theseregulatorsthesystempressureis above atmosphericpressure.Theregulatorsarelocatedalongthecircumferenceof theTPCandfeedthecooling liquid to thecooling lineswhich areconnectedto the front-endelectronics.Theyadjustthepressurein the returnlines to a valuebelow atmosphericpressure.The regulatorsguaranteethatin all subsectionsof thecoolingcircuitsanequalpressurebelow atmosphericpressureis maintained.Thus,any leakin theselinesandconnectionswill not leadto a lossof coolingliquid (leaklesscooling).In addition, the regulators(4) definethat a constantpressuregradientof 200 mbardrives the coolingliquid throughthecoolingpipeswhich areintegratedinto theFECs. A vacuumpump(5) in the returnline dischargesany excessair collected,i.e.duringdrainage,andsustainsa pressurebelow atmosphericpressure.

The cooling supplyandreturnlines aremountedat the outercircumferenceof the servicesupportwheel. Theconnectionsfrom thesupplylines to theFECwill bemadeby 4 mm Viton tubes.In orderto optimizetheheattransferfrom thecoolingagentto theFECa rectangularbrasstubeof dimensions9 E 2D 5 mm2 is solderedor screwed directly onto the groundplaneof the FEC. In this way the heattransferis provided by conduction.This allows thecooling systemto be operatedunderthe followingconditions:With apressuregradientof 200mbarin thesystemandaserialconnectionof 6 FECswegeta flow of 50 l/h anda temperaturegradientbetweenthesupplyandreturnline of 1.5 \ C. Theturbulentfluid velocity is 1.2m/s.

TheFECthermallyconsistsof four regions.Therearethepreamplifier/shaperswhichcontribute16%of the thermalload, the ADCs (64%), the ALTROs(14%),andfinally the transducersandthe voltageregulators(16%).Therectangularbrasscoolingtubewill bemounted30mmabovetheconnectorsto theflexible cablesat theheightof thepreamplifier/shapers.Thusthecoolingtubeis verycloseto thelargestheatsource,which minimizesthe heattransferthroughthe padplaneinto the drift volume. The very


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Figure 4.34: Equipotentiallinescloseto thegatinggrid of theouterreadoutchamber:Vanode T 1700V, VG T 100V.

stringenttemperaturestability in thedrift volumerequiresa very constantandcontinuousoperationofthefront-endelectronics.Theoverall temperaturedistribution will bemonitoredby asetof temperaturesensors.In additionto sensorson the padplaneandin the drift volume,a PT100temperaturesensorwill be integratedinto eachFEC. All this information is monitoredby the slow control system. Thisinformationis thenusedfor thefine-tuningof thetemperaturedistribution, andnecessaryadjustementswill bemadeby local heaterswhichareintegratedinto thecoolingsystem.

4.1.5 Link betweenfield cageand readoutchambers

4.1.5.1 Electrostatic matching of readoutand field cage

To obtaina smoothtransitionfrom thedrift field to the readoutchambersthepotentialon thestripsofthe field cageneedsto be matchedwith thepotentialof thegatinggrid of the readoutchambers.Thisis doneby keepingthe last resistorof thevoltagedivider chainvariable,allowing thestrip potentialtobe shiftedtowardsthatof thegatingplaneof the readoutchamber. Furthermore,thegatinggrid offsetvoltageis adjustedby thegatinggrid pulser(compareSection4.2). The correctvalueof the potentialto beadjusteddependson thedrift field itself, thepositionof thereadoutchamberin z, andthetransferfields betweenthe wire planesof the readoutchamber. Drift field distortionscausedby field leakagefrom theamplificationregion aresmallandconfinedto thevicinity of thegatinggrid wires,asshown inFig. 4.34for adrift field of 400V/cm andananodewire voltageof 1700V (outerchamber).


Figure4.35: Three-pointmountingof thereadoutchambers.

4.1.5.2 Mounting and sealingof the readoutchambers

The readoutchambersaremountedon end-platesclosing the drift volumeon eithersideof the TPC.Eachreadoutplaneis subdivided into 18 inner and18 outer readoutchambers.The inner chambersextend from 83.0 to 133.2 cm in the radial direction, the outer onesfrom 133.5 to 247.7cm. Theelectronicreadoutcomponents,including their cabling and cooling services,are mountedon servicesupportwheelsplacednext to the end-plates.This considerablyreducesthe weight and, hence,thedeformationof theend-plates.To reducedeadspacebetweenthereadoutchamberbordersandtheend-plateframes,a specialinsertiontool hasbeendesignedto mountthereadoutchambersfrom insidethefield cage.During the installationprocedurethis tool is attachedto theservicesupportwheel. Prior toinsertionof a readoutchamberinto the field cagevolume, the readoutchamberis rotated90\ aroundits two planaraxes. Insidethedrift volumethechamberwill be rotatedbackto its original orientationandpulledbackinto its corresponding‘slot’ of theend-plate.It will thenbesecuredby threepointsontheend-plate(kinematicallyfreemounting,seeFig. 4.35)andalignedwith shimming(seebelow). Thismethodensurestheplanarityof thechamberwith thecentralelectrodeto within b 100µm.


Figure 4.36: Schematicview of theend-platewith readoutchamberandO-rings.

Thechosenthree-pointmountingschemecannotprovide a gas-tightsealbetweenthechamberandtheend-platewithout overconstrainingthepositioningof thechamberwith respectto theend-plate.Toclosethe crackbetweenthe chamberand the end-plate,a flexible Mylar gasket is mountedwith twoaluminiumbracesaroundthe chamberborderand the end-plateframe, respectively (Fig. 4.36). Thissealingtechnique,alreadysuccessfullyemployed in the CERESTPC [7], leaves the readoutchamberstress-freein its sectorof theend-plate.

4.1.5.3 Handling and replacementof readoutchambers

For mountingandhandlingof thereadoutchambers,amobileplatformof aminimumsizeof 1 E 3 m2 isforeseenwhichallowsaccessto theend-platesandservicesupportwheels.If areadoutchamberneedstobereplacedin situ, thecorrespondingservicesmountedontheservicesupportwheelneedto beremovedprior to accessingthechamberto bereplaced.

4.1.5.4 Deformation of the end-plate

To estimatethe deformationof the end-platethreeprincipal load factorsneedto be consideredin thecalculations:(a) theweightof theend-plateitself andthe readoutchambers,(b) theweightof the ITSbeingsupportedby theTPC,and(c) afloor inclinationof 1.39%(0D 796\ ) in theexperimentalcavern.Tothisend,thestaticbehaviour of theendplatewasanalysedin three-dimensionalspace,i.e.its deformationin its own plane(x, y) andalsodisplacementsin z, primarily dueto thefloor angle.Theresultsareshownin Fig. 4.37. Theconclusionsarethatall deformationsof theend-platearewithin elasticlimits anddonot exceed100 µm in the xy planeand600 µm in z. The displacementsin z will be compensatedbyan end-platebracket counteractingthe sagforce arisingfrom the floor angle. The TPC is designedtomaintainits mechanicalstability andthealignmentof its mountedcomponentsevenwhenit is removedfrom thespaceframewhichwouldbenecessaryfor maintenanceinterventionsat theITS.


0.07 mm

0.07 mm

0.08 mm

0.08 mm

0.60 mmZX

y

Figure 4.37: Deformationof theend-platein thex-y plane(left panel)andin thez-direction(right panel)com-putedwith finite elementmethods.

4.1.5.5 Survey and alignment

In this sectionwe addressthe problemof aligning the individual componentsof the TPC with respectto eachother. Theoverall survey of theTPCwithin theALICE spaceframeandits positionrelative tothebeaminteractionpoint arenot thesubjectof this study. Theprincipal componentsof theTPC,thepositionandalignmentof which needto be known with high accuracy, are the HV centralelectrode,the potentialdegraderstripsandthe readoutchamberswith their individual padandwire planes.Theintrinsic precisionof all the individual componentsof the TPC is ensuredduring the manufacturingprocess(qualitycontrol). It is thustheknowledgeof theircollectivebehaviour within specifiedtolerancesthat leadsto theexpectedperformanceof theTPC.Theprincipal referencepoint for positioningall theinternalTPCcomponentswith highprecisionis thepositionof theHV centralelectrode.Thiselectrode,built extremelyflat andstiff from compositematerial,is mountedinto theTPCfield cagewith aprecisionof 100µm. Thisprecisionmustnotdeterioratebecauseof misalignedcomponentsmountedbetweentheelectrodeandtheend-plates.Thus,thepositioningof theintermediatepotentialdegradernetwork mustbeof suchprecisionthat the initial referenceis preserved over thedistanceof 2.5 m from thecentreoftheTPCto thereadoutplanes.Therefore,theplacementandgluing of thestripsupportrodsis surveyedthroughouttheentireinstallationwith a high degreeof accuracy. With anintrinsic precisionof therodsof z 20 µm the internalreferencepositionof thecentralelectrodeis thustransferredto the level of theend-plates.

Theexternalstiffnessof thecompositestructureof theTPCvesselslendsitself to thehighstructuralintegrity of theinternal,field defining,network of theTPC.

4.1.5.6 Alignment of the readoutchambers

To determinethepositionof thereadoutplanewithin thechosenTPCreferenceframe,thesurfacesof theend-platesserveasaccessiblesurvey pointsfor positioningthereadoutchambers(Fig.4.38).Thisallowsthereadoutchamberstobealignedwith respectto thestripsupportrodsandthecentralelectrode.Sinceitis known thattheend-platesdeformundertheweightof thereadoutchambers(Section4.1.5.4),theend-


Figure 4.38: Referencemarksdistributedalong the outerand inner circumferenceof the end-plateallow theorientationandplanarityof theend-plateto besurveyedvia photogrammetry.

plateswill first beloadedwith dummychambersto estimatetheamountof adjustmentto bemadein eachindividual end-platesectorprior to installationof therealchambers.This is donein thesurfacebuildingwith a methodcalledphotogrammetry. This methodis often appliedin otherexperiments[21] andissensitive to aprecisionof z 10K 5 comparedto thedimensionof theapparatussurveyed.Adjustmentstothechamberpositionaremadewith shimonly. Thereafter, no otheradjustmentsarepossible.The iso-staticthree-pointmountingof thechambersis designedto leadto anoverall readoutplanaritywith thecentralelectrodeof z 100µm. Combinedwith theinternalmechanicalaccuracy of thepadplane,eachindividualpadcoordinatewith respectto thecentralelectrodewill beknown to aprecisionof z 100µm.

4.1.6 Testsand prototyping

Thissectiondescribesthestepsto betakento validateandtestexperimentallytheoptimizationdescribedabove obtainedfrom electrostaticsimulations(Section4.1.3). In addition,all the mechanicalaspectsof the proposedlayout, e.g.the strengthof the aluminium body againsttorsion, the mountingof theelectronicsand the cooling schemeetc.have to be verified. Finally, the operationalparametersandcharacteristicsof thereadoutchambersunderrealisticconditions,e.g.their stability underhigh particleloadatagivengain,mustbedetermined.Ourstrategy is to usebothexistingreadoutchambersandsmalltestchambersto investigateaspectswhich appearto beindependentof thefinal layoutor dimensionsofthe readoutchambers.In addition, full-size prototypechamberswill be built. The chamberswill betestedemploying dedicatedtestbeamsboth at GSI andCERN, aswell asparasiticallywithin CERNheavy-ion experiments(NA49, CERES/NA45) duringleadbeamtime. Whenever appropriate,testswillbeperformedusinglaserbeams,radioactive sourcesor cosmicrays.


Small test chambers

Several small testchambers(active area5 E 5 cm2) with differentwire andpadgeometrieshave beenbuilt. Thesechambersaremountedon a 50 cm long field cageinsidea steeltubeof 20 cm diameter,whichenclosesthedrift gas.Thesteeltubeprovidesseveralportswherelaserlight or particlesfrom ara-dioactive sourcecanbeinjectedinto thedrift volumeatdifferentpositions.Thereadoutschemeconsistsof standardpreamplifier/shapers,whichareconnectedto theADC systemdevelopedin theframework ofRD32[22] andaredescribedin Section8.1.Thisset-upallowstheevaluation(Section4.1.2and4.1.3)ofdifferentwire andpadgeometriesandpotentialsettingsandtheir influenceon thechamberperformanceto beverified,in particular:

F gasgain,

F gatingefficiency andvoltagesettings,

F signalcollectionandshape.

Existing readoutchambers

A sparereadoutchamberof the NA35 TPC (active area52 E 78 cm2) hasbeenmountedon a 15 cmlong field cageand a plastic gasbox. The chambercan be readout with ALEPH-type NA35 TPCelectronics[23]. Thepurposeof thisensembleis

F to scrutinizesourcesof instabilities— which hadbeenobserved,but not yet understoodin detail— suchasdischargesoccurringpresumablyat thebordersof thechambers;

F to investigatetheresponseof thechamberto highparticlesfluxes;in particularthelimits of stableoperationshouldbe mappedout as a function of the gasgain, particle density, meanangleofincidenceandrate.

Full scaleALICE TPC readoutchamberprototype

Thebehaviour of a chamberis specificto a particulardesign.Thus,someof theabove testsneedto berepeatedwith achamberof theactualALICE design.It is plannedto build duringtheyear2000two full-sizeprototypesof the innerchambersasdescribedin Section4.1.1. Thechamberswill beconstructedat GSI. Subsequenttestswill be performedemploying GSI testbeams.Finally the chamberswill bemountedandtestedin theprototypefield cage,which is describedin Section3.1.5.This schemeallowsusto

F investigatethemechanicalpropertiesof thechambersincludingtheproposedsealingandmountingschemeinto theend-plate;

F investigatetheboundaryproblems(field distortions,gating)betweentwo adjacentchambers;

F performtestsin arealisticheavy-ion environment(astheproposedALICE TPCelectronicswill notbeavailableby theendof 2000,it is plannedto startwith modifiedversionsof theCERES/NA45or NA49 TPCelectronics);

F testtheALICE TPCelectronics,whenthey becomeavailable,on theprototypechambersin con-junctionwith thepropercoolingscheme.


4.2 Gating system

Thesensitivemodeof theTPCshouldbeactivatedonly for valid triggersandfor thedurationof thedrifttime(100µs). Therefore,agatinggrid is installedbetweenthecathodegrid andthedrift region [24] (seeSection4.1 ). During the‘closed’ state,potentialsVG | ∆V areappliedon alternatingwiresof thegrid,so that the resultingelectrostaticfields renderthegateopaqueto thepassageof chargedparticles.Thevalueof ∆V dependson themagneticfield, thewire spacing,anddrift field [25]. It is lessthan100Vfor B z 0D 5 T for a coolgas,suchas90%Ne,10%CO2 adoptedfor theALICE TPC.In thetransparentstateall wires of the gatinggrid arekept at VG enablingthe passageof drifting electrons.The gatingcircuit (usuallycalledthe‘pulser’) hasto placethelargevoltageswings∆V on thegatinggrid asfastaspossiblewith minimalpick-upon thereadoutelectronicsresultingfrom theseswings.

4.2.1 Designconsiderations

For signalswith a rise time greaterthan100 ns the gatinggrids canbe consideredascapacitors(ne-glectingeffectsdueto thefinite size).Thus,we arefacedwith theproblemof charging anddischargingthecapacitanceof thegatinggrid which consistsof themutualcapacitancebetweenwiresCM , andthecapacityto thegroundC0. Thevoltagechangeon thecapacitorCM is 2∆V andthereforethecapacitance‘seen’by thepulseris C H C0 2CM . For thechambermoduleof dimension0.7m2, and1.25mmwirepitch,CM b 2D 75 nF, C0 b 0D 5 nF thereforewe canexpecta maximumcapacitanceof C = 6.0 nF. Thechamberswill beconnectedto thepulsersvia 40m longcablessotheinterconnectionshave to betreatedastransmissionlines andspecialcarehasto be taken not to producereflectionscausedby impedancemismatch.In orderto have agatingpulsewithout oscillationswe musthave apulserwith animpedanceequivalent to the impedanceof the cable. The rise-timeis in this casedeterminedby that impedanceandthecapacitanceof thegatinggrid. Fromthe theoreticalpoint of view, we have a transmissionlineterminatedby the internalimpedanceof thepulseron the input endandby a capacitorof about6.0 nFat theoutputend.If theoutputimpedanceof thepulsermatchestheimpedanceof thetransmissionline,thecharging of thecapacitoris determinedonly by its capacityandtheimpedanceZ0 of thecable(typ-ically 50Ω). Thevoltageon thegatinggrid will have 95%of its maximalvaluein 3τ, (τ H CZ0). ForC = 6.0nFandZ0 = 50 Ω this impliesthatabout900nsareneededfor thegateto befully transparent.

The only way to reducethe rise-timefor a given cableimpedanceis to compensatethe capacitiveload. This canbedoneby usingpassive elements(resistors,capacitorsor coils). For our purposestheonly convenientway to compensateis with an inductancebecausethis way a reductionof the pulseamplitudeis avoidedandit doesnot introduceanadditionalload. Themaximalvalueof theinductanceis determinedby therequirementof having acritical dampinggivenby

L H Z20C4

DIn this casethevoltageon thecapacitoris givenby

V H V0 1 y L1 t

τ G 2 N exp y tτ G 2

D (4.7)

From this equationoneconcludesthat the time for a pulseon the capacitorto reach95% is given by2D 37τ, i.e. 2D 37 E 300 H 700ns. Theinductancefor critical dampingis 3D 75µH. It follows thatthecoilsreducetheopeningtimeby about20%.Thecompensationcoilsalsoreducetheinitial capacitorchargingcurrentspike andpick-up, andhenceareusefuleven if the switchingspeedis not of primary interest.Theinductanceis smallenoughto allow theuseof coilswithoutaFeroxcoresothatthey canbeusedina magneticfield.

4.2 Gatingsystem 81

4.2.2 Gating circuit design

Commerciallyavailable high-speedMOSFET drivers are usedtogetherwith n-channelpower MOS-FETsasswitchingdevices.This eliminatestheneedfor additionaladjustmentof thetiming of theFETopening.A blockdiagramof thegatingpulseris shown in Fig. 4.39.

Figure4.39: Block diagramof thegatingpulser.

The input TTL pulseis transformedinto short (lessthan500 ns) startandstoppulsesusingdualmono-stableflip-flops74HCT221.Therestof thepulseris separatedfrom groundby capacitivecouplingsothatany offsetvoltageVG canbeapplied.Its absolutemaximumis limited by thevoltagelimit of thecapacitors.Themaximal∆V is limited by themaximalVDS of theMOSFET(200V for IRF640). Thegatingpulseis regeneratedusinga resetableflip-flop (HEF 4013),anddistributedinto four inputsof thetwo dual MOSFETdrivers(IR2100). Small time differencesin the MOSFETswitchingtimes(of theorderof 20 ns)arecorrectedusingdifferentvaluesfor thegateresistors.Fasterturningoff is achievedby addingfastdiodesin parallelwith theseresistors.Additional fine tuning is not necessary, exceptforMOSFET2, whereit couldbedonewith a50pF trim capacitorat thedriver input. A largermismatchinswitchingtimescanbecorrectedby carefulselectionof thecomponents(especiallyMOSFETdrivers).

The impedanceof thepulseris determinedby the resistorsRO (for gateopening),andRC (for gateclosing). The closingof the gatecanbe slower andit is not necessaryfor RC to matchZ0. It canbebiggerin orderto compensatetheasymmetryof thesignalscausedby thetime differencewhenopeningandclosingtheFET (lessthan50 ns). This reducespick-upduringgateopening.TheoptimalvalueisbetweenRC H 150Ω andRC H 200Ω.

4.2.3 Performance

A switchingspeedof lessthan150nshasbeenachieved(it hasto belessthan2td, wheretd is thecabledelay, sothat thepulsereflectedfrom thechamber‘sees’thematchingimpedanceon thepulsersideofthecable).For acablewith 5 ns/mdelaythisgivesaminimal lengthof 10m, well below theactualcablelengthof 40 m. Themaximallengthof thecablesis only limited by their ability to transmitsignalswith


a frequency below 10MHz. Thesymmetryof thepositiveandnegativepulsesduringswitchingfrom theclosedto theopenstatecanbetunedto betterthan1%.

The internaldelayof thepulseris lessthan400 ns. This implies that theTPC gatecanbe openedabout1 1µs after thestartof the gatesignal,with possiblya small pick-up in thefirst two time slices.The minimal pulsedurationis determinedby the start-stopgenerationcircuit andthe flip-flops andislessthan1µs. Themaximalpulsedurationis infinite. Themaximalrateis determinedby themaximalsupplycurrent,andpre-gatingusingthefirst level triggeris possible.

Up to now, pulsersbasedon this designareusedfor TPCsof theNA49 [6] andNA45(CERES)[7]experiments.Standard50 Ω 20 m cableswith BNC connectorsareused.The costper channelof theNA49 pulserwaslessthan300CHF.

Consideringthegatingcircuit, thereadoutchambers,andthenecessaryinterconnectionsasawhole,thesystemcanbedesignedfor aswitchingtimebetween1µsand1 5µsevenfor largeareaTPCmodulessuchasthe ALICE experiment. Owing to the simplicity of the design,the internalpropagationdelaycould be reducedto lessthanhalf of that achieved with otherexisting models. The robustnessof thecomponentsmakesthemvery reliable,requiringminimal maintenance,so that they canbe installedinareaswhich arenot easilyaccessible.Remotecontrol is only neededfor switchingandadjustingthesupplyvoltages.

4.2.4 Implementation

It is foreseento install the gatepulsersystemoutsidethe L3 magneton eithersideof the TPC. Thisrequirescablesof about40m lengthto connectto thereadoutchambers.Following theNA49 designthepulsercardswill behousedin standard3U Eurocrates,accommodating10channelsandoneextramodulefor distributing thetriggersignalacrossthebackplane.Alternatively, a6U Eurocrateis beingconsideredwhich would leadto a somewhatmorecompactunit with 20 channels.In bothcasesonecompleterackwill be neededon eithersideof theTPC which will alsoaccommodatethe remotely-controlledpowersupplysystem.

Beforeinstallationthesystemhasto undergo stringentquality tests.As for thefront-endelectronics(seeSection5.1.9)aburn-inprocedurelastingfor about10 h at50 C is foreseen.

4.3 Calibration

In thischaptercalibrationissuesrelatedto theoperationof theALICE TPCaredescribed.In thefirst partthe lasersystemis presentedaddressingthedeterminationof electrondrift velocity, residualdrift-fielddistortions,distortionsrelatedto E B effectsandcorrectionsof mechanicalmisalignmentof thereadoutchambers.In the secondpart the electronicpulsersystemfor the gain and time equilibrationof the570000readoutchannelsis described.Thethird partdealswith thecalibrationof thegasamplificationvariationsacrossindividual readoutchambersdueto mechanicalimperfections.

4.3.1 Laser system

4.3.1.1 Intr oduction

Narrow and short-durationUV laserbeamscan be usedto simulateparticle tracks in the TPC [8].Nd:YAG lasers(1064nm) with two frequency doublers,generatinga beamwith 266 nm wavelength,havebeensuccessfullyappliedfor thispurposein NA49 andin CERES/NA45 at theSPS.Theionizationoccursvia two-photonabsorptionandthusthetransversedistribution of theionizationdensityis in everydirection 2 timesnarrower thanthe light-intensitydistribution. On the otherhand,the squareof thefluctuationof thelaserintensitywill enterinto theionization.

4.3 Calibration 83

4.3.1.2 Designconsiderations

Theaimof thelasercalibrationsystemis to measuretheresponseof theTPCto straighttracksatknownposition. In particular, the lasersystemshouldallow the TPC readoutelectronicsandsoftware to betested,distortionscausedby misalignmentof thesectorsto bemeasured,temporalandspatialchangesof thedrift velocity down to 10 4 to be monitored,apparenttrack distortionsdueto E B andspace-chargeeffectsto bemeasured,andthetransitionregion betweenthedrift volumeandwire chamberstobeunderstood.

Thefirst two objectivescanbeachieved by merepresenceof several lasertrackscrossingtheTPCvolumeundervariousangles,basedon the fact that the lasertracksarestraight. We plan to have 168beamsin eachof theTPChalves.For monitoring,reproducibilityof thelaserbeampositionis important,while thestudyof E B andspace-chargeeffectsrequiresknowledgeof absoluteposition.Wecurrentlyassumethat theabsolutepositionwill beknown up to 2 mm at theentranceof the track into theTPC,andup to 2 cm at its end-point,andthat therelative positionwill beknown up to 0.2 mm and2 mm attheentranceandend-point,respectively.

The laserinstallationshouldnot contribute a significantamountof material. Following the STARapproach[26] we decidedto illuminate clustersof small (1 mm diameter)mirrors by a wide beam(Gaussianprofile with σ 2 cm). The profile of the reflectedbeamis thendeterminedby the mirrorratherthantheincidentbeam.This is anefficient way of producingmany narrow beamsandit doesnotrequireinstallationof opto-mechanicaldevicesinsidethedetector.

Monitoring drift velocity will be realizedby runninglasersin parallel to the normalphysicsdata-taking.Theotherquestionswill beaddressedby meansof dedicatedlaserruns.Monitoringandcalibra-tion will thusbethetwo modesin which thelasersystemwill beused.

4.3.1.3 Implementation

Theplannedoverall layoutof the lasersystemis shown in Fig. 4.40. Eachhalf of theTPChasits ownlaser. With thehelpof remotelycontrolledmirrors,andwith CCDcamerasandposition-sensitive diodesusedfor diagnostics,thelaserbeamis transportedto theedgeof thecorrespondingend-plate.

Subsequently, thebeamsarereflectedseveral timesby prismssuchthat they follow theouteredgesof theend-plates(Fig. 4.41).

At six of the18positions,wherethebeampassescloseto theendsof therodswhichsupportthefield-cage,partially reflectingmirrorsareusedto sendpartof thelight into thehollow rods.A mirror locatedat theoppositeendof eachrodandperpendicularto its axishelpsto adjustthesix beamsparallelto theirrespective rods. This is doneby requiringthat the incidentandthe reflectedbeamsoverlap. Insidetherods,at four differentdepthsin z, micro-mirrorclustersareinstalledsuchthat they do not shadow eachother(Fig. 4.42).Themicro-mirrorsare1 mmdiameterglassfibres,cutatanangleof 45 , polished,andcoated.A mirror clusteris a bundleof sevensuchfibres.Eachfibre in a bundleis slightly rotated.Thisway eachbundlegeneratesa fanning-outpatternof sevenbeams.Therodwall hasanopeningsuchthatthereflectedbeamscangetoutof therodandentertheTPCvolume.

Oncethebeamsarealignedin therods,thepositionsof all theraysin theTPCaredeterminedby thepositionsandanglesof themicro-mirrors.Theanglesof theindividual micro-mirrorsin amirror bundleneedto bepreciselycalibratedin thelaboratoryprior to installationof thebundlein a rod. Thepositionsandtheorientationsof thebundlesin therodswill bemeasuredby photogrammetricmethods.Someofthesemeasurementswill berepeatedaftertherodshave beeninstalledin theTPC.

Themeasurementof thelaserbeampositionwill bedifficult becauseof thelargebeamdiameter. Weconsiderusinglensesto focusthebeamon thesensitive areaof CCD cameras(or diodes).Thecameras(or diodes)will have to belocatedsomewhatbeforethepositionof thefocus.Alternatively onecanusethePoissonline technique[27].

The lasersandmirrorswill becontrolledvia a PC.Thediodedatawill be readoutandfed into theTPCdata-acquisitionsystem.


Figure 4.40: Overall layoutof thelasersystem.Eachhalf of theTPChasits own laser. Thelaserbeamsfollowtheperimetersof theend-plates,passingthroughpartially reflectingmirrorswhich sendpartof the light into thesupportrods.For clarity, only tracksoriginatingfrom onerodareindicated.

Figure 4.41: Distribution of thelaserbeamson theTPCend-plates.Also shown is thepatternof lasertracksatoneof thefour z-positionsat which themirror bundlesaremounted.

4.3.1.4 Production and survey

Themicro-mirrorbundleswill bebuilt in collaborationwith thegroupof A. Lebedev (STAR Collabo-ration). We have testedonesuchmirror bundle. The transversebeamprofile of thebeamreflectedoffoneof the micro-mirrors,measuredat differentdistancesfrom the mirror, is shown in Fig. 4.43. Thereflectedbeamprofile,measureddirectlyafterthemirror, dependsonthemirror shapeandquality. Aftersome60–80cm thebeambecomesnarrow andGaussian.This is thewaistof thebeam;from thereonthebeamdiameterincreases.

4.3 Calibration 85

Figure 4.42: Bundlesof micro-mirrors,mountedinsidethefield-cagesupportrods.Thesubsequentbundlesarealignedsuchthatthey do not shadow eachother.

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Figure 4.43: Evolution of the beamprofile after a reflectionoff a micro-mirror. Directly after the mirror thebeamhasa box profile, with a sizeequalto thatof themirror (1 mm). About 60–80cm later thebeamacquiresa perfectGaussianshapewith diameterd 0.6 mm. (For Gaussianbeamsdiameteris definedasfour standarddeviations,d 4σ.) Subsequently, thebeamstaysGaussianandits diameterincreasesto d = 1.6 mm at 300cm,which correspondsto a divergenceof 0.5 mr. For comparison,the bottomrow shows the profile of the incidentbeamwe used.(For thismeasurementthemicro-mirrorbundlewasremoved.)


4.3.2 Electronicscalibration

Thepurposeof theelectroniccalibrationsystemis threefold:

General operational testsof thecompletereadoutchain,i.e. preamplifier/shaper, digitizer, zerosuppression,buffering, datatransfer, etc. In particular, deador defective channelsshouldbe de-tected.

Gain equilibration of all electronic channels: The positionmeasurementin the azimuthaldi-rectionrelieson theprecisedeterminationof theclustercentroids,i.e. theweightedmeanof thecharge seenby neighbouringpads. In addition the requirementof 7% resolutionin dE/dx putsconstraintson theallowed variationof theelectronicsgain. Thegain factorsof individual chan-nelsor groupsof channelsshouldthereforenot vary by morethan2%. Owing to variationsin thechip manufacturingprocessdifferencesin gainof 10%have beenobserved [28]. A relative gaincalibrationis thereforeindispensable.In additionthepulsercanbeusedto determinethelinearityof theelectronicsby generatingasequenceof increasingamplitudes.

Equilibration of the readouttiming : Thedeterminationof thedrift coordinaterequiresarelativeprecisionof thetime measurementbetweendifferentchannelsto bewithin 7 ns(200µm). Varia-tionsin thetiming arecausedby varyingcablelengthsin thedistribution of thetriggersignalovertheTPCreadoutplaneandby variationsin theshapingtimeof thepreamplifier/shaperchips.Thelattercaneasilyreach30ns(correspondingto 850µm).

4.3.2.1 Principle of operation

For the gain calibrationthe injection of an equalamountof charge into individual readoutchannelsiscrucial. Onemethodto achieve this is thepulsingof a wire planeof a readoutchamber. Owing to themechanicalprecisionof around50µm averyhomogeneouscapacitivecouplingbetweenthevariouswireplanesandthepadplaneis ensured.Evenin thecaseof distancevariationsbetweencathodeplaneandpadplaneby morethanthespecifiedvalueover thefull width of achamber, thelocal variationsareonlysmall.Thereforeit is alwaysguaranteedthatthevariationof chargeinjectedinto adjacentpadsis in factwell within 1%.

Thecapacitive couplingbetweenthecathodewire planeandthepadsleadsto adifferentiationof thesignal.Thereforethesignalgeneratedby thecalibrationpulserhasto betheintegral of thedesiredpulseshapeat thepads.This is schematicallyshown in Fig. 4.44. Thevoltagereturnsto zeroonly after the100µs readouttime of theTPC.

100 100time (us)

curr

ent

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itude

Figure 4.44: Pulseshapegeneratedby the pulsersystemandfed into the cathodewire grid (left). This pulseinducesacurrentin thepadssimulatingthesignalfrom achargedparticle(right).

4.3 Calibration 87


Becauseof thedesignof theALICE readoutchambers(seeSection4.1.1)theinjectionof chargeis mosteasilydoneby pulsingof the cathodewire plane. Insteadof connectingit directly to grounda 50 Ωresistoris introduced,actingastheterminationresistorof thecableconnectedto thedriver stageof thecalibrationpulser.

Thepulseritself canberegardedasanarbitrarywaveformgenerator. A schematicdiagramis shownin Fig. 4.45.It is foreseento besetup in a3U Eurocrate.It consistsof thefollowing components:

An interface to theALICE detectorcontrol systemvia a field bus (CAN bus,Profibus). Duringnormaloperationthepulseris switchedon/off andtheamplitudesaresetvia this communicationlink.

A field programmablegatearray (FPGA) actsascontrollerof thesystem.Thepulseshapeofthedesiredoutputsignalis storedasasequenceof amplitudesin aSRAM.OncommandtheFPGAswitchesfrom a dormantinto a pulse-generatingstatetaking into accountthe desiredamplitudeandthestoredreferenceshape.TheFPGAalsocontrolswhich of thedriver channelsareactuallyactivated.

A fast digital-to-analog converter (DAC) with 16bit resolutionand30nssettlingtimegeneratesthepulseshapeaccordingto theamplitudesstoredin memory. This referencesignalis distributedvia acommonbusline to thecabledrivers.

The output dri vers areconnectedvia a high-impedanceinput to the DAC. Their function is todrive the50 Ω cablesconnectedto individual TPCreadoutchambers.Eachboardaccommodates

(9 boards, 4 channels each)

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Figure 4.45: Schematicdiagramof the main componentsof the calibrationpulsersystem. Onesuchunit isforeseenfor eachsideof theTPC.


fourchannels.An additionalcontrolfunctionallowstheactivation/deactivation of individualdriverchannels.This featurewill allow individual chambersor groupsof chambersto bepulsed.

Onepulsermoduleaccommodatesthe36channelsnecessaryto supplyonereadoutplaneof theTPCwith its 18 innerand18outerchambers.

Thepulseris connectedto theclock andtriggerdistribution systemof the front-endelectronics.Itis triggeredby thenormalTPC triggerexceptthat theFPGA internallydelaysthepulsegenerationbya presetnumberof clock ticks. The useof the generalTPC clock ensuresthat the calibrationpulseissynchronizedto thereadoutof theTPC.

A prototypeof the systemdescribedhasbeenbuilt in Frankfurtand is at presentundergoing per-formancetests[29]. A similar systemhasbeenbuilt for the NA49 experiment[28], [30]. The maindifferenceis the larger flexibility of thesystemunderdevelopment:In theold systemtheshapeof theoutputpulseis determinedby anetwork of threeRCcircuitsandthusthepulseshapecannotbechangedeasily. It is alsonotpossiblewith thismethodto simulateproperlythe1/t tail of thetypicalwire chamberoutputsignal.

The following performancefigureshave beenachieved in theNA49 systemandareexpectedto bereproducedwith thenew system:rangeof outputamplitudes0–4V with a nonlinearityof 1%,andastability of 0.25%.Theamplitudevariationsbetweendifferentoutputchannelsare0.04%.

It is foreseento install thetwo calibrationpulsersystemsin therackspacecloseto theL3 magnetinthenonaccessiblecavernareaUX25.

4.3.3 Krypton calibration

Variationsof thegasgaindueto mechanicalimperfectionsof thereadoutchambers(wire sag,deforma-tions)canbe investigatedby thereleaseof a fixedamountof charge into theamplificationregion. Theadditionof radioactive 83Kr to thedrift gascanserve this purpose.It hasbeenusedextensively by theNA49 experiment(seeRef. [6] andreferencestherein).

A 83Rb source(producedat theISOLDE IsotopeSeparatorat CERN)is mountedin a bypassof theTPCrecirculationsystem.It decayswith ahalf-life of 124daysinto anisomericstateof thestableisotope83Kr, which decaysinto thegroundstatewith a half-life of 1.8 h via anexcitedstateat 9.4 keV with ahalf-life of 147 ns. (A detaileddescriptionof the decaychaincanbe found in Ref. [31].) This decaychainprovidesseveralelectronenergiesspreadovera largedynamicrangewell suitedfor thecalibrationof the readoutchambers.In Fig. 4.46a Monte Carlo simulationof the resultingcharge distribution iscomparedto themeasurementin oneof theTPCsof theNA49 experiment.Theshortlifetime of 83Krallowsnormaloperationof theTPCseveralhalf-livesafterthegasflow is cutoff from thebypasshousingthe 83Rb foil. Sinceit is not possibleto derive a trigger from individual decaysit is necessaryto starttheTPCreadoutby a randomtrigger. Theconsequenceof this is thattheinformationaboutthepositionof individual decaysin thedrift directionis lost. Thechargemeasuredis thereforeaffectedalsoby theelectroncapturein thedrift gas,althoughfor reasonablysmallelectronlossesthisposesno problem.

The NA49 experienceshows that variationsof gasgain can reachup to 10% over the surfaceofindividual readoutchambers.Theapplicationof thekryptoncalibrationreducesthis to 0.5%.

A new andthusrelatively strongsourceof 180 MBq leadsto 100083Kr decaysduring readoutof50 µs in NA49. Sincethe ratecapabilityof ALICE is abouta factor100 higher thanthat of NA49 adata-takingperiodof a few hoursshouldallow theproductionof high-statisticschargespectrafor eachchannel.

It is worth mentioningthat thekryptonmethodis well suitedto studylong-termchangesof thegasamplification,in particularageingeffects. In additionit alsoallows thestudyof gasflow patterninsidetheTPCvolumewithin certainlimits. Owingto thelossof informationaboutthedrift coordinate,only aprojectionof thechargedistribution asa functionof time afterinjectionof 83Kr is possible.In Fig. 4.47anexampleis shown from theNA49 Main TPC. In thetopleft panelthegasstartsto entertheTPCfromthebottompassinga deflectorplate. Thegasdiffusesaroundtheplateandpopulatespreferentiallyone

4.3 Calibration 89

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Figure 4.47: Distribution of chargeinsidetheNA49 Main TPCat six time intervalsafter injectionof 83Kr intothegasstream.Shown is aprojectionof thedrift volumeonthereadoutplane.Thesizeof theboxesis proportionalto thenumberof clustersreconstructed.Total timecoveredis aboutnineminutes.

sideof theTPCbeforebecomingmoreuniformly distributedover thewholearea.TheTPCvolumeis18 m3 andtheflow rateof thegascorrespondsto a30%volumeexchangeperhour.

Someinformation about the distribution in the drift direction can be obtainedby monitoring thecurrentsin thereadoutchambers.Uponarrival of 83Kr in theamplificationregionsof thewire chamberstheir dark currentgoesup. This allows an estimateof the time it takesfor the gasto passthroughtheentireTPCvolume.

91

5 Front-end electronicsand readout

5.1 Front-endelectronics

5.1.1 Intr oduction and overview

This sectionreviews the requirementsfor the front-endelectronicsand discussesbriefly the generalarchitectureandthebasicbuilding blocks.

5.1.1.1 General requirements

As detailedin Sections4.1.2and4.1.4,the front-endelectronicshasto readout thechargedetectedby570000 padslocatedon the readoutchambersat the TPC end-caps.Thesechambersdeliver on theirpadsacurrentsignalwith afastrisetime(lessthan1 ns),andalongtail dueto themotionof thepositiveions.Theamplitude,which is differentfor thedifferentpadsizes,hasa typicalvalueof 7 µA. Thesignalis deliveredon thedetectorimpedancewhich, to averygoodapproximation,is apurecapacitanceof theorderof a few pF.

Themainrequirementsfor thereadoutelectronics,listedin Table5.1,arebriefly discussedbelow.

Diffusion and electronstatisticslimit the resolutionboth in the drift and azimuthaldirections.MonteCarlostudies(Section4.1.2)indicatethatto reachthedetectorresolutionlimit a signal-to-noiseratio of 30: 1 is required.

Themaximumpadandtimebin for ahit correspondstypically to achargeof about4.8fC (3 104

electrons)for a minimum ionizing particle, leadingto a maximumacceptablenoise(r.m.s.) ofabout1000electrons.

Theionizationto bemeasuredin a singlepadcanbe large. Themostprobableprotontransversemomentumis expectedto be about550 MeV/c; 280 MeV/c and340 MeV/c protons,e.g.,have

Table5.1: Front-endelectronicsrequirements.

Parameter Value

Numberof channels 570132Signal-to-noiseratio (MIP) 30: 1Dynamicrange 900: 1Noise(ENC) 1000eConversiongain 12 mV/fCCrosstalk 0 3%Shapingtime about200nsSamplingrate 5.66(–7.69)MHzTail correctionafter1 µs 0.1%Bandwidthto DAQ/Level-3 8.4GByte/sMaximumdeadtime 10%Powerconsumption 100mW/channel

92 5 Front-endelectronicsandreadout

about15and8 timestheenergy lossof aMIP, respectively. Owingto Landaufluctuations,anotherfactor2–4 in dynamicrangehasto be considered.Thus,the electronicsshouldnot saturateforsignalsup to 30 MIP (Section7.1),leadingto adesireddynamicrangeof at least10 bits.

The amplifier conversiongainhasto be suchthat the maximumoutputsignalmatchesthe inputdynamicrangeof theADC. An ADC with 2 V dynamicrange,e.g.,requiresa conversiongainofabout12 mV/fC.

In general,theshapingtimehasto beacompromisebetweentheneedfor achieving ahighsignal-to-noiseratio (bandwidthlimitation) andfor avoiding overlapof successive signals.It shouldbenotedthat,owing to longitudinaldiffusionandto thetrack inclination,thesignalspreadsin time.Therefore,assoonastheshapingtime becomessmallascomparedto thewidth of thepulse,thecurrentpulsesdueto thedifferentprimaryelectronswill bevisibleat theshaperoutput.Thesignalwidth variesfrom 124 ns (r.m.s.), for 90 -tracks,to 400 ns (full width), for 45 -tracks. It canbe shown [1] that a shapingtime of about200 ns, comparableto the signalwidth (FWHM), iscompatiblewith thelow-noiserequirement.

Theshapingtimeof about200nsmakesasamplingfrequency of 5–6MHz plausible.Wethereforedivide the total drift time of 88 µs into about500 time bins, leadingto a samplingfrequency of5.66 MHz. Eachof the 500 time bins correspondsto a drift distanceof 5 mm. Simulationsshow that with 130 ns time bins a somewhat bettermomentumresolutioncanbe achieved (seeSection4.1.2.3)albeitat theexpenseof a largerdatavolume.Wekeeptheoptionopento runwithasamplingfrequency of up to 7.69MHz, possiblyjust for thesmallerradii.

Owing to the high channeloccupancy, in order to minimize pile-up effects, a very precisetailcancellation,at thelevel of 0.1%of themaximumpulseheight,is requiredin thefront-endstage.Thiscanbedoneeitherbeforeor aftertheanalog-to-digitalconversion.

The large granularityor theTPC(3 108 pixels for 500 time bins) leadsto eventsizesof about84 MByte afterzerosuppression(132MByte for time binsof 130ns). To achieve thenecessaryratecapabilitythezerosuppressionhasto bedonein thefront-endbeforethedatais transferredtotheDAQ system.In Pb–Pbrunning,event rateswill reach104 minimumbiaseventspersecond,while in pp running the maximuminteractionrate will be of the order of 105 interactionspersecond.A few percentof theseratescorrespondto centralcollisionsthatwill betriggerselected.Zero suppressionat the front-endwill reducethe datavolumeby a factor2.5, leadingto a datathroughputof 8.4 GByte/s(13.2 for the shortertime bins) with 100 events/stransferredto theDAQ/Level-3 processing1. Thereductionfrom theLevel-1 rateof up to 200Hz to 100Hz afterLevel-2 is dueto pileup protectionagainstfurther interactionsduring theentiredrift time of theTPC.Weimaginethatthiscouldberelaxeddependingonoccupancy andexperiences(from STARandalsoour own) to a shorterprotectionperiod. Taking this into account,at least140 DigitalDataLinks (DDLs) shouldbe foreseenfor thedatatransferfrom the front-endelectronicsto theDAQ (asoutlinedin Section5.1.1.2actually180links will beused).In pp modethedetectorwillproduceadatavolumesmallerby a factor5.

A critical aspectin theTPCoperationis thetemperaturestability; to ensureaconstantdrift velocityit hasto be controlledat the level of about0 1 C over the whole volume. The large numberofchannels(570000)requiresdevelopmentof a systemwith low power consumption.Theaim is tokeepthetotal power consumptionbelow 60 kW (100mW/channel).We planto remove theheatfrom thereadoutmoduleswith awatercoolingsystem(seeSection4.1.4.2).

1Thecompressionfrom 10-bit datato nonlinear8-bit andaccordingpackagingcouldbedoneat thefront-endlevel leadingto 66 MByte perevent.For thedataformatalsoseeSection5.1.3.5.

5.1 Front-endelectronics 93

Theradiationloadon theTPCis low, with a totaldosereceivedover10 yearsof lessthan300radanda neutronflux of lessthan1011 neutrons/cm2. Thusstandardradiation-softtechnologiesaresuitablefor theimplementationof thiselectronics.Nevertheless,somespecialcareshouldbetakento protectthesystemagainstpotentialdamagecausedby SingleEventEffects(SEEs).

Theelectronicswill belocatedin anareawith limited access.High reliability is thusaconcern.

The front-endelectronicssystemhasto satisfymany otherconstraintswhile meetingthe requiredperformancespecifications.Mainly, thereadoutelectronicsneedsto fit into theoveralldetectorstructureand,in particular, into theavailablespace,which hasimportantconsequencesfor our requirementsonreliability, power, andcooling.

The following sectionsprovide moredetailson theserequirementsandshow how we plan to meetthesechallenges.

5.1.1.2 Systemoverview

The front-endelectronicsfor the ALICE TPC consistsof 570000 channels.A singlereadoutchannelis comprisedof threebasicunits(Fig. 5.1andColourFig. IX): a chargesensitive PreAmplifier/ShAper(PASA); a 10-bit 10 MHz low-power ADC; andan ASIC which containsa shorteningdigital filter forthetail cancellation,thebaselinesubtractionandzero-suppressioncircuits,andamultiple-eventbuffer.

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DDL(18 GByte/sec)

Figure5.1: Basiccomponentsof theTPCfront-endelectronics.

The imagecharge inducedon the TPC padsis amplifiedandintegratedby a low input-impedanceamplifier. It is basedonaCharge-SensitiveAmplifier (CSA)followedbyasemi-Gaussianpulseshaperofthesecondorder. Theseanalogfunctionsarerealisedby acustomintegratedcircuit, to beimplementedinCMOStechnology, whichwill contain16channelswith apowerconsumptionperchannelbelow 20mW.Thecircuit hasaconversiongainof 12mV/fC andanoutputdynamicrangeof 2 V with deviationsfromlinearity of 1%. It producesa pulsewith a rise time of 120 ns anda shapingtime (FWHM) of about200ns. Thesinglechannelhasa noisevalue(r.m.s.) below 1000e anda channel-to-channel crosstalkbelow about60dB.

ImmediatelyafterthePASA, a10-bitADC (oneperchannel)samplesthesignalatarateof 5.66MHz.ADCs of the requiredconversiontime, dynamicrange,andprecisionarecommerciallyavailable. Thestrict limitation on thepower budget(100mW/channel)calls for low-power ADCs. Single-,double-orquadruple-channel ADCs, with power consumptionbelow 50 mW, areavailable. Furthermore,mostofthemfeaturestandbypowerconsumptionbelow 10mW. This featureis very importantsincetheALICE


TPChasaduty cycleof lessthan10%.A studyof severalcommerciallyavailableADCs hasbeencom-pleted,showing thattheALICE TPCrequirements,in termsof performanceandpowerconsumption,arefulfilled in thefrequency rangeof interest.

Thedigitizedsignalis processedby a setof circuitscontainedin anASIC namedALTRO [2]. TheALTRO (ALice Tpc ReadOut)is a customCMOSchip which containsthecircuitry, for eightchannels,to performtail cancellation,pedestalsubtraction,zerosuppression,formatting,andbuffering.

Thecircuit that removesthe long tail is basedon thecascadeof two pole-zeroshorteningfilters ofthe first order. This filter, implementedby a 16-bit fixed-pointarithmetic,achievesa tail cancellationbetterthan0.1% of the maximumpulseheight. After the tail cancellation,the ALTRO performsthesubtractionof thesignalbaseline.This is doneby a circuit, basedon a look-uptable,whichcorrectsthesystematicinstability of thesignalbaseline,allowing thesubtractionof (drift) time-dependentpedestalvaluesfrom theinput datavalues.Rejectingthesamplesof valuesmallerthana constantdecisionlevel(threshold)basicallydoesthe zerosuppression.To reducethe noisesensitivity of this pulsedetectionscheme,a glitch filter checksfor a consecutive programmablenumberof samplesabove the threshold.Moreover, in orderto keepenoughinformationfor furtherfeatureextraction,a programmablesequenceof pre-samplesandpost-samplescanalsoberecorded.Thezero-suppresseddataarethenformattedinto32-bitwordsaccordingto aback-linkeddatastructure.Trigger-relateddataarestoredin amultiple-eventbuffer. Themultiple-event buffer is a RAM of 1 k words,32 bits wide, partitionedin a programmablenumberof up to eightfixed-lengthbuffers. TheALTRO chip interfacesto theexternalworld througha40-bit controlbusbasedonadatatransferprotocolthatsupportsadatarateof 160MByte/s.

Thecompletereadoutchainis containedin theFront-EndCards(FEC)pluggedinto cratesattachedto thedetectormechanicalstructureassketchedin Fig.4.29onpage69. EachFECcontains128channelsandis connectedto thereadoutpadplaneby meansof eightflexible cableswith 16signalseach.32FECsarecontrolledby aReadoutControlUnit (RCU),which interfacestheFECsto theDAQ, thetrigger, andtheDetectorControlSystem(DCS).TheRCU broadcaststhetrigger informationto theindividual FECmodulesandcontrolsthereadoutprocedure.Both functionsareimplementedvia acustombus,basedonlow-voltagesignallingtechnologywhich providesa databandwidthof 160MByte/s. TheinterfacingoftheRCU modulesto thetriggerandto theDAQ follows thestandarddata-acquisitionarchitectureof theexperiment[3].

In summary, for eachof the36TPCsectors,thefront-endelectronicsconsistsof 125FECs,5 RCUs,and5 DDLs.

5.1.2 Inter connectionto the readoutpad plane

5.1.2.1 Electronicsmapping

Severalconstraintsdefinethemappingof thedetectorinto front-endboards.

Thetotal lengthof theconnectingwireshasto beminimized.

Thecablesusedfor thisconnectionshouldbeall of thesamelength.Thisshouldavoid theproduc-tion of several typesof cablesand,especially, shouldguaranteethehomogeneityof performanceover thedifferentregionsof thedetector.

Cablecrossingshouldbeavoidedtominimizetherisk of errorandasolutionasuniformaspossibleis desirable.

Theseconsiderationsleadto theinterconnectionschemeshown in Fig. 4.29onpage69. Thesignalscomingfrom 16 padsaregroupedtogetherandtransportedto the front-endcardsvia a cablesoldereddirectly ontothepadplane.Thehighgranularityin theconnectivity allows to keepsufficiently shortthetracesby which thesignalsarecollectedfrom thepadsandconcentratedin theareawherethecablesaresoldered(transportpoints). Only thepadslocatedat thetop andbottomedgesof thereadoutchambers


which,dueto themechanicalframe,arenotdirectlyaccessiblerepresentanexceptionandrequirelongertraces.

As eachsectoris segmentedinto threeregionswith threedifferentpadsizes,the first two require-mentscall for atleastthreedifferenttypesof boards.Toavoid afurtherincreasein thenumberof differenttypesof boards,we optedfor a radialorientationof theboardsasshown in Fig. 4.29on page69. Theboardsaregroupedin fivecratesof (from theinsideout)20(C1),25(C2),22(C3),26(C4),and32(C5)boards.All FECshave thesamefunctionalityandthesamenumberof channels(128)but slightly differ-entgeometry. TheFECsof thefirst two cratesareconnectedto thepadsof theinnerreadoutchamberandarethesmallestones.Theboardsof thenext two cratesareconnectedto thecentral-sectorpadsandthelastcrateis connectedto theoutersector. Thenumberof differenttypesof boardsandtheir dimensionsaresummarizedin Table5.2.

Table5.2: Classificationof thefront-endelectronicscards.

Group No. of boards Dimensions[cm2]

C1 20 19 14C2 25 19 14C3 22 29 14C4 26 29 14C5 32 40 14

5.1.2.2 Cablesand connectors

To keepthe crosstalkwithin the specifiedlimit all the connectionsinsidethe connectorsmustbe lowinductance.The crosstalkof the whole chain is currentlyundersimulation,but a finite-element3-Dmodelof thecableandconnectorhasalreadybeencreatedandsimulated.

The cablesusedto transferthe signalsfrom the padsto the input of the amplifier arepolyimidestriplines,especiallydesignedto minimisecrosstalk,pick-up noise,andheattransferto thepadplane.Eachcablecontains16 channelsandhasa lengthof about7 cm. As is shown in Fig. 5.2, the signal

epoxy glue (35µµm)

cover lay (60 µµm)

cover lay (60 µµm)



polyimide (200µµm)

polyimide (170µµm)

copper gr id (18µµm)

copper gr id (18µµm)

copper (18µµm)

SIGNAL GROUND GROUND

GROUND

GROUND

150 µµm 265 µµm 120 µµm

Figure5.2: Cross-sectionof thefront-endcable.


tracesare120µm wide with a pitch of 0.8 mm. Thegroundreturns,150µm wide, arealternatedwiththesignalstripsfrom which they areatadistanceof 265µm. Two solidgroundplanesshieldcompletelythestriplines.Thecrosstalkhasbeensimulatedto belessthan0.1%.Theresultsof thesesimulationsareshown in Table5.3.

Theproposedconnectoris amatchedimpedanceconnector(QSE-SAMTEC)[4] developedfor high-bandwidthapplications(higherthan1 GHz) andlow crosstalk.Theconnectorconsistsof a socket andterminalsetwith an integral groundplanerunningbetweentwo rows of pins. The signalpins have apitch of 0.8mm. Themaincharacteristicsarelistedin Table5.4.

Table 5.3: Electricalparametersof thefront-endcable:impedance(Z0), propagationdelay(td), capacitance(C),mutualcapacitance(CM), inductance(L), mutualinductance(LM), andcrosstalk.

Z0 td C CM L LM crosstalk[Ω] [ns/m] [pF/m] [pF/m] [nH/m] [nH/m] [%]

64 6.4 99.70 0.06 410 0.03 0.05

Table5.4: 50 Ω connector, S-G-Sconfigurationwith centerplanegrounded.

0.8mmpitch (QTE) 100MHz 500MHz 1 GHz

Impedance[Ω] 50.7 55.0 60Attenuation[dB] -0.0305 -0.2429 -0.4607Crosstalk[dB] -60.6 -49.0 -39.5Propagationdelay[ps] 58.4 58.4 58.4

5.1.3 Front-end basiccomponents

5.1.3.1 Preamplifier/shaper

The basicrequirementsfor the preamplifier/shaperchip have beenoutlined in Section5.1.1.1and inTable5.1on page91. Thetypical numberof electrons(beforeamplification,etc.) generatingthesignalonthemaximumpadandtimebin of aclusteris shown in Fig.5.3for thethreepadsizesandthreerangesin polarangles.As canbeseenit variessystematicallywith padareaandfor thelargestpadsthesignalsareabouta factor2 biggerthanfor the smallestinsidepads. In comparison,the dependenceon polarangleis small. In orderto achieve a signal-to-noiseratio of 30: 1 for all padsizesa factor2 variationingasgainis required.However, workingwith thesamegasgainentailsonly amoderatelossin resolutionasdetailedin Section4.1.2.As far asthepreamplifier/shaperis concernedwe envision onegainsettingthatputsthenoiseequivalentof 1000electronsin channel1 of anADC with 2 V full scale.Thisdefinestherequiredgainas12 mV/fC.

To achieve anoverall systemnoiseof 1000electronsperchannelfor a typical input capacitanceofabout4 pF for the padsplus about10 pF for the on-boardtracesplus the polyimide cablesrequiresalow-noisedevice.

We intendto take the designof the NA45 CERESTPC preamplifier/shaper[5] asa startingpointand to optimize it for the ALICE needs,migrating from 0.8 µm CMOS technologyto 0.35 µm andincorporatingagain16 channelson onechip. Theschematicsof this circuit areshown in Fig. 5.4. FortheALICE applicationthe tail-cancellationfunctionalitywill be removed from thepreamplifier/shaperchip andbeimplementedin thedigital chip (seebelow).


0

200400

0

20 40 60

Mean 8.221

coun

ts

0

100

200

300

0

20 40 60

Mean 12.42

75o<θ<90o 75o<θ!<90o 75o<θ

!<90o

4x7.5mm2 6x10mm2 6x15mm2

0

100

200

300

0

20 40 60

Mean 17.68

0

200

400

600

0

20

40"

60

Mean 8.659

0

100

200300 400"

0

20

40"

60

Mean 13.94

60o<# θ<75o 60o<θ!<# 75o 60o<# θ! <# 75o

0

100

200

0

20

40"

60

Mean 20.58

0

200400"

0

20 40 60

Mean 10.05

0

100

200300

0

20 40 60

Mean 15.80

45o<# θ<60o 45o<θ!<# 60o 45o<# θ! <# 60o

0

50$

100

150

200

0

20 40 60

Mean 22.80

nelectrons

Figure 5.3: Numberof electronsgeneratingthe signal in the maximumpadand time bin of a clusterfor thedifferentpadsizesandaveragedoverdifferentregionsin polarangle.

Figure 5.4: Schematicsof theNA45 CERESTPCpreamplifier/shapercircuit.

In contrastto e.g.thepreamplifier/shaperdevelopedfor theSTAR TPCwhichhasapulsedreset[6],the CERESdesignuseda MOS transistor(MF) with a feedbackcapacitanceCF that is continuouslydischarged with a decaytime τd % CF & Rds(MF) (seeFig. 5.4). This continuouslysensitive designisparticularlysuitablefor a detectorwith high occupancy. Noiseconsiderationsdictatea large feedback


resistanceRF (for e.g.apeakingtimeof 400nsavaluelargerthan4 MΩ) andtheassociateddrain-sourceresistanceRds of aMOSFETtransistor. ThishastheadvantagethatRds dependson thebiasingcondition(the self-adaptive biastechniqueis used[7]) andthat larger signalsaredischargedwith a fasterdecaytime (seeFig. 5.5). This is particularlyrelevant in anenvironmentwheresignalsof minimum-ionizingparticleshave to bemeasuredin thepresenceof heavily ionizinglow-energy particles.Nearmid-rapidityit is expectedthattheabundanceof protonsandantiprotonsis about8% of pionsandkaonswith a mostprobablymomentumof 0.56GeV/c, implying that15%of thesehave a meanenergy lossof morethan8 MIP andLandaufluctuationswill leadto even muchlarger signals. Therefore,beyond the questionof dynamicrange,in the designof the chip attentionhasto be payedto the responseof the circuit tosaturatingsignals.

Figure 5.5: Responseof theCEREScharge-sensitiveamplifierto input signalsof increasingmagnitude.

Particularattentionin thedesignhasto begivento thegain linearity in thepresenceof a nonlinearresistancein the feedbackloop. In theCERESdesignthis hasbeenachieved by theself-adaptive biastechnique[7] leadingto anonlinearityof thepreamplifierof lessthan2%for up to 350fC input charge.For theALICE applicationthemaximumsignalcorrespondsto 166fC. In termsof noiserequirements,theCERESpreamplifier/shaperfulfills therequirementwith anENC of lessthan600electronsfor inputcapacitancesbetween15 pF and19pF.

In termsof powerconsumption,theCERESchiprequiresabout60mWperchannel.For timereasonsandbecausethis wasnot a critical issueit hasnot beenoptimizedfor low power consumption.Alreadyremoving the tail cancellationandgoing to 0.35µm technologyshouldreducethepower consumptionto about1/3 andwe areconfidentthat thegoalof lessthan20 mW perchannelcanbeachieved for anoptimizeddesign.

5.1.3.2 Analog-to-digital conversion

The analogsignaloutput of the preamplifier/shapercircuit is sampled,at a rateof 5.66 MHz, by ananalog-to-digitalconverter (ADC) with 10-bit dynamicrange. Fast-conversionADCs of the requireddynamicrangeandprecisionarecommerciallyavailable. Conversiontimesof theorderof 100ns canbe achieved with flash-ADCsor with successive approximationpipelinedADCs. The requirementonthepower consumption(below 100mW perfront-endchannel)callsfor low-power ADCs, limiting thechoiceto pipelinedADCs. Fortunately, dueto thecontinuousgrowth in wirelesscommunicationsystemsandportabledeviceswherepowerconsumptionis amajorconcern,10-bit20MHz ADCswith low power


0

100

200

300

400

500

600

700

800

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

Year

Po

wer

(m

W)

Harris

Philips

Thomson

SPT

An. Dev.

Burr Br.

AKM

Fujiysu

Sony

Figure 5.6: Powerconsumptionof 10-bit 10 MHz commercialADCs.

consumptionarenow widely available. Fig. 5.6 shows thereductionof power consumption,for 10-bitADCs,over thelast10years.

We notice that, since1997, several commercialADCs with power consumptionbelow 100 mWhave beenavailable, and that in the last year a few devices with power consumptionbelow 50 mWhave appearedon themarket. Moreover, the trendshown by thecurve in Fig. 5.6 indicatesthatwe canreasonablyexpectdeviceswith a power consumptionof 20 mW by the year2002. In addition,mostof themfeaturestandbypower consumptionbelow 10 mW. In this respectit shouldbe notedthat theALICE TPChasadutycycleof about2%,asthedigitizationoccursfor 88µs (TPCdrift time), triggeredby theLevel-1decision(200Hz). In ‘low-powermode’theclocksignalis disabled,causingadropin thedevice’s staticpower. Fromthe ‘low-power mode’ thedevice canberecoveredalmostinstantaneously.Wewill comebackto thispoint laterin thissection.

Table5.5lists themostrecentdevicessuitablefor theTPCapplication.Concerningtheperformanceof thesedevices,thedocumentationprovidedby themanufacturersis notalwaysexhaustive. As amatterof fact,while thed.c. accuracy of thedevicesis normallywell documented,often thedynamicperfor-

Table5.5: CommercialADCssuitablefor theALICE TPCreadout.

ADC Manufacturer Channels Max. rate Powerat max.rateperchip [MHz] [mW/channel]

HI5710 HARRIS 1 20 140AD9200 AnalogDevices 1 20 80TDA8766 Philips 1 20 53ADS901 Burr-Brown 1 20 48MByte40C360 Fujitsu 1 25 40CXD3300R Sony 1 20 40AD9201 AnalogDevices 2 20 100SPT7852 SPT 2 20 80SPT7852 SPT 3 20 130XRD6414 EXAR 4 20 30


manceof thedeviceis reportedonly for afew valuesof theinputsignalfrequency andadefinedsamplingfrequency. Besides,thecharacterizationof thedevice by themanufactureris oftendonein ‘ideal’ con-ditions, far from the userapplication. In the front-endelectronicsfor the TPC, the ADC is closetotheanalogcircuit PASA which providesthesignalto besampled,andto thefastdigital circuit ALTROwhichprocessesthesignal.Therefore,it is importantto studytheeffectof theADC ontheanalogcircuitperformance,andits susceptibilityto thefastdigital circuits.For thispurpose,anADc EvaluationBoard(ADEB) [8], containingthe completereadoutchain,hasbeendeveloped. The ADCs aremountedonsmallprinted-circuitboardspluggedinto themotherboardasdaughtercards.Up to four (different)ADCchipswith differentanaloginputscanbetestedat thesametime in anautomatedtest.Thestudyof fiveoff-the-shelfADCs (in bold in Table5.5) from differentmanufacturershasbeenaccomplished.

Therearemany reasonswhy an ADC with N bits doesnot perform like an ideal N-bit converterunderdynamicoperatingconditions. Several parametersareuniversallyusedto definethesedifferentsourcesof errors:DifferentialNonLinearity(DNL), IntegralNonLinearity(INL), offseterror, gainerror,Total HarmonicDistortion (THD), aperturejitters, aperturedelay, etc. The valuesof theseparameterscharacterizeanADC andarereportedonthedevice’s technicaldatasheet.To getin-depthknowledgeofthesedevicesandanindependentverificationof theinformationprovidedby themanufacturers,all theseparametershave beenmeasuredfor thefivecommercialADCs underevaluation.For a detaileddescrip-tion of themeasurementsandtheresultswereferto [9,10]. Herewefocusonly onthemeasurementof aparametercalled‘Effective NumberOf Bits’ (ENOB),which in a way includesall theeffectsdescribedseparatelyby theparametersmentionedabove. Thisnumbermaybeconsideredasthenumberof bits inaperfectADC whoser.m.s.‘quantization’errorwouldbeequalto thetotalr.m.s.errorfrom all sourcesintheADC undertest.Figure5.7shows theENOB versusthefrequency of theinput signal.We concludethatall thedevicesundertestwould fulfil theALICE TPCrequirementsin termsof performancein thefrequency rangeof interest.

0

1

2

3

4

5

6

7

8

9

10

0 2 4 6 8 10 12

Input Signal Frequency (MHz)

EN

OB

HI5710TDA8766 CRIAD1 (0-512 mV)AD9200ADS-901

Figure 5.7: Input signalfrequency versustheeffectivenumberof bits (ENOB) for fivedifferentADCs.

Oneof the five ADCs (AD9200 by Analog Devices)testedhasbeenusedin the readoutelectron-ics [11] for theRCCprototype(about1400channels)discussedin Section8.1. This readoutelectronicsis basedon a VME 48-channelFront-EndDigitizer Card(FEDC)shown in ColourFig. X andXI. Theresultsachievedwith this readoutsystem,in termsof digitizationresolution,areexcellent. TheSignal-to-NoiseRatio(SNR)measuredis below 0.4ADC countsfor all channels.

In theFEDCsystemit ispossibletoputall ADCssimultaneouslyin ‘low-powermode’tobeactivatedonly for 88 µs after the trigger signal. What mattersfor the recovery time is how long the device has


Figure5.8: Time evolutionof thesignalpedestalafterpower-on for theADC AD9200.Time binsare100ns.

beenin theoff-state.Whenthedeviceis in ‘low-powermode’thecapacitors,whichprovidethereferencevoltages,startto discharge. Whenthe device is switchedbackto runningmode,the nominalvalueofthereferencevoltageis recoveredin a time thatdependsonhow long thedevice waskeptin ‘low-powermode’. Figure5.8shows thetime evolution of thesignalpedestalwhentheADC is poweredon aftera‘sleepingtime’ of 10ms. It canbeseenthatthediscrepancy is nothigherthan1.4mV evenfor ‘sleepingtimes’ of the orderof 10 ms, correspondingto the typical distancebetweentwo consecutive Level-1triggersactivating the TPC readout.It shouldbe notedthat, sincethis error dependsonly on the timefrom thelasttrigger, thedigital processorcircuit couldapplya correctionbeforethezerosuppressionisperformed.

Anotherpotentiallyinterestingoption is thedevelopmentof a customADC. A customdesignpro-videsthe possibility of integratingthe ADC with the restof the readoutchain,e.g. the ALTRO chip,if the technologyfor thetwo circuits is thesame.Anotheradvantageofferedby a customdesignis thepossibility to adaptthe ADC transferfunction to the accuracy requiredin the different regionsof thedynamicrange. For this purposea specialversionof the CRIAD [12], a low-power ADC designedatCERNfor otherapplications,wasdevelopedfor theALICE TPC.TheCRIAD is amultiple-rangelinearADC with four ranges.In eachrangetheresolutionis definedby an8-bit linearconversionperformedbetweentwo references,the lower onebeingground(0 V). The rangeselectionis automaticwith thesignalamplitude.For comparisonwith thecommercialADCs, Fig. 5.7 shows alsotheperformanceofthe CRIAD in the first sub-range.Despitethe very promisingresults,this experiencehasshown thatit is very difficult to develop, in house,a device which is competitive, in termsof performance,powerconsumptionandprice, with the commercialdeviceswhich, dueto the wide market, aresignificantlyimprovedyearafteryear.

Whatmaybemoreattractive hereis thepossibilityof usinga semi-customADC. More andmore,CMOS low-power ADCs areincludedin the standardlibrariesprovided by the silicon manufacturers.As anexample,wementionherethecaseof AustriaMikro Systeme(AMS), aEuropeansiliconfoundry,whichincludesin its 0.35µm CMOSlibrary a10-bit20MHz ADC that,at5 MHz, hasapowerconsump-tion of 100mW. Thisdevicefeaturesapowerconsumptiononly slightly higherthanthebestoff-the-shelfdevices.

In conclusion,at presentthebaselinesolutionis theuseof a commercialADC, however, thesystemdesignis opento thepossibility of including theADC, boughtasa cell, in theASIC (ALTRO) whichcontainstherestof thereadoutchain.


5.1.3.3 Digital readout

After theanalog-to-digitalconversiontheTPCsignalis processedby a setof digital circuits integratedin achipnamedALTRO (ALice TpcReadOut)[2]. TheALTRO chip is acustomintegratedcircuit ded-icatedto theprocessingof a 10-bit digital signalfor the readoutof the trigger-relateddata. It containseightprocessingchannelsoperatingconcurrently, at theADC samplingrate(upto 20MHz), on thedigi-tizedsignalcomingfrom eightindependentinputs.Startingfrom atriggersignaleachchannelperforms,on a streamof up to 1000samples,tail cancellation,pedestalsubtraction,zerosuppression,formatting,anddatastoragein a multiple-event memory. As sketchedin Fig. 5.9, the ALTRO’s processingchainis basedon five mainunitsdescribedin somedetail in thefollowing sections.First the tail cancellationoccurs,followed by baselinesubtractionandzerosuppression.Then the dataareput into a compactformatin theDataFormattingUnit (DFU) andfinally storedin a multiple-eventbuffer.

Ped.Subtr

Tail Canc.

ZeroSuppr.

DataFormatt.

Multi -Ev.Buffer

10 10 10 32

Ped.Subtr

Tail Canc.

ZeroSuppr.

DataFormatt.

Multi -Ev.Buffer

10 10 10 32

Ped.Subtr

Tail Canc.

ZeroSuppr.

DataFormatt.

Multi -Ev.Buffer

10 10 10 32

32

32

32

Data / Addr (32)

CTRLS & INTERFACE

Chip Ctrls (13)

ADC Ctrls (3)

MCLK40 MHz

32

10 MHz 40 MHz

AD

C IN

TE

RFA

CE

G/H

A/B

C/D

#0

#1

#7

Figure5.9: ALTRO blockdiagram.

5.1.3.4 Tail cancellation

As mentionedabove, thesignalcomingfrom theALICE TPCpresentsa long tail. A techniqueto cancelthe tail of the signalarising from the amplificationregion wasdevelopedfor a proportionalchamberand implementedby a circuit basedon discretecomponents[13]. The sametechniqueunderliesthetail-cancellationschemeusedin thepreamplifier/shapercircuit developedfor thereadoutof theCERESTPC [5]. In this method,the measuredTPC signal is approximatedby the sumof threeexponentialterms:

i ' t (*) e' t (,+ I0 - A - expt

α - τ1

.B - exp

tβ - τ2

.C - exp

tγ - τ3 / (5.1)

Hence,thecancellationof thesignaltail canberealizedby adeconvolution filtering technique[13].Thesignalispassedthroughalinearnetwork with aresponsethatcancelsperfectlyoneof theexponential


R2

R1

C1

Vout(t)V in(t)

τ = R1 · C1

k = (R1 + R2) / R2

Figure 5.10: Analogpole-zeronetwork.

terms.Let usconsiderapole/zeronetwork with transferfunction

F1 ' s(*+ 1.

s - τk.

s - τ / (5.2)

It canbeshown [13] that if the time constantτ is adjustedto be τ + τ3, k canbeadjustedin a waythat all the termscontainingτ/k add to zero. In this way the pole τ3, correspondingto the long tail' τ3 0 τ2 0 τ1 ( , is perfectlycancelled.Similarly, asecondpole/zeronetwork with transferfunctionF2 ' s(canbeaddedwith constantsadjustedsothattheremainingtermsonly containthetimeconstantτ1. As aresult,theconvolution of thetwo filters with thechambersignalis

i0 ' t (,+ i ' t (21 h1 ' t (21 h2 ' t (*3+ I0 - expt

α - τ1 / (5.3)

Thenetwork with transferfunctionEq. (5.2) is shown in Fig. 5.10,whereτ + R1 - C1 andk +4' R1.

R2 ( /R2. A circuit diagramof thecompletetail-cancellationfilter is shown in Fig.5.11.Figure5.12detailsthebehaviour of thecircuit assimulatedby usingSPICE.Theaccuracy of ashorteningfilter realizedbyananalognetwork is limited by thetolerancesof thenetwork components.Owing to thepoorprecisionin the matchingof the passive components,provided by the actualintegratedcircuit technologies,theshorteningfilter describedabove cannotreacha high accuracy ' 3 105 3 ( if it hasto be includedin anintegratedcircuit with noexternaltunablecomponents.

On the otherhand,a digital systemprovidesmuchbettercontrol of accuracy requirements.Thisresultsin specifying the accuracy requirementsin termsof word length, floating-pointversusfixed-pointarithmetic,andsimilar factors.Furthermore,adigital systemallowsflexibility in reconfiguringthedigital signal-processingoperationssimplyby changinganumberof programmablecoefficients.This isindeedextremelyimportant,consideringthat theexactshapeof thesignalwhich dependson thedetailsof thechamberandpadgeometryis difficult to predictandis known with high precisiononly whenthedetectoris operated.Also, thisallowssomeflexibility in thechoiceof thegascompositionanddrift fielddependingonfirst runningexperiences.Thefirst-orderfilter expressedby theEq.(5.2)canbeexpressedin thediscretetime domainusingthez transform.Weobtainthefollowing z transformfunction:

F ' z(,+ L0.

L1 - z5 1

1.

K1 - z5 1 6 (5.4)

whereL0 6 L1 andK1 areconstantsthat arefunctionsof A6 B6 C 6 α 6 β 6 γ. This circuit is representedin aschematicway in Fig 5.13.


R15

500k

2k

R2

2kR3

1G

R5

2k

R7

2.2kR13

25pC7

1G

R11

1G

R20

10uC4

+

-

+5V2

+

-

-5V1

C210u

2kR4

25pC6

C1010u 1G

R19

2.2kR14+

-V3-12

PARAMETERS:A 290

CdCAd

100p800p

Rg

1G

R24

Rd1

10uC11

R260.01

R2120k

1kR16

R9 2.3k

R18 2.83E-6/CAd

CAdC5

Rd2R12

PARAMETERS:Rd2 6k

6kRd1

-2

+3

6V+

7

V-

4OPA651/BB

U12

-2

+3

6V+

7

V-

4OPA651/BB

U11

+

-V4

+12

+3

-2

V+

7

V-

4

6B2

5

B1

1

LF356/NS

S+

-

I3

STIMULUS=signal

-2

+3

6V+

7

V-

4OPA651/BB

U2

C9 Cd

R23 A*1E-9/Cd

R8 5k

R171k200

R22

C12 50p

R6

500k

2pC1

-2

+3

6V+

7

V-

4OPA651/BB

U3

0

0

0

+5 -5

0 0

0

00

+12 -12

0

+5

-5

+5

-5

-5

+5

-5

+5

-12

+12

V

Vt1 Vt2

Vrc2Vrc1

Vdiff

Vintgr

File name: Filter3Exp

2 poles of integration7

1/t tail cancellationIntegration + differentiation+ pole zero cancellation8

List of stimulus:

fastsig9fastersig9tail:troncTail:expon;

signal<

Figure5.11: Block diagramof theanalogtail-cancellationfilter.

= > ? @ A B C D E F G H I

J K J L M N K O L J N K O L M N K P L J N KQ A R DS = S > E T P ?

U J J R S

P J J R S

J R SV M J R S

= P L J J J J N W X F Y L M F Z N ?= O L J J O M N W P L X Y M Y R ?

= P U Z L M O M [ W F J O L P P U R ?

Figure5.12: SPICEsimulationof theanalogtail-cancellationfilter.


Z-1+

L0

L1

K1

+

IIR filter

• IIR filter

– 4 N-bit registers

– 2 N-bit adders

– 3 N-bit multipliers

• For N = 16 (0.25 µm):

– prop. delay = 25 ns

– area = 1 mm2

1

1

11

10)( \

\]^ ]^_zK

zLLzF

Figure 5.13: Digital pole-zeronetwork.

It consistsof threemultipliers,two adders,anda register. Eventhoughthereareno negative valuesin theinput, their useis neededfor theinternalcalculationsof thefilter, thereforethetwo’s-complementarithmeticis used.TheVerilog implementationof this circuit wasperformedandtheresultsareshownin thissection.In orderto work in thedigital domain,theanaloginputsignalaccordingto Eq. (5.1)wassampledat 10MHz, and16-or 32-bit ‘quantizations’wereperformed.

Figure5.14is anoverlayof threesignals:input, responseafterthefirst filter, andfinal output.Theseplots show that we canachieve a tail suppressionwith the three-exponentialapproximation.The finalresultwasnormalizedso that themaximumvalueis equalto one. Theerrordefinitionwasto considertheratio betweena givensampleandthemaximumvalue. Thefilter coefficientswereslightly adjustedto have a smallundershootproviding a fastdeclinetowardsthebaseline.Betweenthesamples,thereisasimplelinearinterpolationin orderto improve readabilityof theseplots.Only the16-bit caseis shown(seealsoTable5.6).

To estimatethe size of the digital filter circuit, the Verilog descriptionhasbeensynthesizedandtargetedto a0.25µm CMOStechnology. Theresultsareshown in Table5.7.

INPUT

OUTPUT1

OUTPUT2BASELINE

Time (ns)

Nor

mal

ized

am

plit

ude

Figure 5.14: Digital filter: inputsignal,responseafterthefirst filter, andfinal output.


Table5.6: Shorteningfilter performance.

Input Output1 Output2

After 1 µs 0.036 0.011 ` 1/ 83 a 105 4

After 2 µs 0.0153 0.0088 ` 9/ 17 a 105 4

Area[0-1 ms]AU 1.0773 0.8565 0.8270

Table5.7: 16-bit fixed-pointfirst-orderdigital filter.

Slow Fast

Propagationdelay 25ns 10nsArea 0.5mm2 1 mm2

5.1.3.5 Baselinesubtraction, zero suppression,data formatting and buffering

The baselinesubtractionis performedby the circuit schematicallydepictedin Fig. 5.15. It hasthreemodesof operation:

b Subtraction mode. The circuit performsthe subtractionof pedestalvaluesfrom the input-signalvalues. The subtractedvaluecanbe eitherfixed or time-bin dependent.In the former case—fixedsubtraction mode— the value to be subtractedfrom the input signal is a constantstoredin a register (DIN in Fig. 5.15). In the latter case— variable subtraction mode— the time-bin-dependentpedestalvaluesto be subtractedarestoredin the pedestalmemorywhich, in thisconfiguration,is addressedby a time counterstartedby the trigger signal. Clearly the variablesubtractionmodeis usefulwhenever thesignalis affectedby somesystematicbaselinevariationcorrelatedwith thereadoutcycle.

TIMECOUNTER

DIN

If(B≤≤ A) Y = A - Belse Y = 0

PED.

SIGNAL

READ/WRITE REGISTERS

FROMADC

DO

ADD

PED. MEM.

DI DO

AD

A

B

Y

SETPOL.

Figure 5.15: Baselinesubtractionblock diagram.


THRESHOLD

FLAG BIT

Basic detection scheme

THRESHOLD

FLAG BIT

Glitch filter

MINSEQ = 2

THRESHOLD

FLAG BIT

Feature extraction

PRES = 2POSTS = 3

THRD

FLAG BIT

Merging of close clusters

PRES = 0POSTS = 2

Figure5.16: Zero-suppressionscheme.

b Conversionmode. Thecircuit canperforma memoryless(static)conversionof theinput signalofthetypeyn + F ' xn ( . At any instantn, theoutputyn dependsat moston theinput samplexn at thesametime, but not on pastor futuresamplesof the input. Theoutputvaluesyn arestoredin thepedestalmemoryaddressedin this caseby the input xn. Concurrentlythe subtractionof a fixedvaluefrom theconvertedsignalcanbeperformed.

b In thethird modeof operation— testmode— thepedestalsubtractionunit canbeusedto generatea patternto be injectedinto the processingchainfor testpurposes.This is an importantfeaturethatallows acompletetestof theoverall digital readoutchain.

Oneobviousway to compressthedatastreamis to discard‘zero’ data,i.e., samplessocloseto thereferencelevel (pedestal)that they areconsideredto containno usefulinformationbut ratherto bedueto noise.Thebasicpulse-detectionschemeis basedon therejectionof sampleswith valuesmallerthana constantdecisionlevel (threshold).Whena sampleis found above the threshold,it is consideredtobethestartof a pulse(Fig. 5.16).To reducethenoisesensitivity, a glitch filter checksfor a consecutivenumberof samplesabove threshold,confirmingtheexistenceof a realpulse. The minimum sequenceof samplesabove thethresholdthatdefinesa pulsecanvary from oneto three.In orderto keepenoughinformation for further featureextraction, the completepulseshapemust be recorded. Therefore,asequenceof samples(pre-samples)beforethesignalovercomesthethresholdandasequenceof samples(post-samples)after the signal returnsbelow the thresholdcanbe recorded. The numberof pre- andpost-samplescanvary independentlyin therangebetweenzeroandfour.

Thedataformatis asillustratedin Fig. 5.17anddescribedin thefollowing. Thepulsethusidentifiedand isolatedmustbe taggedwith a time stamp,in order to be synchronizedwith the trigger decisionfor validation. Otherwisethe timing informationwould be lost by the removal of a variablenumberofsamplesbetweenacceptedpulses.This requiresthe additionof a time word to the setof sampledata.Since1000is themaximumlengthof thedatastreamthatcanbeprocessedby theALTRO chip,thetimeinformationcanbeencodedin a 10-bitword. Theprincipleis to labeleachpulsewith a time stampthatdefinesthetimedistancefrom thetriggersignal(e.g.‘T06’ in Fig.5.17).Sothesamplesof theprocesseddatastreamarenumberedstartingfrom 0 to 1000.

SincetheALTRO dataformatdoesnot make useof extra flag bits to distinguishthesamples’datafrom the time data,we introducea further word for eachacceptedclusterto representthe numberofwordsin thecluster(e.g.‘07’ in Fig. 5.17). Sincea clustercanbe aslong asthecompletedatastream


T00 T01 T02 T03 T04 T05 T06 T07 T08 T09 T10 T11 T12 T13 T14

S00 S01 S02 S03 S04 S05 S06 S07 S08 S09 S10 S11 S12 S13 S14

THRD

ADC SAMPLES

TIME

SAMPLES

MEMORYWRITE EN

S02 S03 S04 S05 S06 T06 07 S10 S11 S12 T12 05

S02, S03, S04 S05, S06, T06 07, S10, S11 S12, T12, 05

FORMAT DATA

S93

S02S03S04

S05S06T06

007S10S11

S12T12005

S88S89S90

S91S92

T93008XXX

Total nr. of 10 bits words

Hardware AddSoftware Add

BACK LINKEDDATA STRUCTURE

Figure 5.17: TheALTRO dataformat.

thenew word is alsoa10-bitword.

The DFU formatsthe streamof zero-suppresseddataby adding,to eachsetof samples,two extra32-bit words,andencodingthe10-bitwordsinto 30-bit words.Thetwo 32-bitwordsform thetrailer ofeachdatapacket. The first word is fundamentalfor the decodingof the datapacket andexpressesthetotalnumberof 10-bitwordsin thepacket. Indeedthiswordprovidesthepositionof thelast10-bitwordin thedatapacket. Startingform the last10-bit word theclustersareback-linked, aseachsetcontainsthepointerto thefirst word of thepreviouscluster(Fig. 5.17). Thesecondtrailer word containsin the16 LSB asoftwareidentifier, which is readfrom oneof theconfiguration/statusregisters,while thebyteafter thatcontainsthehardwareaddress.This addressrepresentsa geographicaladdressandis usedinthedatapacket to identify unambiguouslyto which channelthedatapacket is associated.Thesoftwareidentifier is a programmablenumberthat canbe usedto specify to which datasourcethe processingchannelis connected.

Finally the dataarestoredin a multiple-event buffer, a 512 a 32 RAM that can hold up to eightevents. TheALTRO chip interfacesto theexternalworld througha 40-bit controlbus basedon a datatransferprotocolthatsupportsadatarateof 160MByte/s.

5.1.3.6 ALTRO prototype

In its final versiontheALTROwill beimplementedin adeepsub-micronCMOSprocess:at themoment,0.25µm seemstheprocessfeaturesizesuitablefor thisapplication.At thepresentphaseof theprojectthechip functionsarenot frozenyet. Thusin orderto have anestimateof thesizeandpower consumption,thiscircuit hasbeenstudiedusingthestandardcell library for a0.25µm CMOSprocess,leadingto adiesizeof 8 mm2/channelandapowerconsumptionof 8 mW/channel.With theexceptionof theshorteningfilter, all theALTRO’s functionshave beenimplementedin aFPGA,asusedin theFEDCcards[11] forthereadoutof theRCC(Section8.1). Finally, for thepurposeof studyingthesystemaspectsof theFEC,a four-channelprototypehasbeendevelopedasa0.6µm doublemetalCMOSprocessfeaturedby AMS(AustriaMikro Systeme).It hasasilicondieof 40mm2 andis packagedinto aplasticcasewith 100pins(PQFP100).

The layout of the chip is shown in Fig. 5.18 whereone can recognizethe four channelsand thecommoncontrollogic. Thesupplyvoltagesaredistributedto theinternalcellsstartingfrom two external80 µm wide ringsthatsurroundthecircuit bodycompletely.


Figure 5.18: Layoutof theALTRO chip prototype.


5.1.4 Front-End Card

The Front-EndCard (FEC) containsthe completereadoutchain for amplifying, shaping,digitizing,processing,andbuffering the TPC signals. The FEC musthandlethe signaldynamicrangeof about10 bits with minimal degradationof precision,storethesignalsduring theLevel-2 trigger latency, andprovide a deadtime below 10% at the maximumLevel-2 trigger rateof 100 Hz. The designprovides128channelsperFEC,with anestimatedmaximumpowerconsumptionof approximately12 W.

Thelayoutof theFECis shown in Fig.5.19andhasthefollowing flow. TheFECreceives128analogsignalsthrough8 flexible cablesandthe correspondingconnectorsasshown in the figure. The inputsignalsarevery fast,with a rise time of lessthan1 ns. Therefore,to minimize thechannel-to-channelcrosstalk,thePASA circuitshave to beverycloseto theinputconnectors.Thesignalsarefirst amplifiedandthenprocessedby a band-passfilter. Thelatteroperationis doneto limit thebandwidthof theADCinput signalandto reducethesignal-to-noiseratio. A 16-channelchip (PASA) containsthecircuits toimplementtheseanalogfunctions.EachFECcontainsthereforeeightPASA chips.CommercialADCsconvert the PASA outputsignalsinto digital signals. ADCs with two or four convertersper chip areconsideredsuitablefor thisapplicationleadingto 64or 32 ADCsperFEC,respectively.

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÷ ø ù øú û ü ý

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Figure 5.19: Sketchof theFront-EndCard(FEC).

Thedigitized signalsareprocessedandstoredin a memory, wherethey wait for a Level-2 trigger,to be eitherreadout or discarded.In the former case,the buffer will be frozenandprotectedagainstoverwriting until the dataare transferredto the ReadoutControl Unit (RCU). The detailsof the datatransmissionprotocolarediscussedin Section5.1.5.2. In the latter case,the buffer is madeavailableto storea new dataset. All thedigital circuitsareimplementedin theALTRO chip that containseightchannelsandtheoutputmultiplexing circuitry.

The FEC channelsaremultiplexed, at the boardlevel, via a LVTTL (Low-VoltageTTL) bus. Itfeaturesan asynchronousVME-lik e protocol,which is enhancedby a Clocked Block Transfer(CBT)thatprovidesa bandwidthof 160MByte/s. TheFECis interfacedto theRCU througha 40-bit bus thatis basedon the GTL (GunningTransceiver Logic) standard.At the boardoutput the bus signalsaretranslatedfrom LVTTL level to GTL level by bidirectionaltransceivers.Theconfiguration,readoutand


testof theboardaredonevia theGTL bus.However, theFECcontainsacircuit, namedBoardController(BC), implementedin a FPGA,which providestheRCU with an independentaccessto the FECvia afield-bus. Althoughthissecondaryaccessis normallyusedto monitortheboardactivity, power suppliesandtemperature,it canalsobeusedfor theconfigurationandreadoutof theFECat aslower rate.

The boardoffers a numberof testfacilities. As an example,a datapatterncanbe written into theALTRO chip and readback exercising the completereadoutchain. It is also possibleto perform aboundaryscantestof theALTRO chipsunderthecontrolof theBC. TheBC allows thebusactivities,thepresenceof theclock,andthenumberof triggersreceivedto beverified.

The ALTRO chips and the BC work synchronouslyundera masterclock frequency of 40 MHz.TheALTRO circuitsusuallyperformthesameoperationssimultaneouslyunderthecontrolof theRCUwhich canalsocontrola singlechannelat a time. This is performedin theconfigurationphaseandfortestpurposes.TheRCU broadcaststhetrigger informationto theindividual FECmodulesandcontrolsthereadoutprocedure.Both functionsareimplementedvia theGTL bus.

The boardlayout of the FEC canbe seenaspartitionedin nine differentsections.The first eightsectionsareidentical,eachonecontainingthe readoutchainfor a groupof 16 channels.Eachchainiscomprisedof onePASA, eight(or four) ADCs,two ALTROsandfour voltageregulators.ThelastsectioncontainstheLVTTL–GTL translatorsandtheBCs. As detailedin Section5.1.8theFECis poweredbymeansof four power cablescarrying threesupplyvoltages(

.4/ 3 V,

2/ 5 V) anda commonground.

From the threemain supplyvoltages,for eachof the eight sections,four supplyvoltagesarederivedanddistributed(

.3/ 3 V for theADCs,

.3/ 3 V for thedigital circuitsand

1/ 5 V for thePASA). The

lastsectionis poweredindependentlywith.

3/ 3 V. Thevoltageregulatorsfeaturea power modethat isremotelycontrollable.TheON/OFFpin of thevoltageregulatorsrelatedto thesamesectionis controlledvia theBC whichmonitorsthecurrentsuppliedby eachvoltageregulator. It canbeprogrammedto powerdown thesectionswherethemonitoredquantitiesexceedanupperlimit.

Theactuallayoutof theFECshasjust beencompleted.This is animportantstepbecauseit demon-stratesthateven for thesmallestboardson the innerchamberwith dimensionsof only 19 a 14 cm2 allcomponentsfor 128 channelscanfit on the board. The actuallayout for the two sidesof the boardisshown in Fig. 5.20andFig. 5.21.

5.1.5 Readoutbus

5.1.5.1 Requirements

The communicationbetweenthe RCU andtheFECsis implementedvia a custombus (front-endbus)basedon ashieldedribboncableandacustomprotocol.Thefront-endbusis essentiallyanextensionoftheFEC’sinternalbusthatallowstheRCUto accesstheFEC’sinternalcomponents.It is amultiple-dropsingle-masterbus wheretheRCU is themasterunit andtheFECsaretheslaves. In orderto minimizethe lengthof thebus cablesthe bus controllersupportstwo branchesrunningin oppositedirectionsasshown in Fig. 5.22.

Thefront-endbushasto satisfythefollowing requirements.

b Fromthefunctionalpointof view thefront-endbusis themeansby which theRCUbroadcaststheLevel-1 trigger informationto all theFECsin a crateandsteersthe readoutprocedure.Besidesthesebasicfunctions,theRCUwill alsodownloadtheFEC’sconfigurationparameters(pedestals,thresholds,tail-cancellationcoefficients,etc.)andrun testproceduresvia thesamebus.

b To matchtheperformanceof theDDL it hasto provide a bandwidthof at least100MByte/s.

b It mustcontaina numberof slotsthat rangesfrom 21, for the innermostcrate,to 33 for theout-ermostone. Physicallythe front-endbus is composedof two branches,eachonewith half thenumberof slots.


Figure5.20: Top view of thelayoutof thesmallFEC.


Figure5.21: Bottomview of thelayoutof thesmallFEC.


FEC128 CH

Loc

al

Con

trol

ler

DD

L - INT

Slow-C

ontrolInterfa

ce

TTC-RX

BOARDCTRL

RCU

MU

X

1

2

16

17

18

32

Front-end Busbi-directional

(160 MByte/sec)

Slow- Controlserial l ink

Front End Card(#4500)

ON DETECTOR COUNTINGROOM

100 Mbyte/secDetector L ink

(#180)

Slow- Control1 Mbit - serial linkFEC

128 CH

FEC128 CH

FEC128 CH

FEC128 CH

FEC128 CH

Figure 5.22: Front-endelectronicssystemarchitecture.

b The front-endbus cablesarevery closeto theanalogfront-endelectronics;thereforelow signalnoiseandElectroMagneticInterference(EMI) areanissue.

5.1.5.2 Readoutprotocol

The readoutof oneevent is performedin two separatephaseswhich areconsecutive for a givenevent,but canotherwisebe activatedconcurrently. In a first phase,the trigger informationis received by theRCUandbroadcastto all modulesin thesubsystem,startingthedigitizationof eachchannel,whichlastsfor 88µs(theTPCdrift time). During thisphasetail cancellationandzerosuppressionareperformed.Inthesecondphase,informationis movedfrom themultiple-eventbuffersto theRCU.Thetimeneededtocompletethesecondphasedependson thesizeof theevent,but othertriggerscanbeprocessedduringthe readoutof the previous event, as long asthe multiple-event buffers in the FEC arenot full. Deadtime canbegeneratedonly whenthis conditionoccurs.The transferof datawordsis synchronousandmodulescontainingvalid unreaddataareenabledto assertdataon the bus by individual addressing.To beableto performonlinehardwaredatacompression,theorderin which thedataarepresentedforoptical transmissionis not arbitrary. The bestsequenceis to sendall time bins of one channelin achannel-by-channel fashionwith neighbouringchannelsbeingalsotransmittedadjacently. However, thedetaileddataformatmayhave to berevisedwhenmoredetailedstudiesof thepossibledata-compressionscenarioshave concluded.

The front-endbus implementsa VME-lik e protocolwith an asynchronoushandshake betweentheRCU andtheALTRO chipsin theFECs.A synchronousblock transferenhancestheprotocolwhereaswordsaretransferredwithout any acknowledgement.Thebus consistsof 32 bidirectionallines for thetransmissionof dataandaddresses,andsix control lines. Speciallinesareusedto distribute thetrigger(Level-1) andclock signals.TheALTRO chip recognizesa setof eight instructions.By meansof theseinstructionstheRCU canaccesseachALTRO chip in thesystemto

b write/readaConfiguration/StatusRegister(CSR),

b issueaCOMMAND,

b readthemultiple-eventbuffer.


TheALTROchipacknowledgestheexecutionof any instruction.A specialcaseis representedby thedatareadoutprocedurewheretheALTRO actsasMASTERandstrobesdataontheRCUFIFO(SLAVE)synchronouslywithout any acknowledgement.Thesignalsusedfor the implementationof theprotocolareBD[31:0], WRITE*, CSTB*, ACKN*, TRSF*,DSTB*, GRST*,TRG*, MCLK.

In the CSRaccessor COMMAND instructions,themostsignificant16 bits of theBD bus specifythechip andchanneladdresses,theCSRaddressor theCOMMAND code.Theleastsignificant16 bitsareusedto input datain theCSRWRITE andCOMMAND instructions,andto outputdatain theCSRREAD instructions.Thewrite/readaccessto a CSRis definedby theWRITE* signal.A rising edgeonCSTB* (CommandSTroBe) informs the ALTRO chip that the addressinganddatainformationin theBD[31:0] lines is stableandcanbecaptured.On a WRITE* or COMMAND cycle, theALTRO drivestheACKN* signalto indicatethat it hassuccessfullyreceived all thedatacalledfor by the instructiontype.Onareadcycle, theALTRO drivestheACKN* signalto indicatethatit hasplaceddataonthedatabus. During the readoutof themultiple-event buffer the chip announcesthe datatransferby meansofthesignalTRSF* (TRanSFer)which is kept low until thedatapacket hasbeencompletelytransferred.Thedatatransferis not necessarilycontinuousandfor this reasoneachsingleword beingtransferredisvalidatedby the signalDSTB* (DataSTroBe). TheGRST* (Global ReSeT)initializes all the internalregisters,counters,andstatemachines.TheTRG* (Level-1 TRiGger)signalstartstheprocessingof thedigitizedsignals.TheMCLK (MasterCLocK) is aclocksignalgeneratedby theRCUanddistributedtoall FECs,which is usedonly by thereadoutcircuits.

A secondclock signalSCLOCK (SamplingCLOCK), with muchtighter constraintson the maxi-mum skew andjitters, is distributedvia a dedicatedpoint-to-pointconnection.This is theclock signaldistributedto theADCsandit hasa frequency between5 MHz and10 MHz.

Thefront-endbusprovides,with aclock frequency of 40MHz, abandwidthof 160MByte/soneachof the two branches,leadingto an aggregatebandwidthof 320 MByte/s. It is clearthat a bus master-clock frequency of 20 MHz which correspondsto a bandwidthof 160 MByte/s would be adequatetomatchthebandwidthcurrentlyforeseenfor theDDL. Howevera front-endbuswith ahigherbandwidthwill allow to supportpossiblefutureupgradesof theDDL performance.

5.1.5.3 Electrical and physicalspecifications

For theelectricalimplementationof thebustransceiversweplantouseGTL+. GTL, inventedbyW. Gun-ning at Xerox Corporationandstandardizedby JEDEC,is a low-swing input/outputdriver technology.It hasbeenfurthermodifiedby Intel andTI (TexasInstruments)who have increasedthevoltageswingto createGTL+. Thetypical swingfor GTL+ is from 0.6V low (VOL) to 1.5V high (VOH) atmost.TIusestighterthresholdregionsto provide bettersignalintegrity in its stand-alonedevices.

It hasbeenshown [14] thatthenewestTI GTL+ devices(GTL161612A)canoperateupto 100MHz,providing bit ratesof 3.2 Gbit/s in a 32-bit wide backplane.They aredesignedwith a slow rising andfalling edgeto offer significantsystemfrequency improvementin heavily loadedbackplanes.TheGTLoutputconsistsof anedge-controlcircuit thatprovidesoptimizedriseandfall times,typically2.6ns(20%to 80%), for backplanesundervariousloadingconditions. Device power canbe switchedoff withouthaving to remove thedevice from thesystembecausetheinputsandoutputsareat high impedanceand,therefore,areableto tolerateactive bussignals.

ThebasicGTL outputstructureis anopen-draintransistor, whereastheinput is a differentialdriver.Besides,theGTL I/Os have beendesignedto minimizetheir capacitance,anextremelyimportantfactorfor distributed-loadhigh-performancebackplanes.Figure5.23shows thebackplaneelectricalmodelfora 17-slotsystemlocatedin theoutermostcrate(C5).

For theTPCfront-endelectronicsarigid backplaneis notapraticalsolutionastheFECsaremountedon thedetectorfirst, andthebackplaneshouldbepluggedonto17 boardssimultaneously. Thereforeweplan to implementthe front-endbus in a polyimideflexible cable.The40 bus linesarealternatedwithgroundlines,with apitchof 1.27mm,andlie aboveasolidgroundplane.Theelectricalperformanceof


VTT VTT

40-bit Bus

SLOT1 SLOT2 SLOT3 SLOT17

R R

2cm 2cm 2cm 2cm

2.4 cm 2.4 cm 2.4 cm

40 cm

Figure 5.23: Backplanemodelfor onebranchof thefront-endbusfor theoutermostcrate.

this systemis equivalentto a standardrigid backplane.

5.1.6 Readoutcontrol and services

The overall TPC electronicsarchitectureis sketchedin Fig. 5.22 on page114. The electronicsfor128 channelsis mountedon a Front-EndCard (FEC) as describedin Section5.1.4. The functional-ity performedhereincludes,asidefrom amplificationanddigitization,digital filtering, zerosuppression,andbuffering of up to four black events. For a motivation why four eventsareadequaterefer to Sec-tion 5.1.7.BasicallytheFEChostsall electronicsnecessaryfor onechannel.In addition,it alsoperformsnecessarypowermanagement,whichhasto beascloseaspossibleto theanalogcircuitry.

Therewill be a total of about4500FECs. Assumingblack eventsandthemaximum200 Hz TPCreadouteachcardwill produceadatastreamof about11MByte/sif thedatanot falling into thenominalacceptance(32%) are not readout (otherwise16 MByte/s). The acceptedALICE datatransmissionstandardoff the detectoris DDL [15,16] supporting1 Gbit/s. Thereforemultiple TPC FECscanbemergedinto oneDDL. This functionality is performedby thereadoutcontrolunit sketchedin Fig. 5.22on page114. However, dependingon theradialposition,not thetotal depthof thechamberwill bereadout andalsoposition-dependentcompressionfactorsareexpected.Thereforethenumberof FECsperreadoutcontrollervariesfrom 20 to 32 suchthat theaveragethroughputper link is constant.Therearegoingto befive readoutcontrollerspersector.

Thereadoutcontrollerhasthefollowing functionalrequirements

b mergedigitizeddatafrom FECsinto theDDL datastreamin orderedform (time bin, padnumber,padrow);

b distributetriggerandclock signalsto FECs;

b interfacedetectorcontrolsystem(DCS)to FECs;

Thefollowing paragraphsdiscussthefunctionalityandinterfacesof thisdevice.

5.1.6.1 Clock and trigger interface

TheacceptedALICE standardhow to distribute triggerandclock informationis theRD12TTC systemwhich will be usedhere[17]. Every readoutcontroller implementsan appropriatereceiver (TTCRX)


which producesthephase-correctedLHC clock,Level-1 triggerandLevel-2accept/reject;thesehave tobe forwardedto all FECsvia the readoutbus. Dueto themultiple-dropnatureof thereadoutbus therewill besomeclock skew betweenthedifferentFECs.With acablelengthof lessthan30cm perreadoutbus link theclock skew betweenany FECrelative to theTTCRX chip which canbe individually phasecorrected,canbekeptbelow 5 nswith lessthan200pselectronicjitter. Thiscorrespondsto asystematicpositionerrorof lessthan200µm with acontribution to thepositionresolutionof lessthan10 µm.

In addition,a4-bit triggertypeneedsto beprovidedto theFECswhichcanbeusedto issuepulserortestreadouttriggers,suchasthereadoutof a pre-loadedevent for testpurposesof thedatapath. Thesetriggerscanbe issuedasbroadcastcommandsthroughtheTTC. NormalphysicsLevel-1 triggershavethedefault readouttypezero.

ALICE implementsamultiple-deadtimesystemallowing triggersto somesubdetectorswhile others,suchastheTPC,arebusy. Therefore,aneventnumberis notanunambiguousmeansto identify aneventandto ensuredataintegrity. The TTC receiver providesa bunch-crossingnumberwhich, however, isonly uniquefor oneLHC turn. Thereforeit wasdecidedto usethecombinationof thebunch-crossingnumberandanorbit counter. Here24 bits correspondto 20 minutesof real time, which is longerthanany eventis expectedto remainwithin theDAQ andthereforeadequateto ensureuniquenessof theeventID. Theorbit counterhasto beimplementedexternallyto theTTCRX andbeableto runat12 kHz.

5.1.6.2 Interface to the digital optical links

The ALICE standardto transmitdataoff the detectoris the DDL technologydevelopedby the DAQgroup[15,16]. It implementsa gigabitoptical link which is drivenby a SourceInterfaceUnit (SIU) onthereadoutcontroller. Sincethis device is beingusedby multiple subdetectorsthis SIU will bebuilt bytheDAQ groupandbeimplementedasamezzaninecardonthereadoutcontroller. TheprotocolbetweenthereadoutcontrollerandtheSIU is asynchronous40 MHz 32-bit bus.

Thereadoutcontrollerwill startsendinganeventassoonasit hasreceiveda Level-2 acceptfor theevent guaranteeingthat the event will not be aborted. Sincethe orderof eventsis maintainedby theLevel-2acceptor rejectcommandsnospecialeventidentificationlogic is requiredbut ratherthequeuedeventscanbehandledin theorderthey weretriggered.Thewholearchitecturecanbeimplementedasasimplepusharchitecture.Any Level-2 decisionreceivedby theTTC will beforwardedimmediatelytotheFECswhich thenwill startsendingthenext eventuponanacceptor will freetheappropriatebufferin their round-robinbuffer scheme.Thereforethereadoutcontrolleractsmainly asa multiplexer for theopticaltransmitter(SIU).

Thedataformat to beshippedis variablelength. Every eventwill beencapsulatedby at leastthreecommand/statuswords, the begin-of-event, the begin-of-dataand the end-of-event commands. Thebegin-of-event commandis followed by a fixed-lengthheaderthat further definesthe event type andsize. Following standardpractice,the first bytesof the subevent headerdefinethe total subevent size.The second32-bit word definesthesubevent type andsubtypeincluding someversionnumber. In ad-dition the event headerhasto containthe trigger type, event ID (the bunch-crossingnumberandorbitnumber),and the readoutcontroller ID. Following the headerare the zero-suppressedraw data. Theexactheaderdataformatis not critical andwill bedefinedjointly by theTPCandDAQ groups.

5.1.6.3 Detector control relatedinterfaces

Thereadoutcontrollerhasto have two detectorcontrol relatedinterfaces.Oneis theconnectionto theFECs,which is requiredin orderto configureandmonitorthedevices,andtheotheris theDCSinterfaceitself. Thefunctionalrequirementsfor theDCSinterfacearethefollowing

b configurepowerstateonall FECs;

b monitorpower andtemperatureon all FECs;


b read/writeany configurationparameterin the front-end,including zero-suppressionthresholds,andconfigurationof thedigital filter;

b asynchronouslyfire exceptionsor warningsoutof thefront-end;

b readstatusparameters,suchaserrorconditions;

b uploadaneventinto thefront-endfor testing;

b downloadaneventfrom thefront-endasredundantfall-back.

Thelargestdataitemsfor up/downloadarecompleteevents.However, sincethis is donefor testingpurposesonly, thereareno particularperformancerequirementshereandit would beacceptableif theevent datatransmissiontook several seconds.All otherparametersetsarecomparatively small. Theslow control interfacehasto be full duplex supportingthe ability to addressindividual registersin thefront-end.It is thegoalto implementany front-endconfigurationregisterin read/writemodesothattheasis statuscanbereadbackandverified. Further, thefront-endneedsto beableto assertasynchronouseventssuchasalarmslike over-power, currentor temperature.

OneacceptedALICE standardfor slow control is thefieldbusCAN (ISO 11898–2).ThemaximumCAN datatransmissionrateis 1 Mbit/s at a maximumdistanceof 40 m. This would correspondin caseof a centralPb–Pbevent to lessthan20 secondsfor the readoutof a completesector. CAN is a multi-mastermultiple-dropserialbusimplementingtheCSMA/CDarbitration.Therearelow-cost,single-chipdevices available today, suchas the SiemensC167 16-bit microcontroller, which have an integratedCAN interface.This or a similar microcontrollerwill behostedon thereadoutcontrollerto performallnecessarydetector-controlfunctionality. Theparticularprotocolto addressthevariouscontrolandstatusregistersis notyetdefinedin detailasis thecasefor theCSRregisters.Thiswill bedonein collaborationwith the detectorcontrol systemsgroup. Thereareno particulartechnologicalchallenges.In ordertoreducethesinglepoint of failureriskstherewill beoneCAN busperTPCsector.

The readoutcontrollerneedsto be ableto communicatecommandsto andreceive statusfrom theFECs.Most of thecontrolandstatusregisterswill bepartof anFPGAon theFEC.Thereforea simplesynchronousserial interfacewould be adequateand would allow to keepthe numberof signalsandcablesto a minimum. This simpleinterfacewill bepartof theprogrammablelogic on theFECandalsobe implementedon the readoutcontrollerasinterfaceto the CAN microcontroller. The detailsof thisserialinterfacearenotdefinedyet andwill befinalizedastheprototypingof theFECprogresses.Giventhespeedof lessthan10MHz, therearenoparticulartechnologicalchallengeshere.

5.1.7 Front-end electronic readoutefficiency

This sectiondealswith thereadoutefficiency of thefront-endelectronics.In particular, it will beshownhow thesystemdeadtimedependson thedimensionsof thefront-endmultiple-eventbuffer.

WeassumeherethattheTPCwill beoperatedat a rate(Level-1 trigger)of about200Hz for Pb–Pbcentralcollisions.At thereceiptof aLevel-1 trigger, thefront-endelectronicsprocessestheTPCsignalsfor 88 µs, storing the zero-suppresseddatainto a multiple-event buffer. We assumethat the Level-2trigger (accept/reject)hasa fixed latency which correspondsto the TPC drift time. It shouldbe notedthat,owing to thetriggerpast–futureprotection,for theTPCa new Level-1 triggeronly comesaftertheLevel-2 triggerrelatedto thepreviousevent.

In thecaseof aLevel-2accept,thememoryregion wheretheeventdatahasbeenstoredis protectedagainstoverwriting until the datahasbeenshippedto the RCU. Otherwise,in the caseof a Level-2reject,this region is immediatelyfreedto storeanew dataset.Thereforethemultiple-eventbuffer is nota pipelinedmemorybut rathera memorysubdivided in regions(buffers) to be accessedrandomly. Toavoid fragmentationeachbuffer is deepenoughto containa black (nonzero-suppressed)event. Whenall memoryregionsareoccupiedthefront-endelectronicsgeneratesdeadtime.


Thedeadtimegeneratedby theTPChasthereforetwo contributions:detectordeadtime,i.e. thedrifttime, andfront-endelectronicsdeadtime (readoutdeadtime). While themultiple-event buffer schemecanreducethesecondcontribution, it cannoteliminateor reducethefirst. This obviousfactimpliesthatthetotal deadtimehasa lower limit determinedby thedetectordrift time.

In orderto studythereadoutperformanceof thefront-endelectronicsandto dimensionproperlythemultiple-event buffer, a Verilog modelof the front-endchainhasbeencreated.Verilog is a hardwaredescriptionlanguagesuitablefor the stochasticanalysisof digital systems.The resultsaredisplayedin Figs.5.24and5.25which show how thesystemdeadtime dependson thenumberof buffers for twodifferentLevel-1rates:1 kHz and200Hz, respectively. ThesimulationsassumeaLevel-2rateof 100Hzandanaverageoccupancy of 25%.

L1 = 1K Hz - L2 = 100Hz

9.31 9.089.089.089.0910.38

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Figure5.24: TPCdeadtimeversusthenumberof buffersin thefront-endmemory. Thesimulationis madeunderthefollowing assumptions:Level-1rate 1 kHz, Level-2 rate 100Hz, averageoccupancy 25%.

L1 = 200 Hz - L2 = 100 Hz

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Figure5.25: TPCdeadtimeversusthenumberof buffersin thefront-endmemory. Thesimulationis madeunderthefollowing assumptions:Level-1rate 200Hz, Level-2 rate 100Hz, averageoccupancy 25%.


5.1.8 Low-voltagepower suppliesand distrib ution

For thecalculationof thenumberof low-voltagesupplylines for theFECsandtheir crosssection,it isassumedthateachcarddissipatesat most100mW/channelof power (in total 12.8W), where20%areconsumedby thepreamplifier/shaperat

1/ 5 V and80%areconsumedby thedigital partat

.3/ 3 V. In

total thesupplysystemhasto supplyabout21 kA of current. It is assumedthat thepower suppliesarelocatedoutsidetheL3 magnetonbothsidesof theTPCin theareanotaccessibleduringLHC operation.Thecablelengthrequiredin thisscenariois 40 m.

To avoid coolingof thecablesbut keepthecostof cablesmoderateavoltagedropof 0.5V hasbeenassumedasa designvalueleadingto acceptablelevelsof heatdissipationin theL3 cavern. In Table5.8thecharacteristicsof thecablesarelisted.

Table 5.8: Characteristicsof the low-voltagesupplycables.Thevoltagesarenominalvoltagesat theload. Thepowerdissipationquotedrefersto thepowerdissipatedin 40 m of cables.

Parameter Analogsupply Digital supply

Nominalvoltage.

1/ 5 V/ ` 1/ 5 V.

3/ 3 V/GNDCrosssection 2 a 4 mm2 2 a 10mm2

Current 2 a 0/ 85 A 2 a 3/ 1 APower 1700W 6200W

Two scenariosareconsidered.In the first the power for eachFEC is provided individually via aseparatecablewith four wiresanda commonshield(in total 2232cablesperend-plate).This assumesthat thecableshave to passthroughtheexisting gapsat theL3 magnetdoorswith its chicaneandthushave to beflexible enoughto accommodatea bendingradiusof lessor about5 cm. If theopeningscanbeenlargedtheneedfor flexible cablesis alleviated. In thesecondscenario,therefore,thesuppliesforaboutfive cardsarecombinedinto a thicker, lessflexible but moreeconomiccable. Therewould be atransitionfrom thethick cableto individual cablesatapatchpanelmountedat theservicesupportwheel(seebelow).

To allow accessto the inner tracking systemthe TPC and its supply lines have to accommodatemovementsof about4.5 m alongthe beamdirection. Due to spacelimitations no distribution schemeallowing suchamovementwhile at thesametimekeepingthesupplylinespermanentlyconnectedcouldbe found. It is thereforeforeseento connectthe low-voltagecablesvia patchpanelsmountedon theservicesupportwheel.For serviceinterventionsall cableshave to bedisconnectedat thepatchpanel.

Thecrosssectionsof all low-voltagecablesconnectingthepatchpanelwith theindividualFECscanbe reducedeasilyby a factorof two comparedto the 40 m long main sections.The estimatedpowerdissipationcloseto theend-plateis then200W perend-plate.

To avoid groundloopsit is foreseenthat thepower suppliesareleft floatingon thesupplysideandconnectedto groundonly at the detectorside. A schematiclayout of the low-voltagesystemwith itsgroundingschemeis shown in Fig.5.26.SinceeachFECis connectedto thepadplanewith eightcables,eachof which carriesthe groundfor eachof the 16 channelsplus a commonshield,a relatively largecross-sectionis availablein caseof aninterruptionof thegroundreturnline.

TheDCSwill monitor thevoltageson theFECsbeforeregulation. If a voltagedropis detectedthepower suppliescanbe powereddown sufficiently fast(on a time-scaleof ms) to avoid damageduetoexcessive currents.

With regardto the stability of the power suppliesno specialrequirementsarenecessarysincethefinal regulationis doneon theFECsthemselves. Quite strict limits on noisehave to be adheredto thegiven sensitivity of the preamplifier/shaperpart. The cablesarethereforeshieldedbetweenthe powersuppliesandthepatchpanel.At this point no analysishasbeenperformedregardingthebestgroupingof power-supplylinesandthereforethemaximumoutputcurrentsrequiredpersupplymodule.


ground connection toTPC readout chamber

40 m

+1.5 V

-1.5 V

electronicsfrontend

TPC

low voltage power supply

analog

on service support wheelpatch panel

GND

+3.3 V

shield

(ground free)

digital

Figure 5.26: Connectionof thelow-voltagepowersuppliesto thefront-endelectronicsof theTPC.

5.1.9 Systemimplementation

5.1.9.1 Quality assurance

The inaccessibilityof the ALICE experimentduring the entireyearof LHC runningmakes stringentquality testsof the readoutelectronicsmandatorybeforeinstallation. To minimize the failure rateallFECswill betestedin adedicatedtestset-up.A schematicdiagramof thistesterisshown in Fig.5.27.Animportantpartof thetestprocedureis aburn-in process,whichwill bemadein anovenata temperatureof 50 C for a periodof approximately10 hours. All crucial functionsof the circuits will be cycledaccordingto a preselectedscheme.During the 10 hoursof burn-in typically thousandsof cycleswillbe completed.The resultsof the testswill be automaticallyloggedto file for later investigationof theerrorsoccurring.A malfunctioningcomponentwill bereplacedwith a new oneandthefull testwill be

Front-end Bus

PulserCharge

Cha

rge

inpu

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module with128 channels

Front-endPower

Pulsercontrol

Control and

distributionSignal

SupplyPower

and storing of test andComputer for control

calibration results

2

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10

Inside of oven1

Front-end Bus

Figure5.27: Functionaldiagramof theFCtesterwith burn-in feature.


repeatedbeforeacceptingtheboardfor installation.Specialcarehasto betakento control theexternalnoiseinfluenceon theanalogcircuits.

Thetestset-upforeseenwill hold tenunits to betestedandtwo testroundscouldbecompletedper24 hours. A total numberof 570000 readoutchannelsgroupedin about5000FECs(including 10%spares)thenrequiresabout240daysto completetheburn-in acceptancetestusingoneteststation.It isforeseento duplicatethetestset-upin orderto speedup theburn-inprocess.

Oncethe boardsareacceptedandinstalledon the TPC an in situ testingprocedurehasto be im-plemented.All thedigital partscanbetestedthroughtheDAQ and/ortheDCS.To testtheanalogpart(preamplifier/shaperandADC) it is necessaryto include the TPC itself or usethe calibrationpulser.During operationthefull systemhasto bemonitoredfor malfunctioningchannels.

5.2 Readout

TheALICE Trigger/DAQ architecturewasinitially designedfor a setof requirementsdescribedin theTechnicalProposal[3]. TheserequirementsandthecorrespondingTrigger/DAQ architecturehave beenrelatively stablesincethenandhave beenrefinedfor thepreparationof thefirst TDR. This architectureis now facingtwo importantmodificationsof theoriginal requirements.First, theoccupancy of theTPCis expectedto be higherthanoriginally assumed.Second,a new trigger detector(the TRD) hasbeenintroducedinto theALICE detectorsystem.Both modificationsrequirerevisiting thebaselinearchitec-ture in orderto ensurethat thechangingrequirementscanbemetby theTrigger/DAQ architecture.Inthenew scenariotheTPCandTRD detectordataaresentdirectly into thehostprocessorsof a Level-3Trigger/DAQ processorfarm.Thesystemis scalablein termsof processingpower andbandwidthin or-der to meetthecomputingrequirementsof thevariousphysicsprogrammes.It is absolutelymandatorythatzero-suppressedraw dataarerecordedwithout any furtherprocessing,especiallyin thefirst yearofrunningin orderto ensureacompleteunderstandingof theTPCresponse.Theintelligentreadoutsystem(Level-3) guaranteesthereadoutof unprocessedfull TPCeventsat a rateof 10 Hz. Having understoodthe TPC responseto centralPb–Pbcollisions, the Level-3 systemwill take over more functionalities.Almost losslessdatacompressionandselective readoutfor TRD eventscanbeimplementedeasily, sincethesetechniquesdo not needlargecomputingpower. Finally, by addingmorecomputingpower, onlinetrack finding in thewholeTPCandthereforevery effective datacompressionmethodsandTPC-basedselective readoutschemesbecomefeasible.Shouldtherebea needfor a higherrateof unprocessedfullTPCevents,thebandwidthof thesystemcanbeincreased.Thissystemis referredto asLevel-3 trigger,Level-3 filter andLevel-3 processorsin variousplaces,dependingon thefunctionalityaddressedin thecontext.

Extensive simulationsof track-findingmethodsanddataflow have beenperformedfor theproposedLevel-3 Trigger Systemof the STAR [18] experimentat RHIC. A large scaleprototypeof an Level-3systemis currentlybeinginstalled. Thereis a large overlap in the physicsobjective, detectordesign,readoutelectronicsandpersonellbetweentheSTAR andtheALICE Level-3systems.

5.2.1 Physicsrequirements

5.2.1.1 Event size

Thelatestsimulationsfor centralPb–PbcollisionsindicatethattheaverageTPCoccupancy will beabout25%(seeTable5.13on page132in Section5.2.3).Thereadoutchambershave agranularityof 570000pads,thenumberof time bucketsperchannelis assumedto be512. Theexactnumberof time bucketswill bedeterminedafter thefirst Pb–Pbcollisionshave beenrecordedandtheclustershapeshave beenanalysed.Dependingon occupancy it may increaseto 768 at the most. The 10-bit amplitudecanbecompressedinto a nonlinear8-bit number. This correspondsto a raw event sizeof 292 Mbyte. Theeventsizecanbereducedby roughly30%–50%by removing thetwo inner45 conesof theTPCvolume

5.2 Readout 123

duringreadout.Differentsolutionsfor codingthezero-suppresseddataarecurrentlybeinginvestigated.Assuminga 15%–40%overheadfor encodingthe zero-suppresseddatayields an event sizeof about66 Mbyte 15% per centralcollision. Minimum-biaseventsareassumedto be smallerby roughly afactorof two (in NA49: centralPb–Pb100%,mediumbias85%,p–A 30%andpp 20%).

5.2.1.2 Event rates

In general,the requiredcentralevent samplesfor variousphysicstopicsvary between 106 for mostinclusive hadronicobservablesto 107 for rareprocessessuchasopencharmandopenbottomproduc-tion, quarkoniaproduction,andthedetectionof direct realandvirtual photons[3,21–23]. Thedetailedeventratesto betakenundergo continuousdiscussionwithin thecollaboration.

In thefollowing we present,for a quantitative evaluationof theproposedreadoutschemes,anillus-trative setof event ratesexpectedfor hadronandelectronphysicsin the TPC.However, the proposedreadoutschemeis sufficiently generalthatphysicsdictatedvariationsin theratesshouldbeaccomodatedwithout changesin thearchitecture.

An eventrateof about1 Hz leadingto morethan106 eventsperALICE yearis currentlyconsideredsufficient for mosthadronicobservables.An examplefor anobservablethatwould requirehighereventrates( 10 Hz) is thereconstructionof D mesonsvia their hadronicdecayproducts.TheALICE DAQwasoriginally dimensionedto handleupto 50Hz of eventswith full readoutandin additionupto 1 kHzof muontriggers,with a total bandwidthto massstorageof up to 1.3 Gbyte/s. With the introductionof theTRD andthe increaseof occupancy in theTPC,theeventsizehasgrown substantiallysincethepublicationof theTechnicalProposalandwould currentlylimit thedatarateto lessthan30 Hz withintheforeseenbandwidth.

As discussedabove, the final bandwidthof the systemcanbe increasedbeyond 10 Hz if the cor-respondingphysicsrequirementsare there. However, by introductionof the Level-3 system,onlinetrackingwill make it feasibleto performpartialreadout,eventrejectionandsophisticateddatacompres-sion. It cansignificantlyreducetheeventsizefor full TPCreadout(potentiallyby morethanoneorderofmagnitude)and/orreducetheeventrate(by onlinefiltering) andthereforerestoretheoriginally foreseenrateswithin or below theoriginally foreseenbandwidthto massstorage.TypicaleventratesandtheTPCeventsizefor diffent compressionschemes(discussedin detailbelow) anddifferenttriggersamplesaregivenin Table5.9.

Therequiredeventratesfor thefull dielectronprogrammeandanimprovedopen-charmprogrammewith aLevel-3 triggerareasfollows.

Full dielectronprogramme:

TRD triggersTPCreadoutat 60 Hz;

Selective inspectionof a few TPCsectors;

Onlineprocessingof electrontrackcandidates,limited processingcomparedto full TPCtracking;

Outputrateis 30Hz, eventsize0.5Mbyte.

Improvedopencharm(hadronicdecays)programme:

TPCreadoutat10 Hz–100Hz;

Onlineprocessingof full TPCdata,largeprocessingpowerneededfor clusterandtrackfinding;

Selectinghigh-transverse-momentum tracks;

Outputrateis 10 Hz–100Hz, eventsizeis 4 Mbyte.


Table 5.9: Typical TPCeventratesanddatarates.It hasto benotedthatall otherdetectorscontribute10 Mbyteto thesizeof aneventfor a centralcollision. Themuonprogrammewill increasethebandwidthby approximately250Mbyte/s.

No. of events Eventrate Eventsize Bandwidth[Hz] [Mbyte] [Mbyte/s]

1 — no Level-3, full TPCreadoutHadronicobservables 106 1 66 66Opencharm(hadronicdecays) 107 10 66 660

2a— Level-3, losslessdatacompressionHadronicobservables 106 1 47 47Opencharm(hadronicdecays) 107 10 47 470

2b — Level-3,slightly lossydatacompressionHadronicobservables 106 1 19 19Opencharm(hadronicdecays) 107 10 19 190

3 — Level-3,TRD triggerandselective TPCsubsectorreadoutDielectrons 6 107 60 1 – 2 60– 120

4 — Level-3,TRD trigger, selective TPCtrackingandeventrejectionDielectrons 3 107 30 0.5– 2 15– 60

5 — Level-3,TPCtrackingandhigh-pt trackselection( 0.5GeV/c)Opencharm(hadronicdecays) 107 10 20 200

6a— Level-3,TPCtrack/clustermodelingandfull datacompressionOpencharm(hadronicdecays) 107 10 4 40

6b — Level-3,TPCtrackinganddatacompression,DE tracks,TPCsummaryeventfor correlationsDielectrons 3 107 30 5 150

5.2.2 Data rate reduction

Froma triggerpoint of view thedetectorsin ALICE canbedivided into two categories: fastandslow.Fastdetectorsprovide information for the trigger systemat every LHC bunchcrossing. Decisionsattrigger Levels-0, -1 and -2 are madeusing information from thesedetectors. Fast detectorsare theForward Multiplicity Detectors(FMD), the Zero DegreeCalorimeter(ZDC), the TransitionRadiationDetector(TRD) andthemuontriggerchambers.

The slow detectorsare tracking drift detectorsand needa longer time spanafter the collision todeliver their data. TheTime ProjectionChamber(TPC) andthe InnerTrackingSystem(ITS) areslowdetectors.Their slownessis compensatedfor by the detailedinformation they provide. The Level-3systemis intendedto takeadvantageof TPCinformation(up to 66Mbyte/eventat ratesof up to 200Hz)in order to reducethe datarate as far as possibleto have reasonabletaping cost. The dataare thenrecordedontoanarchival-qualitymediumfor subsequentoffline analysis.

A key componentof the proposedsystemis the ability to processthe raw data— performing,forexample,track finding in real time — besidesits baselinerequirementof forwardingzerosuppressedeventsinto theDAQ datastream.Level-3 is designedto utilize the informationfrom the TPCandthefastdetectors,e.g.theTRD. Thesystemwill beflexible enoughto beexpandedto includeothertrackingdevices.

Datareductioncanbeachievedin differentways:

Generationandapplicationof asoftwaretriggercapableof reducingtheinput datastream.

5.2 Readout 125

Reductionin thesizeof theeventdataby selectingsubevents.By analysingthetrackinginforma-tion or by utilizing TRD information,RegionsOf Interests(ROI) canbedefined.Thedatavolumeis significantly reducedby recordingonly summaryinformationandraw dataof the ROIs (e.g.electrontracks).

Reductionin thesizeof theeventdatacanalsobeachievedby compressiontechniques.Generallosslessor slightly lossymethodscancompressTPCdataby factors2–3[20]. By onlinetrackingandacompresseddatarepresentationaneventsizereductionby afactorof 15canbeachieved[24].This datacompressionmethodis basedon a clustermodelanda local trackmodel. It is assumedthat all TPC informationcan be reducedto a local track with its helix parametersand a meancharge,thecorrespondingclusterinformation(chargepercluster, clusterposition)is storedasthe(quantized)deviation from the local model. Theresultmaybefurthercompressedby a Huffmancodingscheme(seee.g.[25]). Sinceall necessaryinformationis preserved,TPCdatacanlaterbeoncemorereconstructedby theoffline chain.This methodmaybeusefulin thelaterphaseof theexperimentwhenthedetectorandthereconstructionperformancearewell understood.

5.2.2.1 Volumereduction

The raw datavolume per event can be reducedby compressiontechniquesor by selective or partialreadout. Thesesubeventscontainonly the raw dataof a few tracks. Zero suppressionis performedin the front-endelectronicsby pedestalsubtraction(a thresholdoperation)andone-dimensional(timedirection)hit-finding. The resultinglong zerosequencesbetweenhits canthenbe compressedby, forexample,run-lengthencoding,which is equivalent to storing only the hits and their positions. Thistechniqueachievescompressionratioswhich areroughly inverselyproportionalto theTPCoccupancy.The thresholdingand hit-finding operationsare lossy techniqueswhich could lead to a loss of smallclustersor tails of clusters. The following encodingmethodsof hit sequencesare lossless.All theseoperationsareperformedon thefront-endelectronicsboard(seeSection5.1.3.5).

Selective or partial readout

Subeventsor ROIs canbedefinedon thebasisof roughtrackinginformation,includinga PID suppliedby theTRD. All raw datainsidetheseregionsarewrittenon tapeandall otherdataaredropped.For theleptonmeasurementsthedatavolumecanbereducedto candidatee e tracks,whichwouldyield a fewtracksperevent.

Data compression

Generaldatacompressiontechniqueshavebeenappliedto TPCdata,i.e. boththeNA49 raw data[20,26]andtheSTAR simulationdata[18,19,27]. ZerosuppressionreducestheTPCdataby a largemargin (de-pendingon occupancy). Losslesstransformationslike variablelengthcodes(e.g. Huffman coding)orlossycompressionmethodslike vectorquantizationcancompresstheADC databy factorsof up to 3.Thedatavolumecanbereducedevenfurtherby usingdatamodellingtechniques,andstoringonly quan-tizeddifferencesto a datamodel,i.e. a clusteranda local trackmodel.This resultsin reductionfactorsof about15.

Losslesscompression– variable length coding Thecrucialobservation to reducethesizeof thedatais that theADC valuesarenot equallyprobable.SmallADC valuesoccurvery oftenin thedatastreambut larger onesarerare(Fig. 5.28). The distribution is approximatelyexponential. The expectedsizeof the datacanbe reducedif shortwordsareusedfor frequentvaluesandlongeronesfor rarevalues.Anotheradvantageof this techniqueis thatoneis no longerrestrictedto 256symbols— asoneis withbytes— or powersof two. Onecanchooseasmany codesasnecessary. Thismeansthatif oneenhances


Figure5.28: Distributionof ADC valuesfor a centralPb–Pbcollision.

theADC resolutionfrom 8 to 9 bits thesizeof thedatais no longerdoubled,but increasedby 1/8 in theworstcase.

Thereis a theoreticallower boundon the averageword sizethat canbe achieved by this strategy.This lowerboundis calledentropy E of thedatasourceandcanbecomputedas

E ∑x Σ

p! x" log p! x"$# (5.5)

whereΣ is the setof all possiblewordsthat areoutputby the datasource,p is the probability of theoccurrenceof anADC valuex. It canbeshown that this boundis tight for stochasticdatasources[28].Therearecompressiontechniquesthatapproachthis boundarbitrarily close.An exampleis arithmeticencoding,which is rathercomplex andalsopatented.An alternative to arithmeticencodingareHuffmancodes. They areeasierto implement,needlessprocessingpower andachieve goodresultsunlessthedistribution of input valuesis too extreme. It is importantto notethat theaveragesizeof aneventwillbe reducedby this approach,but one hasto considerthat sometimeseventscan becomelarger thanuncompressed.

A TPC is obviously not a stochasticdatasource,as adjacentADC valuesare highly correlated.Thereforeit is possibleto compressthedatato a lower bit ratethantheentropy of theADC values.Inotherwords,therearerepresentationsof the TPC datathat have a lower entropy thanthe formatsde-scribedabove. Variousmethodslike differentiation,prediction,etc. have beenevaluated,but noneoftheseapproachesyieldsresultsthataremuchbetterthantheplainHuffmancoding[25,29].

Lossycompression Thereis a somegainin usingthelosslesstechniquessuchassimpleHuffmanen-coding.Betterresultscanbeachievedwhenverysmall,noise-like changesof thedataaretolerated.Thisleadsto theso-calledlossycompressionschemes.Onekind of lossycompression,thezerosuppressiontogetherwith a run-lengthencoder, is alreadyin useat thefront-end.Thefollowing sectionfocusesonthefurtherreductionof theremainingdata.

Vector quantizationVector quantization[30] is a sophisticatedtype of quantization. Here, statisticaldependenciesbetweensuccessive datasamplesareexploited: Insteadof quantizingdatasamplesindependently,several samplesaregroupedtogetherto form a vectorof datasamples.Thenthis vectoris com-paredto entriesin a codebookandthenumberof thebestmatchingvectorfrom this codebookis

5.2 Readout 127

output,wherebestmatchingmeanslowestdistanceusingEuclideanor any othermetric, asbestsuitedfor the application. If the distribution of the input datais known in advance,the vectorquantizercanproducedatawith a very low distortionrelative to the bitrate. To be optimal, thecodebookhasto betrainedonthestatisticpropertiesof typical inputdata.This is usuallydonebe-fore thecodebookis takeninto usewith analgorithmknown asthemodifiedLBG algorithm[31].Theadaptedcodebookthenremainsunmodifiedthroughouttheactualquantization.

Vector quantization with residualencodingThoughthis approachoffers a very low bitrate, it is obvious that thereis almostno possibilityfor thevectorquantizerto changebehaviour: Sincethecodebookis preproduced,only thegivenvectorsare available to representthe output data. Even if the codebookwere well adaptedtothe data,sometimeslarge quantizationerrorscould occur(Fig. 5.29). To prevent this, we needto store the differencesbetweenthe input dataand the selectedcodebookentry, the so-calledresiduals.Theseresidualsarethenquantizedandentropy encodedto achieveanevenlowerbitrate.A quantizationof the residualsis especiallyeffective, sincethe distribution of errorsbetweencodebookentry and input datais very steep: If the codebookis sufficiently trained,the vectorquantizerwill alreadyabsorbmuchof theinput signalenergy, sotheresidualencoderwill mostlygetvaluesaroundzero. Whenthesevaluesarequantized,evenmorevaluesaremappedto zero.Here,arithmeticcompression[25] is themethodof choice.

105

104

103

102

10 0 4 8 12 14

Error

F

req

ue

ncy

p-p (1.5 bits / sample)

Pb-Pb (2.2 bits / sample)

Figure 5.29: Errordistributionof vectorquantization.

Cluster and track ModellingThebestcompressionmethodis to find a goodmodelfor the raw dataandto transformthedatainto anefficient representation.Informationis storedasmodelparametersand(small)deviationsfrom themodel.Therelevantinformationgivenby atrackingdetectoris thelocal trackparametersandtheclustersbelongingto this tracksegment.Thelocal trackmodelis a helix; theknowledgeof thetrackparametershelpsto describetheshapeof theclustersin a simplemodel[33,34]. Thetrack finder reconstructsclustersandassociatesthemwith local track segments.Note that trackrecognitionat this stagecanbe redundant,i.e. clusterscanbelongto more thanonetrack andtracksegmentscanoverlap.Oncethetrackrecognitionis completed,thetrackcanberepresentedby helix parameters.Thesearecurvaturer, startingpoint ! x# y# z" , dip angleλ, azimuthalangleϕ,tracklength,averagecharge,χ2 of thehelix fit andthenumberof clustersbelongingto this tracksegment(seeTable5.10).


Table5.10: Trackparameters

Parameter Size[byte] Type

Curvaturer 4 floatBegin x 4 floatBegin y 4 floatBegin z 4 floatDip angleλ 4 floatAzimuthalangleϕ 4 floatTracklength 2 integerClustercharge(average) 2 fixedpointχ2 2 fixedpointNumberof clusters 1 integer

Total 31

Table5.11: Clusterparameters

Parameter Size[bit]

Flagemptycluster 1∆ time 6∆ pad 6∆ clustercharge 7∆ shape 4

Total 24

In a secondstep,the deviation of the clustercentroidpositionfrom the track model(residuals),thedeviation from theaveragechargeanddeviationsfrom theexpectedshape(basedon thetrackparameters)a re calculatedfor eachcluster. Thesenumbersare thenquantizedby a nonlineartransferfunctionadaptedto thedetectornoiseanddetectorresolution(seeTable5.11).Remainingclusterscanbeoptionallykeptasraw dataarrays.

Thecompressionmethoddiscussedabove allows for a latersecondpassof calibrationanddistor-tion corrections,trackandvertex findingandfitting, anddE/dx analysis.Sincetheaimof thetrackfinding is not to extractphysicsinformationbut merelyto build a datamodel,which will beusedto collect clustersandto codeclusterinformationefficiently, any inefficienciesin track findinge.g. dueto anunprecisetrackmodelwill resultin aninefficient compression,but not in a lossofclusters;norelevantdataarelost. By fastandredundantonlinetrackrecognition,reductionfactorsof about15 canbeachieved[24].

Experimental results Thedescribedalgorithmswereappliedto a setof TPC eventsfrom theNA49Pb–Pbcollionsaswell asto ppdata.Theresultsfor centralheavy-ion collisionsareshown in Table5.12.Compressionfactorsfor pp datawith a much lower occupancy are higherby 30%–40%. In our ex-perimentswe useda vectorquantizerof lengththree,so that two vectorsmodelthemajority of hits oflengthsup to six time bins. Thesizeof thecodebookis 256entries(VQ3). This leadsto a datarateof2 bit/samplefor the vectorquantizeralone. The algorithmRVQ3 is a vectorquantizerof lengththreewith quantizedresiduals.Allowing anabsoluteerrorof onein the residualquantization,thechangeofthe numberof clusterswaslessthen10 4 andno changein the numberof trackswasobserved. Firstpreliminarytestwith ALICE simulateddataarein accordancewith theabove results[16].

5.2 Readout 129

Table5.12: Performanceof differentcompressionalgorithmson NA49 TPCdata.

Typeof encoder Entropy bit/sample Relative eventsizePb–Pb(NA49) [%]

Zerosuppressedraw data 8 100Huffman 5.8 72Transformationse.g.differentation(gif) 5.3– 6.1 66–76RVQ3 lossless 4.8 60VQ 2.3Residualvalue 2.5RVQ3 lossy(error1 ADC value) 3.8 48VQ 2.3Residualvalue 1.5VQ3 lossy 2.3 29ClusterandtrackModelling 7

5.2.2.2 Event rate reduction— Level-3 trigger

Basedon theresultsfrom theTRD analysisandtheonlinetrackinginformation,the(sub)eventbuildingcanbe aborted. The Level-3 systemis responsiblefor deriving sucha trigger decision. The relevanttracksareidentifiedby theTRD, anda processingcommandis sentto theappropriatesectorprocessorsof theTPC.TheLevel-3processorreceivesthis informationvia its Level-3network from theTRD globaltrigger. This caneasilybeimplementedat a rateof a few hundredHz without presentingany particulartechnologicalchallenges.

5.2.2.3 Modesof operation

The Level-3 systemwill incorporateall datacompressionmethodsmentionedabove. The TPC rawdataareshippeddirectly to commercialoff-the-shelfcomputers,which have FEDC(Front-EndDigitalCrate)andLDC (Local DataConcentrator)functionalities(seeSection5.32). Thenumberof machinesis mappedto the naturalgranularityof the TPC (36 sectors,180 fibre connections).This local layerof theLevel-3 systemis partof a hierarchicalsystem.Datacompression,subevent selectionandtrackrecognitionareperformedon the local level, the global layer is responsiblefor track merging acrossdetectorboundariesanddifferentdetectorsand for deriving software trigger decisions. The modeofoperationfor a combineddielectron/opencharmprogrammeis shown in Fig. 5.30. TheLevel-3 systemutilizesthefull luminosity, while producingmanageabledatarates.

No Level-3 activity — full TPC zero-suppresseddata readout

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Full readout— data exchangewith other detectors

In principle,thenew architecturedoesnotneedto haveaninfluenceonothersubdetectors.Theraw dataof otherdetectorscaneasilybedistributedto therelevantprocessorswithin theLevel-3 farm. It is alsopossibleto combinethedatafrom thereceiver processorsinto oneoverall dataflow afterthetriggerhasbeenreceived andto sendit to theDAQ. This ensuresthat,at any time, zero-suppressedandeven nonzero-suppressedraw datacansimplybereadout, thoughlimited by thetapebandwidth.

TRD Level-3 trigger and selective readout(without tracking)

The electrontracksare identified by the TRD, and a processingcommandis sentto the appropriatesectorprocessorsof theTPC.Thecoordinatesof thetrackcandidatesaredistributedthroughtheLevel-3network. After having receivedtheeventin memory, thesectorprocessorcalculatesroadsin theraw-dataspace(padnumberandtime bin) basedon the track parametersgiven by theTRD trigger. If an eventis to be readout, theprocessorreceivesan appropriatenetwork messageandtheROIs aretransferred.If not, the internaldataaremarkedavailableagainin theFEDCtablesandoverwrittenlaterby anotherevent.

TRD Level-3 trigger and selective Level-3processing

Evenif theTRD shouldperformasdesigned,it will produce,for example,40Hz triggersbut only 20Hzof them will be good events. The intention hereis to achieve this reductionby tracking the high-pt

candidatesin theTPC.If theTRD selectivity is not ashigh asplanned,thesamemethodcouldbeusedfor increasingtheselectivity by trackingthetrackcandidatesin theTPCsothattheresultingtriggerratewould not exceed10 Hz–20Hz. However, this doesnot requirethe full event. The relevant tracksareidentifiedby theTRD,andaprocessingcommandis sentto theappropriatesectorprocessorsof theTPC.

5.2 Readout 131

After having receivedtheeventin memory, theprocessorstartstrackingthedefinedtrackcandidateonlytogetherwith theprocessingcommand.Which trackcandidateto usehasto bedistributedthroughtheLevel-3 network. During thesecalculations,the processorcanalreadyreceive anotherevent to makemaximumuseof theCPU.TheLevel-3processorreceivestheinformationconcerningthecandidateviaits network from theTRD global trigger. This caneasilybeimplementedat a rateof a few hundredHzwithout presentingany particulartechnologicalchallenges.Thetrackingis donerelatively fast.

Level-3 processingfor ROI selectionand partial readout

Owingto thedata,eachreceiveralreadyknowswhatneedsto bedonewith thedatarecord.Up to 16TTCcommandsaccompanying thedatacanbedefined.In caseof a full readoutandLevel-3 processing,theclusterfinderalgorithmhasto beexecutedfirst in orderto determinespacepoints,a taskthat typicallyrequiresthe longesttime. This canbe donein parallelfor eachsector. The receiver processorswouldtakeonthistask.Thearchitectureoffersenoughleewayto beableto provideadequatecomputingpower.For example,four- to eight-fold processormodulescould be usedinsteadof simpleprocessors.Afterthespacepointsaredetermined,they couldthenbesentto a secondlayerfor tracking.This is only onescenario,many othersolutionsarepossible.Thereceiverboard,for example,canbeconfiguredin suchawaythatacertainpartof theevents(everysecond,threeoutof four, etc.)aresentdirectly to thenetworkto obtaina higherdegreeof parallelism.All thesescenarioscanbeput into practiceor evenbechangedlong aftertheinstallation.

Level-3 processingfor data compression

This is similar to the above, but trackinghasto doneonly locally, thoughredundantly, i.e. with veryopenparameters,sincetrackparametersareonly usedfor compressionpurposes.Theaim of the trackfinding is not to extract physicalinformationbut merely to build a datamodel,which will be usedtocollectclustersandto codeclusterinformationefficiently. Therefore,thepatternrecognitionalgorithmsaredifferenlyoptimisedor evendifferentmethodsmaybeused,ascomparedto theoffline tracking.

5.2.3 Ar chitecture

Thebasicideais to addan intelligent layer to theTPCreadoutandto move thedataonly if necessary.Datamoving costsmoney anddatastorageis locationindependent.Customdesignsshouldbeavoidedwherever possibleandmass-producedelectronicsshall beusedwherever possible.Theupdatedarchi-tecturewill allow a very high degreeof flexibility by reducingthe requirementfor customhardwaretoa minimumandusingstandardinterfacesinstead.In thenew scenariotheTPCandTRD detectordataaresentdirectly into the hostprocessorsof a Level-3 Trigger/DAQ processorfarm, thusavoiding thenecessityfor customfront-endsystems.By usinga standardbus, which is commonin any commer-cial off-the-shelfcomputersystemtoday, a very largedegreeof freedomis achieved with regardto theprocessorarchitecture.

TheintelligentLevel-3 layerwill make useof theinherentgranularityof theTPCandTRD readout.Eachsectoris processedin parallel,resultsarethenmergedon a higher, global level. 36 TPC sectorsareprocessedin parallelby a singleLevel-3 processingfarm per sector. For a Level-3 trigger (e einspection)only a few tracksin a few sectorshave to be analysed.A completeevent reconstruction(Level-3 processing)resultsin 12000 trackabletrackstotal, 333 tracksand125000clusterspersector(seeTable5.13andSection7.2).

TheTPCgranularityis adaptedto thegranularityof theTRD. Thereforetherewill be2 18 sectors,whicharecomposedof twosegmentseach(innerandouter).Theanalogdataispreamplified,shaped,anddigitized(10 bits). A digital filter will beusedto achieve a bettertail correction.After this, zeroeswillbesuppressed,andthedatawill beshippedto thecountingroom. In orderto optimizefibre utilizationthefront-endelectronicsimplementsasmallelasticitybuffer.


Table 5.13: Numberof tracksandclustersandsizeof raw dataof a centralPb–Pbevent (dNch/dy % 8000,570000channels,160padrows,approximately66 Mbyte/event).

Total Persector Peroptical link

TPCraw data 292MbyteWithout innercone 146– 208MbyteAfter zerosuppression 37– 52 MbyteIncludingcodingoverhead & 66 Mbyte 1.8Mbyte 0.37MbyteTotal trackabletracks 12000 333Total clusters 4500000 125000 25000Tracksummary(total) 0.4Mbyte 11kbyteCompresseddata(total) 4.2Mbyte 120kbyte

The TPC raw dataareshippedoptically to the countingroom. The protocolproposedhereis theALICE developmentDDL [16]. Appropriatemezzaninecardsareavailable in the PMC form factor.Assuminga total of 180optical links or five links persector, thedatarateper link correspondsto about67 Mbyte/s(at200Hz) leaving plentyof headroom.

In the following, we will show that the FEDC functionality presentedin the ALICE TP can beachievedby usinganarchitecture,whereanew RORCplugsdirectly into thebackplaneof acommercialoff-the-shelfcomputer, suchas a PC. The correspondingoverall architectureof a sectoris shown inFig. 5.31. The raw dataare directly transmittedto a processor’s main memoryvia the widely usedstandardPCIbus(assumingtheuseof a fast66MHz/64-bitversion,which is availabletoday),while theLDC taskrunsontheprocessor’s CPU.TheelasticitybuffersonthePCIFEDCandthemainmemoryofthecomputerfunctionsastheeventbuffer in thiscase.

Theprocessorsthathold thePCIRORCarecalledreceiver processorsin Fig. 5.31.They aregenericstandardprocessingunits,which couldbeoperatedassuch,ignoringor disablingtheRORC,or bepart

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Figure 5.32: Level-3architecture.

of thewholesystemor evenfurtheroperateasa stand-alonesingle-sectorDAQ system.Thesedifferentconfigurationsareonly softwareswitches.

Figure5.32showsasketchof theLevel-3architecture.Thereceiverprocessorsareall interconnectedby a hierarchicalnetwork. Thebusstandardusedto interfaceto theseNetwork (Interface)Cards(NIC)is labelledasPCIasthis is thestandardthatwouldbeusedif thesystemwereto bebuilt today. However,in theunlikely eventof anotherstandardbecomingaswidely acceptedby thetime thedesignhasto befrozen,it wouldbeused.At present,neithertheprecisenetwork topologynorthenetworkingtechnologyaredefined.However, it is obviousthattheTPCsectorgranularityis reflectedin thearchitectureprocess-ing theTPCdata.Therearevariouspromisingcandidatesandasin thecaseof thecomputingnodesthebeststandardnetwork will bechosenatthelatesttimepossible.Thisrequirestheuseof portablestandardnetwork APIs. Following this Level-3 network, thereareonly further processors.Note that the GDCandLDC functionality canbe performedon identicalprocessingnodes. LDC andGDC functionalitycanevenbeperformedonthesameCPUrunningdifferenttasks.Notethatthepermanentdatastorageisperformedby thecomputercentrethroughanappropriatelink. This very samelink canbeusedto feedthis general-purposeprocessingclusterwhenALICE is not runningin orderto allow theutilization ofthis systemasananalysisprocessingfarm.

Thesystemarchitectureprovidesanoverall unit thatcanalsobeoperatedoffline. In thismode,onlysomeprocessorshave an additionalcard,which may or may not be used(RORC). If theTPC is to betestedoffline,anappropriatereadoutprocesscouldthenrunsimultaneouslyin thebackground(similar to


a computerin a network which, today, canreceive andsenddatain thebackground)or a correspondinggroupof receiverPCscouldbetakenoutof theLevel-3cluster. All of thiscanbecontrolledby software,without any needto make changesto the hardware. It is possiblethat the onlineandoffline operationwill beusingdifferentoperatingsystems,somethingwhichcouldbedoneeasily. Eachcomputershouldbeequippedwith asmallharddisk for pagingandswappingin theoffline mode.

5.2.3.1 Estimated computing power

Trackrecognitionin trackingdetectorslike theTPCis usuallydonesequentially. After finding clusters,the positionandcharge of the clustersarecorrectedfor gain differences,distortions,time offsets,etc.Thesespacepointsarethenpassedon to a trackfinderwhich builds trackpiecesout of clusters.Trackpiecesarefinally mergedto globalvertex andnonvertex tracks.

A fastclusterfinderhasbeenoptimizedfor speed[35]. Preliminarytiming testsin theSTAR environ-ment[19,36] showedthatclusterfinding for 600clustersincludingdeconvolution canbedonein 10 mson a 66 MHz versionof a i960 microprocessor[37]. Thesecondstepin thesequentialtrackrecognitionschemeis a trackfinder. Thetracker combinesa numberof spacepointsto form tracksegments.Tracksegmentsarethenmergedto form vertex andnonvertex tracks.Thetrackfindercurrentlyusedin STARis basedonanalgorithmdescribedin Refs.[38,39]. Its mainfeaturesareanoptimizeddataorganizationanda conformalmappingto speedup fitting procedures.It takesabout90 ms on an ALPHA XP1000(500MHz) to find 400tracks.

The processingpower neededcould be estimatedon the basisof thesebenchmarksfor the STARLevel-3system[36]. A farmof theorderof about500PCs(year2005PCs,assumingscalabilityaccord-ing to Moore’s law) would suffice for full tracking. Partial readoutneedseven lesscomputingpower.In view of thehigh occupancy a sequentialtrackrecognitionapproachmaynot work. Trackfinding onraw TPCdataby transformationtechniquesfollowedby a clusterfitter is a possiblealternative [40,41].Shouldtherearisea needfor more computingpower for the track recognition,morenodescould beaddedto the sectorLevel-3 system,or PCI boardswith DSPsor FPGAscould be addedto performspecialfunctionsatvery highspeedon thedatastream.

5.2.3.2 Network bandwidth and latency

Thereceiverprocessorsandall otherLevel-3processorsareinterconnectedby ahierarchicalnetwork. Atpresent,neithertheprecisenetwork topologynor thenetworking technologyaredefined.Thenetworkhasto havea low latency, ahighbandwidth,andshouldallow remoteaccessto datawith low processingoverhead.

5.2.3.3 Simulations and Prototyping

Extensive simulationsof trackrecognitionmethodsanddataflow havebeenperformedfor theproposedLevel-3 Trigger Systemof the STAR [18] experimentat RHIC. A large scaleprototypeof an Level-3systemis currentlybeinginstalled. Thereis a large overlap in the physicsobjective, detectordesign,readoutelectronicsandpersonellbetweentheSTAR andtheALICE Level-3systems.

The RHIC acceleratorat Brookhaven National Laboratory, USA, will start to investigateAu–Aucollisionswith ' s ( 200A) GeV andpp collisionswith ' s ( 500GeV in 2000.TheSTAR experimentis a large scale,cylindrical, symmetric4π-detectorat oneof the RHIC interactionpoints. Datatakingwill startin 2000with a full sizeTPC(Time ProjectionChamber, r in 0* 6 m, rout 2 m) with 24 TPCsectors,6912padseach.TheSTAR Level-3 triggeris a distributedALPHA processorfarm,performingonline tracking of Ntrack 8000 particleswith a designinput rate of R 100 Hz. The components(clusterfinder, track finder) have beenbenchmarked, a large scaleprototypesystem(1/3 of the finaldesign,R 20Hz) is envisagedfor STAR datatakingin 2000.

5.2 Readout 135

The STAR trigger systemis subdivided into 4 hierarchiclevels. The Level-0 input rateis 105 Hz,the first threelevels reducethe rateby oneorderof magnitudeeach. The Level-3 trigger is supposedto reducean input rateof 102 Hz to thefinal DAQ rateof Rtape=1 Hz at anexpectedTPCeventsizeof 15 Mbyte. The tasksof theLevel-3 triggerareevent selectionsbasedupontheonline reconstructedtrack parametersof eachparticle. Two examplesare: for Au–Au collisionsthe online invariantmassreconstructionof J/ψ + e e , andfor pp collisionsthe filtering of 700 pile-up eventsin theTPC peroneLevel-0 trigger.

TheLevel-3 triggerdesignis embeddedinto theSTAR DAQ system[19]. EachphysicalTPCsectoris mappedonto oneVME crate,containinga SectorBroker, i.e. Motorola MVME-2306 VME board,carryinga PowerPC604(300MHz, VxWorks),astheTPCsectormastercontrolleranda low latency,high bandwidthnetwork interface for the raw-datatransferto the main STAR event builder and theconnectionto theLevel-3 trackfinderCPU.Moreover, eachDAQ cratealsocontainssix VME receiverboards,eachcarrying threemezzaninecardswith one Intel i960 CPUs(66 MHz, VxWorks) for dataformatting,initiating theVME raw-datatransferandrunningtheLevel-3 clusterfinder, and4 Mbyte ofdual-portedVRAM for buffering andpipeliningof raw datafor 12events.

TheLevel-3 triggerschemeconsistsof two mainparts:ThesectorLevel-3 part is mappedontoonephysicalTPCsector. It containstheLevel-3clusterfinderandtheLevel-3trackfinder. TheglobalLevel-3 part consistsof onemasterCPU for the whole STAR TPC,collectingall track dataandissuingtheLevel-3decision.Thedevelopmentof theLevel-3 triggercanbesubdividedinto two mainstages.In thefirst stage,envisagedfor 2000,eightTPCsectorsareconnected.TheLevel-3 triggerwill employ TPCdetaonly, andthe input trigger rateis estimatedto be 20 Hz. In the secondstage,all 24 TPC sectorswill beconnected.TheLevel-3 triggerwill employ additionalinformationfrom theSVT (SiliconVertexTracker)andtheEMC (ElectroMagneticCalorimeter),andthefinal designvaluefor theinputtriggerrateis 100 Hz. ProbablymorethanoneCPU perTPC sectorwill have to be used,implying programmingparallelizationtechniques.

Thecluster-finderalgorithmrunson theIntel i960 CPUs,implementedon theDAQ receiver boards.Thenumberof i960sis 18 perTPCsector, 432 for thewholeTPC.Input to theclusterfinderarezero-suppressedTPCraw data,storedin theVRAM. Theoutputclusterdata,i.e. clustercenter-of-gravity andclustertotal charge (ADC sum),aresentvia VME to theSectorBroker, which itself shipsthedataviathenetwork to theLevel-3 trackfinderCPU(expecteddatatransferrateof 3 Mbyte/sperTPCsector).The time constraintis τcluster ( 10 ms (input rate100 Hz). Benchmarkson the i960 wereperformedfor 600 clusters(realisticAu–Au scenario)on the TPC’s most inner padrow. The positionresolution(differencebetweenreconstructedandMonte-Carlogeneratedclusterpositions)of ∆ ! rϕ ", 37 µm and∆z 13 µm couldbeobtainedwith analgorithmwithin τcluster 7* 5 ms. If two clustersaremerged,anadditionaldeconvolution subroutinemustbestarted,consuming6.0%moreCPUtime thanin thecaseof two separatedclusters.

In caseof anAu–Au collision, the trackfinderalgorithmmustbeableto fit at leastN 400tracksper event per TPC sector, eachconsistingof Npoint ( 45 points (given by the numberof pad rows).The fast track finder algorithmhasspecificallybeendevelopedfor the Level-3 trigger project [39]. Itemploys conformalmapping(transformationof a circle 2 into a straightline), followed by a fit with afollow-your-nosemethod.Clusterdataaretheinput to thetrackfinder. Thetrackfindertime constraintof τtrack 110msis givenby thebuffer time of 12 pipelinedevents(12 10 ms),minusthetime beingnecessaryfor clusterfinding τcluster ( 10 ms. Thetrackfindercodewasbenchmarked on severalLinuxCPUs. Accordingto the benchmarkresults,the ALPHA XP1000500 MHz workstationwasthe onlyCPUto beableto fulfill thetimerequirement,if onerestrictsthenumberof CPUsperTPCsectorto one.Basedon theseresults,theALPHA 21264waschosenfor thefirst Level-3 trackfinder implementation.Otherarchitectures(e.g.PentiumIII 733MHz) arecandidatesfor futureLevel-3extensions.

Oneglobal Level-3 CPU receives the track datafrom all sectorLevel-3 CPUs. It is connectedto

2In theSTAR solenoidmagneticfield of B - 0. 5 T chargedparticletrackscanbeparametrisedashelices,beingvisible ascirclesin xy-projection.


thenetwork. TheglobalLevel-3 CPUperformstrackmerging for tracksof differentsectors,a Level-3decisionalgorithmbasedon the track data(e.g. invariantmassreconstruction)andissuesthe Level-3yes/nodecision.

5.2.4 Interfaces

This paragraphoutlinestheperipheralinterfacesof theALICE Level-3 intelligent readoutsystem.Theoverall architecturehasbeenoutlinedin theprevioussection.

5.2.4.1 Interface betweenTPC readoutelectronicsand Level-3.

By far the largestamountof datais producedby the TPC. The shippingof the datahasto be doneoptically in order to avoid any groundpotentialproblems. In order to minimize risk, all unnecessaryelectronicson thedetectorshouldbeavoided. 180optical links (five persector)will ship the raw dataoff thedetectorinto thecountingroom. In orderto utilize theopticallinks optimally asmalleight-eventelasticitybuffer will be implementedon the front-enddigital chip in orderto supporta very streamingdatatransferacrosstheopticalfibre. Theestimatedaggregatedatarateis expectedto beabout73Mbyte/s(at 200Hz). For adetaileddescriptionreferto Section5.1.

ThedecisionabouttheLevel-3processortypeandarchitecturewill bedelayedaslongaspossibleinorderto benefitfrom the rapidly evolving markets. On theotherhandan interfaceis requiredto allowthesecomputers,which arecurrentlynot yet defined,to receive theTPCdatafor processing.Themostobvious approachis to usethe mostwidely acceptedperipheralbus standardin industry, PCI [42], asinterfaceor whatever becomesthe PCI replacementat the time the designhasto be frozen. Basicallyall computerstodayusethis bus standardas their internalperipheralbus. Given the availability of aPCI receiver card,which connectsto theopticalfibresbeingfed by theTCPfront-end,it is conceivableto mountthis cardinto a Level-3 CPU andto directly receive the TPC datainto the processor’s mainmemory. Thisscenarioavoidstherequirementfor any front-endbussystems,suchasVME, andusestheinexpensive hostmemoryaselasticitybuffer.

General description of the data transfer components

The only databeingtransmittedfrom the countingroom to theTPC areslow controls,calibrationandmonitoringdata.Thesedata,however, areto beprovidedby theDetectorControlSystem(DCS).It is anessentialsystemrequirementthat theDCSbeindependentof any othersystemsuchasDAQ or triggerin order to have the necessaryredundancy to debug sucha complex system. Thereforean additionalpath to the front-endwill have to be implementedin any casein order to enabledownloadingof testor calibrationdata,andto readout typical slow-control datasuchastemperature,voltageandcurrentwithin thesystem.Thisexistingdatapathis anidealmethodfor redundantbut slow readoutof any TPCsector, allowing simpleoffline detectortestingwithout the requirementof a DAQ system.Given theseargumentsthereis no requirementfor a full duplex TPCdataup-link. For a detaileddescriptionof theTPCfront-endsystempleasereferto Section5.1.

TheTPC countingroom datalink consistsof threemajor components:thenetwork feed,which ispartof the front-endreadoutcardson thedetector, theactualoptical link itself andtheoptical receivercardin theLevel-3 host. Thenetwork feedis partof theTPCfront-endelectronics.However, in orderto achieve maximumflexibility while permittingcontinuedprogresson theTPC front-endelectronics,thefibre transmittersin thefront-endwill bemountedon appropriatemezzaninecardsusinga standardbus.Prototypeopticaltransmittershave alreadybeendesigned,ashasanappropriateprototypeof aPCIopticalreceiver.

5.2 Readout 137

Figure5.33: DDL logical interface.Notethatthis interfacecanbeoperatedin full-duplex modewith handshakeassketchedabove,which,however, is a feature,which is notbeingrequiredby theALICE TPC.

DetectorData Link

The DetectorData Link (DDL) [16] is the proposedstandardfor optically transmittingdatawithinALICE. The logical interfaceof the DDL canbe seenin Fig. 5.33. The DDL is composedof threehardwareitems: theSourceInterfaceUnit (SIU), thefibre, andtheDestinationInterfaceUnit (DIU). Itcanbeoperatedin full-duplex andhalf-duplex modes.Theoperationmodefor theTPCis half duplex.TheSIU is designedto bepluggedinto thefront-endelectronicsof mostof thedetectors.Thefibre willbetheonly mediumusedto shipthedatabetweentheALICE detectorsin theexperimentalareaandthecomputingroomlocatedin theaccesspit. TheDIU will beinsertedonamothercard(theRORC)insidea computerlocatedin thiscomputingroom.

A prototypeof theDDL DIU andof aVME-basedreceivercardhavebeendeveloped.A prototypeoftheDDL SIU iscurrentlyunderdesign.Itsmaincharacteristicscorrespondto theneedsof theTPC.A testof integrationof theTPCreadoutsystemandof theDDL will beperformedin 2000.Thespecificationsof theDDL will be frozenaftera successfulintegrationtestwith theTPCandtheothermajorALICEdetectors.Twomoduleshavebeenproducedto allow aneasystand-alonetestof thefront-endelectronics.Thesetwo modulessimulatea completeDDL andDAQ systemby simpleandcheapelectronicsboards[43,44].

Read-Out Receiver Card (RORC)

A prototypeFront-EndDigital Crate(FEDC)housingseveralRORCshasbeendevelepodandtestedintheNA49 ForwardTPCdetectorsystem.Thecurrentprototypeof theFEDCconsistsof a VME crate.In this prototype,thedatatransfersystemis performedthroughtheVME backplaneby theLocal DataConcentrator(LDC). TheLDC is currentlyimplementedasa single-boardcomputerrunningtheUNIXoperatingsystem.Thephysicsdatatransferredby theDDL arebufferedin aprototypeRORC,built asaVME moduleandcarryingtwo DDL DIUs [45].

PCI is the proposedstandardinterfacebetweenthe detectorelectronicsand the Level-3 system.Almost every moderncomputerusesPCI asa peripheralinterfacebus. The processorchoicefor theLevel-3systemis not restrictedby thisrequirement.Shouldanotherbusstandardbecomeaswidely used


OpticalReceiver

DataBuffer

Data-Ptr.FiFo

Pu

shr

ea

do

ut

Po

inte

rs

FPGAPCI66/64

PCIHost

memory

PCIHostbridge

Figure 5.34: Thefunctionalblocksof theTPCreceivercard(PCI-RORC).

by thetime theALICE electronicshasto befrozenit wouldbechoseninstead.

ThePCIreceivercardbasicallyconsistsof adataconverterwhichwritestheopticallyarriving datatothemainmemoryof thehostcomputer(receiverprocessor)aftersomeadditionalbuffering. Theinterfaceis PCI66/64(theoreticalbandwidthof 528Mbyte/s).At present,PCI is thebaselinebusinterface.

Figure5.34 shows the functionalblocksof this cardas it is integratedin the computer. The datawhich are received optically are converted into electronicsignals. A small elasticity buffer followsbehind,which is only necessaryin orderto cover PCI latencies.Thearriving datais thencopieddirectlyinto themainmemoryof thereceiver processorvia thePCI bususinga DMA engineon this card. Thehost’s main memory, which now alsoactsasdatabuffer, caneasilybe considerablyenlargedwithoutproducingsignificantadditionalcosts. The target memoryareais marked light grey andworks like acyclic memorybuffer. Thestartandendlong-termaddressesareconfigurable.Thereceiver cardcopiesthe arriving datasequentiallyinto the cyclic memorybuffer of the host. Thesememorymodelsaresupportedby all commonoperatingsystems,includingWindows NT andLinux. Thebuffer principleofanEthernetinterface,for example,is verysimilar. For everynewly startedevent,thestartaddressof thisevent is written into a secondbuffer memory, which is herealsoimplementedasa FIFO for thesake ofsimplicity. Thehostprocessor(s)only need(s)to readthestartaddressof an event from theFIFO. AnemptyFIFO definestheabsenceof furtherdata. If aneventhasbeeneithertransmittedto theDAQ orhasbeenrejected,thecorrespondingreadpointerin theCSRareaof thereceiver cardis updated,t husfreeingthecorrespondingmemoryarea.

This architecturedoesnot requireany flow controlbetweenthedetectorfront-endandthe receivermodules.At every point, therearelarge enoughelasticitybuffers to absorbmaximumreactiontimes.Thedatabuffer in thereceiver processorcaneasilybeenlargedto ensurethattherewill benodead-timeundernormaloperatingconditions.In caseof extremelylong triggerburstsor falsetrigger thresholds,a simplemodel,suchashigh/low-watermarking,would allow to generateLevel-3 dead-timethroughsoftware. Alternatively, a busysignalcanbegeneratedon thePCI-RORCsanddaisy-chainedinto oneTPCcounterbox,which sendsa busysignalto thetrigger. This ensuresthatno datacanbeoverwritten,while the requirementfor flow control hardwareis neverthelessminimal. The resultingsystemcanbedebuggedvery easily.

ThePCI interfaceitself canbe integratedinto theFPGA.TherearecommerciallyavailableFPGAs

5.2 Readout 139

which, today, all support64 bits and66 MHz operatingspeed.For exampleXilinx, LucentandAlteraspecificallyoffer suchchipstoday, which demonstratesthe industry’s large interestin supportingsucharchitectures.All FPGAsnamedabove have a gatecountof or exceeding30000.

Figure 5.35: ThePCI–RORCprototype

In orderto testthefunctionalitydiscussedaboveandto designthenecessarysoftware,aPCI–RORCprototypewas built. Figure 5.35 shows a picture of the device. It containsthe discussedfunctionalblocks.Theinterfaceto theopticalreceivers(DIU) is implementedasacommonmezzaninecardCMC.AppropriateDDL modulesarecurrentlybeingbuilt. Shouldotheroptical transportsbecomeattractivethey couldeasilybeimplementedasall datasignalsto themezzaninecardarefreely programmable.

Thedesignof the device is complete,including necessarydriver software. First performancetestsshow 90–100Mbyte/secof databeingmoved by the DMA engineon the card directly into the hostmemoryof theprocessorusinga 32-bit, 33 MHz PCI system.Furthertestswith a 64-bit and66 MHzsystemareplannedfor thespringof 2000.

5.2.4.2 Interface to DAQ and trigger

Theremaininginterfacesarethoseto DAQ andtrigger. Neitherof thetwo have particularlychallengingrequirements.TheTPCgroupis currentlyworking closelywith theDAQ andtriggergroupin ordertodetailtheseinterfaces.

Interface to DAQ

The detailedinterfacebetweenthe Level-3 systemand DAQ is not yet finalized. In the following afew scenariosarepresentedto illustratethe feasibility andfunctionalityof this interface. Althoughthedataacquisitionsystemandthe Level-3 trigger are functionally different systems,the processorsandnetworking hardware are expectedto be identical. The software executedon a particularnodeat aparticulartime mayhave Level-3 triggeror DAQ functionality. For exampletheprocessorshostingtheTPC receiver cardswill first performLevel-3 clusterfinding on the received zero-suppressedraw dataandforward the spacepoints to anothernodewithin the Level-3 farm, which thenperformstracking.Having acceptedtheevent thesameprocessormay thengo on to performthenecessaryeventbuilding


stepsrunningDAQ routines. Ultimately the whole Level-3 farm canbe usedasa genericprocessingfarmduring timeswhenALICE is not running. This schemerequiresthedatastructuresanddataflowarchitectureto bewell definedbetweentheLevel-3 triggerandDAQ.

Given the architectureof the Level-3 systemoutlinedabove the resultingrequirementspresentnoparticulartechnologicalchallenges.This is mainly dueto thefactthatLevel-3 activity commencesonlyafteranLevel-2 accept.In theALICE detectorsystemall datais copiedinto elasticitybuffersuponanLevel-2 accept.Therefore,thereis no explicit maximumlatency for Level-3. Theraw dataof theTPCis receiveddirectly into Level-3,wherethebufferscanbemadeeasilylargeenoughto accommodatetheappropriatelatencies.Thenthedata,which hasto be moved from theLevel-3 receiver processors,areonly the spacepointsandtrack segmentsin caseof Level-3 processingor somederivative of the rawdatain caseof an accept.The Level-3 spacepoints,however, aresmall andthe datashippedto DAQafteranLevel-3 acceptareeithercompressedfurtheror appropriatelyreducedwith appropriatetriggerselectivity. However, at any point in time the systemcanbe scaledup suchthat it operatesexactly asdescribedin theTP.

Theflow controlandsynchronizationbetweenLevel-3andDAQ requirethedefinitionof amultipro-cessornetwork softwareinterface,supportingthefollowing functionality. After theraw datahave beenreceivedby areceiverprocessorit will startprocessingthemaccordingto eventtype,which is partof thedatastream.After completion,theresultingspacepointsmayhave to beshippedto thenext processinglayer, handlinga completesectorfor track segmentprocessing.This requirestheability to move bulkdata. The processornodeneedsto know the next available target nodedownstream,which is anothernetwork messageto bereceivedandprocessed.At somepoint latertheLevel-3decisionhasto bemulti-castto all receiver processorsholdingtheappropriateraw data.Uponreceiptof theLevel-3decisionfora giveneventit is discardedupona reject,or theLevel-3 compressionandtheDAQ processis activateduponan accept.Oncethe subevent is built it is shippedto the appropriateprocessorupstream,whichagainhasto reportits availability via thenetwork. Otherrequirementsfor Level-3 arethe interfacestootherdetectorsubsystemsin orderto allow processingof a completeevent. Thereforea datainput pathinto Level-3needsto beprovided.

Interface to trigger

SincetheLevel-3 systemis activatedonly uponanLevel-2 acceptthereis no direct feedbackrequiredto the lower trigger layers. Theprocessingfarm couldoperatecompletelydatadriven. But in ordertohave someredundancy andthepossibilityof errorchecking,theLevel-3 systemwill implementa TTCreceiver like any otherALICE subdetector. This receiver will allow matchingof theeventsarriving atthevariousreceiver processorsto thetriggersasissuedby theLevel-1 triggersystem.

5.2.5 Data acquisition and Level-3 software

Several aspectsof the proposedsysteminfluencethe software of the Level-3 system. The proposedLevel-3 systemwill be built from standardlow-costcomponents.The connectionbetweenthe nodeswill be via a standardnetwork. The connectionto the detectorfront-endelectronicswill be designedaroundwhatever bus is widely acceptedat thetime. Sinceit is highly desirableto usetheLevel-3 farmfor real-timeandoffline processingit is mandatoryto make switchingbetweenthesemodesassimpleaspossible.Theexpectedlow price for eachnodeandthehigh numberof nodeswill limit the choiceof operatingsystems.The Level-3 trigger algorithmsarelikely to changefrequentlyandareunlikelyto beimplementedby real-timeexperts.Theinput rateinto thesystemis in theorderof a few hundredHz andthereareseveral levelsof elasticitybuffers throughoutthesystemallowing simpleflow controlmechanismsto beused.Assuminga network thatdoesnot createa largenumberof interruptsthereal-time requirementsfor suchasystemaremoderate.A possiblesoftwarearchitecturethataddressestheseboundaryconditionsis describedin thefollowing.

5.2 Readout 141

5.2.5.1 Operating systems

TheALICE collaborationhasnot yet selectedacommonoperatingsystem.However, for thetimebeingwe will illustratethat therequirementsfor sucha systemcanbemetby at leastonecurrentlyavailablesystem.We chooseRT-Linux for this purpose.Whatever commerciallyavailablecost-effective systemswill beused,it is very likely thatLinux will berunningon them.Severalreal-timeextensionsto Linuxcan be used. Interrupt latenciesbetter than 20 µs have beenreportedfor Intel-basedmachines(see,for example,Ref. [46]). However, becauseof the designof the Level-3 system,no interruptswill begeneratedfor theprocessof moving thedatavia theRORCsinto thememoryof themachines.Thelargepossiblebuffer sizein themainmemoryof thenodeswill reducetheneedof real-timebehaviour evenfurther.

All requiredtools for softwaredevelopmentareavailableon Linux andowing to their similarity tocommonUNIX systemsminimal training for systemsprogrammingwill berequired.Thestability androbustnessof the kernelhave beendemonstratedin many farm andclusterapplications.A UNIX-lik eoperatingsystemhastheadditionalbenefitthatoffline reconstructioncodeswill run with only minimalchangeson theLevel-3 farms.Thiswill allow for adualuseof thesystemfor triggeringandreconstruc-tion work. Becauseof thewidedistribution of Linux systems,machinesfor developmentwill bewidelyavailable.

5.2.5.2 Online processing

Two principalgroupsof taskswill run on theLevel-3 systems.Onethat controlstransportof dataandcommunicationandmanagesthe processingof the events,andonethat performsdataanalysistasks.While thefirst will bequitestablefrom thebeginning,thesecondhasto bemodifiedfrequentlyto copewith changesin runningconditionsandthephysicsprogramme.Thefirst groupwill have to dealwithsomereal-timeaspectsandhasto run only on theLevel-3 systems.This partof thesoftwarestructurecanbecalledcontroltasks.

Thesecondwill morecloselyresembleoffline codeandit is highly desirableto usealmostidenticalcodein Level-3 andin thesimulationof Level-3. This groupof taskscanbereferredto asprocessingtasks,which shouldnot be awareof time. They shouldbe calledinsidea framework provided by thecontrol tasksandin caseof exceedingtheallowedtime limits thecontrol taskwill aborttheprocessingandstarttheclean-upcodeof theprocessingtask.Thiscleanupcodewill bethemaindifferencebetweentheonlineandtheoffline versionsof theLevel-3 processingcode.Thecontrollertaskswill make heavyuseof portablestandardnetwork APIs to allow flexible andearlyimplementationof thesystem.

A key componentof the proposedsystemis the ability to processthe raw data,performingtrackrecognitionin real-time. It is designedto utilize the informationfrom theTPCandfastdetectors.Thesystemshouldbeflexible enoughto beexpandedin a naturalmannerto includetheothertrackingde-vices. Track recognitionof trackingdetectorslike the TPC is usuallydonesequentially. After findingclusters,thepositionandcharge of theclustersarecorrectedfor gaindifferences,distortions,time off-sets,etc.Thesespacepointsarethenpassedonto atrackfinderwhichbuilds trackpiecesoutof clusters.Trackpiecesarefinally mergedto globalvertex andnonvertex tracks.If trackdensitiesespeciallyin theinnerpadrows get too high, clustersstartto overlapso thata simpleclusterfindercannotrecognizeorresolve mergedclusters.Trackrecognitionmethodsfor trackfindingonraw datalike templatematchingor adaptive generalizedHoughtransforms[40,41] have to beemployed. Thesemethodshave beenusedin NA35 andtestedin NA49.


Thecontroltaskswith theircomplicatedstructurewill benefitfrommodellingthesystemusingasoftwareengineeringmethodologylike OMT. To allow a large numberof participantsin the Level-3 effort tounderstandandmaintainsoftware, a widely acceptedandavailable programminglanguagehasto be

used.At themomentthecandidateis ANSI C++, which is usedfor theonlinetrackingin STAR andonwhich theALICE simulationframework (AliRoot) is based.

Someof thecomputationaltaskscanberealizedin hardware.Huffmancompressionchipsarecom-mercially available. The vector quantizercanbe heavily pipelinedwith several parallel stagesand atree-encondingschemewhich reducescomparisionsbetweendatawordsby a factorof four. Roughesti-matesshow thatavectorquantizerwith arbitrary, tablebasedmetricandtablebasedresidualencodercanbeimplementedin oneratherlargeXILINX XC4000FPGAandasmallexternalRAM atarateof about60 million samplespersecond[47]. Eventrackrecognitionprocedures,e.g.Houghtransformation,canbeimplementedin anFPGAif necessary.

143

6 Material budget

Theperformancerequirementsof theALICE TPC aremorestringentthanfor any otherTPC everbuilt. Themajorgoalis to handletheharshLHC environment,in particular, thehighparticledensityandthe high interactionrate. Sincethe amountandpositionof materialtraversedby particlesin the innerdetectorshasan impacton the performanceof the outerdetectorsandconsequentlyon the physics,itrepresentsa majoroptimizationissuein thedefinitionof theTPClayout,aswell asin several technicalchoices.Thesimulationof theTPCmaterialmustbeveryaccuratein orderto provideacarefuldetermi-nationof thematerialin termsof radiationandinteractionlength.In orderto estimatetheimportanceofdifferentcontributionsandhencethetargetsfor optimization,theanalysisof thematerialbudgetis splitinto two parts:

1. The cylinder materialthat is uniformly distributedaroundthe beampipe, i.e. the inner contain-mentandfield-cagevessels,andtheoutercontainmentandfield-cagevessels.This is indicatedinFig. 6.1a.

2. Thefield cagematerialthatis nothomogeneouslydistributedover thecylinder, i.e. in theϕ direc-tion, for exampletheMylar strip supportrods.This is shown in Fig. 6.1b.

Figure 6.1: Schematicof theTPCmaterialin η (a),andϕ (b), space.

Viewedfrom theinteractionpoint, thematerialin thepathof particlesis ‘scanned’in η andϕ slices,takinginto accountthedensityandcompositionof thematerialtraversed(shown in Tables6.1,and6.2)andits positionin space.Thiswasperformedwith theCAD tool Euclid,andtheresultsin termsof X/X0

aregivenin thenext sections.

6.1 Estimate of radiation length in η space

As canbe deducedfrom Fig. 6.1athe materialdistribution in η spaceis generallysmoothandhomo-geneous.Theonly step-like massconcentrationin η spacecomesfrom thecentralelectrodelocatedatη 0. Sinceits volumetricoccupancy is smallcomparedto themaximumη acceptanceot theTPC,wescanthematerialwith andwithout thepresenceof thecentralelectrode.Furthermore,to avoid singular-ities in thematerialdistributionsfrom individual components,theorigin of particletrajectorieshasbeensmearedover theLHC beaminteractiondiamond,i.e. / 5* 3 cm ( z ( 5* 3 cm. Theresultsareshown inFigs.6.2aand6.2b.

144 6 Material budget

Table6.1: Propertiesof thematerialof theTPCvessels,gasandouterthermalscreen.

Thick- ρ X0 λ0 X/X0 λ/λ0

Material ness [g/cm3] [g/cm2] [g/cm2] [%] [%][cm] ([g/l]) ([cm]) ([cm])

Field-cagestructureTedlar 0.04 1.71 44.77(26.18) 84 (49.12) 0.15 0.08Fibre(Kevlar) 0.24 1.45 44.86(30.94) 83.9(57.86) 0.60 0.41Nomex (29kg/m3) 7.5 1* 45 0* 02 41.28(1424) 85 ( 2931) 0.52 0.25Air (within Nomex) 7.5 (1.293) (30420) 90 (107305) 0.02 0.01Mylar 0.05 1.39 39.95(28.7 ) 85.7(61.65) 0.17 0.08Al 0.05 2.70 24.01( 8.9 ) 106.4(39.4 ) 0.56 0.12H2O 0.1 1.00 36.1 (36.1 ) 84.9(84.9 ) 0.27 0.01Ne–CO2 [90–10] 178 (1.116) (30755) (101152) 0.58 0.17CO2 32 (1.977) (18310) 90.5( 45776) 0.18 0.07

Total 3.05 1.20

Table6.2: Propertiesof thematerialof theTPCfield definingnetwork.

Thick- ρ X0 λ0 X/X0 λ/λ0

Item/Material ness [g/cm3] [g/cm2] [g/cm2] [%] [%][cm] ([cm]) ([cm])

SupportrodsectorSupportrod (i) a 2.5 1.20 41.84(34.6 ) 83.9(69.9) 7.225 3.57Supportrod (o) a 2.5 1.20 41.84(34.6 ) 83.9(69.9) 7.225 3.57

HV sectorHV cable/polyethylene 2.7 0.93 44.7 (48.15) 78.8(84.7) 5.61 3.4HV cable/copper 0.31 8.96 12.9 ( 1.43) 134.9(15.5) 21.67 2HV rod (hollow) a 1 1.20 41.84(34.6 ) 83.9(69.9) 2.89 1.43Supportrod (i) a 2.5 1.20 41.84(34.6 ) 83.9(69.9) 7.225 3.57

ResistorrodsectorResistorrod a 2.5 1.20 41.84(34.6 ) 83.9(69.9) 8.71 3.57Strip hook/copper 0.01 8.96 12.9 ( 1.43) 134.8(15.5) 0.69 0.06Resistor/iron 0.05 7,87 13.84( 1.76) 131.9(16.7) 2.84 0.29Resistor/graphite 0.3 2.27 42.7 (18.8 ) 83 (36.5) 1.59 1.64Supportrod (i) (hollow) a 1 1.20 41.84(34.6 ) 83.9(69.9) 2.89 1.43

LaserrodsectorLaserrod (hollow) a 1 1.20 41.84(34.6 ) 83.9(69.9) 2.89 1.43Supportrod (i) a 2.5 1.20 41.84(34.6 ) 83.9(69.9) 7.225 3.57

Field-cagestructure 3.05 1.20

aMacrolon(i) — inner(o) — outer

6.2 Estimateof radiationlengthin ϕ space 145

a b

Figure 6.2: Fractionalradiationlengthof theTPCin η spacewithout (a)andwith (b) thecentralelectrode.

6.2 Estimate of radiation length in ϕ space

Unlike the smoothmaterialdistribution in η space,therearedistinct high-massconcentrationswithintheϕ acceptanceof theTPC(seeFig. 6.1b). Thesearedueto thepresenceof theelectricfield definingnetwork, in particular, the supportrodsof the Mylar strips. Someof theserodsarehollow andhavethereforea differentmassbecauseof additionalfunctions,suchashousingthevoltagedivider resistorchain,thehigh-voltagefeed,andtheopticsfor thelasercalibrationsystem.Weareinvestigatingwhethertheothersupportrods,which aremadeof solid Macrolon,canalsobereplacedby Macrolontubes,toreducepassive materialin theseregionsevenfurther. Theindividual massandpositionwithin theTPCvolumearetaken into accountfor thecalculationsover theentireϕ range(00 to 3600 ). In Fig. 6.3awegive an accountof thematerialconcentrationasseenby particlesof infinite momentumtraversingtheTPCin all ϕ directions.Theϕ-scanincludesthesteelrails of theTPClocatedatϕ 900 and2700 .

Figure6.3bgivesa detailedview (zoom)of theequivalentradiationlengthof a laserrod placedatϕ 1300 in Fig. 6.3a.

a b

Figure 6.3: Map of the fractionalradiationlengthof the materialin ϕ space(a), andan expandedview of thepeaklocatedat ϕ % 1301 , showing thedetailedmassdistribution of theindividual componentsof oneof thelaserrods(b). Thetop distribution is thesumof thethreedistributionsbelow.

147

7 Detectorperformance

7.1 Requirementsand detectorparameters

The Time ProjectionChamber(TPC) is the main trackingdevice of the ALICE experiment. Its tasksare track finding, momentummeasurementsand particle identificationby dE/dx. A good two-trackresolution,requiredfor correlationstudies,is alsooneof the main designgoals. Besides,significantattentionshouldbe paid to the possibility of track matchingwith otherdetectors.Theserequirementsareconflicting. Higher readoutgranularityrequiredfor goodtrack-findingefficiency is limited by thedatavolumeandby thesignal-to-noiseratio. Goodmomentumresolutioncalls for a ‘cold’ drift gasoflow diffusion(diffusionconstantsshouldnot significantlyexceed200µm/' cm). This however requiresa high drift field (400V/cm) to secureanacceptabledrift time of 2 100µs. Largesignal-to-noiseratiorequiresadrift gaswith largeionization(largeZ). Ontheotherhand,agaswith a largeZ leadsto a largespace-charge in thedrift volumeandthusto largerdistortions.Also multiplescatteringis largerfor suchgases.Moredetaileddiscussionon thechoiceof thedrift gascanbefoundin Ref. [1].

TheTPCdesignshouldsecurethefollowing detectorperformancein anenvironmentcorrespondingto dNch/dy 8000:

drift time ( 100µs,which imposesa10%upperlimit for theCO2 content;

dp/p 2 1* 2%–1* 5% for MIP andabout5%for a5 GeV/c electrons(stand-alone);

dE/dx 10%in thehigh track-densityregion;

signal-to-noiseratio for MIP betterthan 2 20:1in thecaseof smallpadsin innersectorsand30:1for largerpadsin outersectors;

track-findingefficiency largerthan90%;

trackmatchingto otherdetectors2 90%;

two-clusterresolution2 1 cm in bothrϕ andzdirections;

thedistortions,in general,shouldnotbesignificantlylargerthanthespace-pointresolution(a fewhundredmicrons)whichcallsfor theuniformity of bothelectricandmagneticfieldsandalsofor agaswith a low Z andlargeion mobility.

7.2 Simulation of TPC response

7.2.1 Micr oscopicsimulator

The ALICE TPC is supposedto operatein ratherextremeconditions,becauseof very high particlemultiplicity, the event geometry(inclination anglesof tracks),anda large drift length. The detectorperformance(i.e. resolutions,trackingefficiency, datavolume,etc.) dependson many parameters,suchas

energy lossperunit length;

diffusionof electronsduringthedrift;

electronattachment;

148 7 Detectorperformance

gasgain,includingfluctuations;

padandtimeresponse,determinedby thereadout-chambergeometryandelectronicsparameters.

In orderto studytheinfluenceof theseparametersandto choosetheoptimalones,areliableMonteCarlosimulationis required.This led to thedevelopmentof a microscopicsimulatordescribedin Ref. [2]. InAliRoot [3], thesimulationcodeusedby theALICE Collaboration,this simulatoris incorporatedintotheGEANT3.21-basedtrackingpackage.Below, thedetailsof themicroscopicsimulatoraredescribed.

7.2.1.1 Physicsof the AliRoot Monte Carlo

Ionization in gas

The ionization in the gasproceedsin two stages.Firstly, electromagneticinteractionsof the primaryparticlewith theTPCgasleadto thereleaseof primaryelectrons.Thestatisticsof theprimary interac-tionsimpliesaPoissondistribution of anumberof primaryelectrons,asshown in Fig. 7.1.Thedistance

Number of primary electrons per 1 cm for MIP0 5 10 15 20 25 30 35 40 45 50

arb

itra

ry u

nit

s

0

1

2

3

4

5

Figure7.1: Distributionof numberof primaryelectronsper1 cmfor MIP in 90% Ne,10% CO2.

betweencollisionsleadingto theprimaryionizationis describedby anexponentialdistribution (see,forexample,Refs.[4,6])

P! s"3 1D

exp/ sD#

wheres is thedistancebetweentwo successive collisionsandD is themeandistancebetweenprimaryionizations:

D 1Nprim ) f ! βγ" #

whereNprim is thenumberof primary electronsper1 cm producedby a MIP, and f ! βγ" is theBethe–Bloch curve. In Fig. 7.2 theenergy-lossdatafor 90% Ar, 10% CH4 [5,6] areplottedtogetherwith theparametrizationproposedby theALEPH Collaboration[6],

f ! βγ"4 P1

βP4) P2 / βP4 / ln P3 5 1

! βγ" P5#

7.2 Simulationof TPCresponse 149

βγ10

-11 10 10

210

36

104

I/I0

0.87

18

1.281.4

1.681.88

2

2.2

2.49

Figure7.2: Bethe–Blochcurvefor 90% Ar, 10% CH4, datafrom Ref. [6]

whereP1 : : : 5 arefreeparameters.Thefit to thesedatagives

P1 0* 762 10 1,

P2 10* 632,

P3 0* 134 10 4,

P4 1* 863,

P5 1* 948.

This parametrizationis alsousedin oursimulation,astheenergy-lossdatafor Ne in the1; β2 regionareratherpoorandthebehaviour of theNe-basedmixturesis very similar to thatof Ar-basedones[5].

Theenergy transferto atomicelectronscanbedescribedby thePhotoAbsorbtionIonizationmodel(PAI) — seefor exampleRefs.[7,8]. In mostcases,if oneneglectstheatomicshellstructure,it obeysthe 1/E2 rule. However, for light gasesthe dataindicatea slightly steeperdependence[6, 9]. In oursimulationsweuse1/E2 : 2 for 90% Ne,10% CO2. Thelowerenergy cut-off is equalto thefirst ionizationpotential,theupperoneis equalto 10keV. Abovethisenergy, theGEANT simulationpackagegeneratesδ-rays.Thecomparisonof oursimulationswith thoseusingthePAI modelshowssatisfactoryagreement.

Generation of secondaryelectrons

With sufficient kinetic energy, theprimaryelectroncanionizeatomsandproducesecondaryelectrons,creatinganelectroncluster. Thetotal numberof electronsin suchclusteris givenby

Ntot Etot / Ipot

Wi5 1 #

whereEtot is the energy loss in a given collision, Wi is the effective energy requiredto produceanelectron–ionpair andIpot is thefirst ionizationpotential.Theclustersareassumedto bepoint-like, and


E (keV)1 2 3

<4 5=

6>

7 8?

9@10 20 30

<40 50=

Eff

ecti

ve r

ang

e (c

m)

108 -2

108 -1

1

Figure 7.3: Effectiverangeof anelectronin Ne.

no distinctionbetweenprimaryandsecondaryelectronsis made.This seemsto bejustified,asmostofthe fractionalenergy lossesin a givensteparesmall (on averageabout3–4electronsareproducedin asinglecollision) andthe effective rangeof low-energy electronsis small (seeFig. 7.3) comparedwiththe lengthscaleof diffusion during the drift towardsthe readoutchambers.Even the mostenergeticelectrons,i.e. thosewith energy equalto theuppercut-off (10keV), createin themagneticfield of 0.2Tablobof radius 2 160µm only. Theenergy lossandtotalnumberof electronsper1 cmfor MIP (πwitha total momentumequalto 400 MeV/c) in 90% Ne, 10% CO2 areplottedin Fig. 7.4 andin Fig. 7.5,respectively.

Diffusion of electrons

During thedrift, theelectronsaresubjectto diffusion. Theelectroncloud,afterdrifting over a distanceLdrift, canbedescribedby the3-D Gaussiandistribution:

PA xB yB zC4D 1E2πδT

exp / A x / x0 C 22δ2

T

F 1E2πδT

exp / A y / y0 C 22δ2

T

F 1E2πδL

exp / A z / z0 C 22δ2

L

B

where A x0 B y0 B z0 C is theelectroncreationpointand

δT D DTE

Ldrift

δL D DLE

Ldrift BDT andDL arethediffusionconstantsin thetransverseandlongitudinaldirection,respectively.

Electron attachment

During thedrift, electronscanbeabsorbedin thegasby the formationof negative ions. This is duetothe presencein the drift gasof substanceswith large electronaffinities suchasoxygen. Accordingto


Energy loss (keV/cm)

0 1 2 3 4 5 6 7

arb

itra

ry u

nit

s

0

1

2

3

4

Figure 7.4: Energy lossof electronsper1 cmfor MIP in 90% Ne,10% CO2.

Total number of electrons per 1 cmG0 10 20 30 40 50 60 70 80 90 100

arb

itra

ry u

nit

s

0

1

2

3

Figure 7.5: Totalnumberof electronsper1 cmfor MIP in 90% Ne,10% CO2.

experiencefrom NA49 [9], the probability for an electrondrifting in our gasto be capturedby an O2

moleculeis 1%per1 m drift per1 ppmof O2 (at thelevel of a few ppmof O2). Thisvaluehasbeenusedin thesimulations.

E H B effectnear the anodewir es

It hasbeenassumedthat the electricandmagneticfields in the drift volumeareuniform andparallel.This, however, is not truecloseto theanodewires,wheretheelectricfield becomesradialwith respectto eachwire. Thustheelectronsexperience,becauseof theLorentzforce,adisplacementalongthewiredirection.If anelectronentersthereadoutchamberat thepoint A x0 B y0 C , it is displacedin thex-direction(assumingthatthewiresareplacedalongthex-axis).Thenew x-positionof theelectronis thengivenby

x D x0 I ωτ F A y J y0 CKB


wherey is the coordinateof the wire on which an electronis collected,andωτ is the tangentof theLorentzangle.

7.2.1.2 Signal generation

Avalancheat the anodewir e

An electronarriving at the anodewire createsan avalanche,the amplitudeof which is determinedbythehigh voltageappliedto thewire. This amplitudeis subjectto fluctuations,which canbedescribed,accordingto Refs.[4,6], by anexponentialdistribution

PA qC3D 1qF exp J q

qB

whereq is theaverageavalancheamplitude.Theresultingavalancheis theconvolutionof single-electronavalanches.

Shapingand sampling time signal

Thetimesignalis obtainedby folding theavalanchewith theshapingfunctionof thepreamplifier/shaper.This signalis thensampledwith a givenfrequency. In thepresentsimulationsa 3σ truncatedGaussianhasbeenusedto describethepreamplifier/shaperresponse.Thepreamplifier/shaperefficiency, i.e. theratio of integratedcharge to thaton thepreamplifier/shaperinput, wasassumedto beequalto 0.4. Themeasurementsby theSTAR Collaboration[10] indicatethatit shouldbepossibleto obtainlargervalues.This wouldbevery welcomeasit couldleadto abettersignal-to-noiseratio.

Chargeinducedon padsand pad responsefunction

An electroncollectedon the anodewire inducesa charge on the padplane. This charge is integratedover the padarea. It is the padresponsefunction (PRF)which characterizesthe readoutchamberanddeterminesthe intrinsic resolution. If the charge inducedon the padplanehasa distribution given byQA xB yC , thePRFis definedas

PRFA xB yCLDSQA xMNB yMOC dS B

whereS is thepadarea.The 2-D induced-charge distribution hasbeencalculatedaccordingto Ref. [11], and is shown in

Fig. 7.6andColourFig. V. The2-D padresponsefunctionfor rectangularpadsis shown in Fig. 7.7andColourFig. VI. Thestepsin onedirectionindicatethepositionof theanodewires.

Themethodwe adoptedfor thePRFcalculationis a generaloneandcanbeusedfor classicalwirechamberswith differentpadgeometries(rectangular, chevron,radiallyoriented)aswell asfor theGEM-typereadout.(Thedetailsof theGEM readoutcanbefoundin Section8.2.) This algorithmalsoallowsthesignalcomingfrom thewireson theneighbouringpadrows (‘crosstalk’)to betakeninto account.

Electronicsnoiseand the conversion gain

It hasbeenassumedthattheelectronicsnoisecanbedescribedby aGaussianwith r.m.s.equalto 1000e.Theconversiongainwaschosensuchthatσnoisecorrespondsto 1 ADC count.

Digitization

Theanalogsignal,obtainedby integrationof signalscreatedby individual electrons,is digitizedusingagivendynamicrangeof theelectronicsandapplyingzerosuppression.


pad direction (cm)P-1

-0.50Q 0.5

Q 1

pad row direction (cm)

R

-0.5

0Q0.5

Q0Q

0.02Q0.04Q0.06Q0.08Q

Figure 7.6: Induced-chargedistributionaccordingto Ref. [11]. Normalizationis arbitrary.

pad direction (cm)P-1

-0.50Q 0.5

Q 1

pad row direction (cm)

R

-0.5

0Q0.5

Q0Q

0.2Q0.4Q0.6Q0.8Q

Figure 7.7: Padresponsefunctionfor rectangular4 S 7T 5 mm2 pads.Normalizationis to unity.


7.2.1.3 Parametersusedin the simulations

In this sectionthemainparametersof thedrift gas(Table7.1), readoutchambers(Table7.2),andelec-tronics(Table7.3)arelisted.

Table7.1: Parametersof thedrift gas.

Item Value

Numberof primaryelectronsfor MIP 14.35First ionizationpotential 20.77eVEffective energy for e–ionpair creation 35.97eVDrift velocityat 400V/cm 2.83cm/µsDiffusionconstants(DT D DL) 220µm/

Ecm

Tangentof Lorentzangle(ωτ) 0.15Oxygencontent 5 ppm

Table7.2: Parametersof thereadoutchambers.

Item Value

Paddimensions:84U 1 cm V r V 132U 1 cm 4 H 7U 5 mm2

134U 6 cm V r V 198U 6 cm 6 H 10 mm2

198U 6 cm V r V 246U 6 cm 6 H 15 mm2

Anodewire spacing 2.5mmAnodewire to paddistance:84U 1 cm V r V 132U 1 cm 2 mm

134U 6 cm V r V 246U 6 cm 3 mmGasgain 2 H 104

Table7.3: Parametersof theelectronics.

Item Value

Noise(r.m.s.) 1000e (in thesystem)Dynamicrange 2 V, 10bitsConversiongain 12 mV/fCShapingtime 190nsFWHMSamplingtime 200nsZerosuppression 3 ADC counts

7.2.2 Background and detector load

The total load on the readoutchambersis estimatedfrom the obtainedionizationdensityin the driftvolume in centralcollision events. The charge doseper centimetreof wire andper yearof operationcanthenbe calculatedby integratingin time alongwith the electronamplificationandthe trigger ratefor eachkind of events. The resultsaresummarizedin Table7.4 for both leadandprotonbeams.Allheavy-ion beamsareconsideredto be leadbeams.A charged-particlemultiplicity for centralcollisioneventsof 8000per unit of rapidity hasbeenassumed.Minimum-biasevents,about90% of the total

7.3 Track reconstruction 155

Table7.4: Estimationof themaximumdosepercmof anodewire for differentcases.

Charge/cma Triggerrateb Runtime Dose Currentc Currentd

[mC/cm] [Hz] [s] [mC/cm.yr] [nA] [µA] (ROC)

Pb–Pb 2U 3 H 10W 13 200(1+1/5) 106 1.1 3.36 7.2Pb–PbROC 5U 8 H 10W 16 8000(0.1+0.9/5) 106 0.018 0.19pp 2U 3 H 10W 16 400 107 0.018 0.15 0.0012ppROC 6U 4 H 10W 19 105 107 0.013 0.008

aPercentralcollisionevent,averagedalongtheinnermostregion 82 X R X 102cm,beforeamplification.bIncludesthemultiplicity correction(onaverageonefifth of central)for minimumbiasevents(90%of all interactions).cTotalcurrentin thedrift volumecorrespondingto oneinnerchamber.dCurrentin oneinnerreadoutchamberafteramplification.

interactions,areconsideredto haveonaverageonefifth of thismultiplicity. Theaveragemultiplicity forppinteractionsis takento be1000timeslower thatfor Pb–Pbminimum-biasevents.Thechargereleasedin the0.7 cm gapbetweenthepadplaneandthegating-gridelectrodeof the innerchambers,which isalwaysactive irrespectively of thetrigger, is takeninto accountseparately(columnslabeledPb–PbROCandpp ROC in Table7.4). Theoverall expecteddose,about12 millicoulombspercentimetreof wire in10 years,is rathermoderate.Giventhegasmixtureandthematerialsforeseenin thedetectorassembly,sucha charge load doesnot challengeeither the short-termor the long-termstability of the readoutchambers.

Thecurrentsin the inner volumeof theTPCarealsogiven,both in thedrift volume(trigger inde-pendent)andin thereadoutchambers,afteramplification.

7.3 Track reconstruction

7.3.1 Tracking envir onment

7.3.1.1 Detectoroccupancy

The performanceof the TPC working in the high track-densityenvironmentstrongly dependson thedetectoroccupancy. The latter shouldbe reducedto the level that securesthe performancementionedin Section7.1. Thus,detailedstudiesof thedependenceof theoccupancy on thedetectorparametersofwereperformed.Theoccupancy O canbedefinedastheprobabilityof having asignal(digit in thepad–time-binplane)abovethreshold(zerosuppression).In thesestudieswedefined(seealsoSection4.1.2.2)occupancy astheratio

O D NABOVE

NALLB

whereNABOVE is thenumberof digits above threshold,NALL is thetotal numberof digits.The numberof digits above the thresholddependson the particle densityF and on the effective

(mean)clusterareaseff . Neglectingcorrelationbetweenclustersoneobtains

O D 1 J exp AYJ F F seff CZUNoticethatin thelow particle-densityenvironment,theoccupancy dependslinearlyon F andseff , whilefor ahigh particledensitysaturatesexponentially.

Theparticledensityat thepositionr from theinteractionpoint canbeexpressedas

F [ K A α C 1r2 cosα B


whereK A α C is givenby theinteractionmechanism,α D]\ (dS,r ), dSbeingtheunit areain thepadplane.The r W 2 dependencesuggeststhepossibilityof decreasingoccupancy by increasingthe lower radiusoftheTPCsensitive area.However, becauseof thesaturationin thehigh particle-densityenvironment,thereductionof occupancy by 50%attheinner-pad-row radiuswouldrequireits increasefrom 84to 140cm.This is incompatiblewith therequirementof goodtrackmatchingwith theInnerTrackingSystem(ITS).Therefore,onehasto reducetheoccupancy, reducingtheeffective clusterarea. Assuminga Gaussianshapeof theclusters,theeffective areaseff canbeexpressedas

seff D 2πσtσp lnKMax

thresholdB

whereσt B σp arether.m.s.of theclusterwidth distributionsin thetime andpaddirections,respectively.Themaximalamplitudewithin aclusteris givenby

KMax D Qch

2πσtσpB

whereQch is theintegratedchargeof acluster.The effective areaof a clusterand thus the occupancy can be reducedby decreasingthe cluster

width in bothpadandtime directions.This width, in general,dependson thediffusion,on theresponsefunctions,andon the padlength. For a given gasanddrift field the diffusion is no longera variablefactor. Thus,in orderto decreasetheclustersizeonecanoptimizethelasttwo parametersonly. This ledto thepresentchoiceof padgeometryandpreamplifier/shaperwidth, andto thereductionof thePRFintheinnerpartof theTPCdown to about2 mm. A detaileddiscussionis presentedin Section4.1.2.1

In Fig. 7.8 the occupancy as a function of z-coordinateis plotted for different pad-row radii. Itreaches,for smallη (centralregion), 44%for thepadrow at a radiusof 90 cm anddropsto 10%–15%for 250cm. In ColourFigs.VII andVIII thesimulatedclustersareshown in theinnermostandoutermostpadrows, respectively. Theaverageoccupancy is about22%.

7.3.1.2 Distortions

In general,the distortionsof theelectrontrajectoryin thedrift volumearedueto imperfectionsof theelectricandmagneticfieldsandtheir relativeorientation.In thecalculationspresentedin thissectionwetook into account

^ thenonuniformityof theelectricfield becauseof thepositive chargepile-upin thedrift volume;

^ thenonuniformityof themagneticfield of theL3 magnet.

Thenonuniformityof theelectricfield becauseof imperfectionsof thefield cageandto edgeeffectsisdiscussedelsewhere(seeSections2.1 and4.1.5). Here,the nonuniformityof the electricfield resultsonly from thepositive chargepile-up.

An electroncreatedin theionizationprocessdrifts in theelectric(E) andmagnetic(B) fields insidetheTPCvolumewith velocity

vdrift D µ1 I A ωτC 2 F E I ωτ

E H BB I A ωτC 2 A E F B C F B

B2 B

whereµ is theelectronmobility andωτ is thetangentof theLorentzangle.For bothfieldsuniform andparallel,thedrift velocity is

vdrift D µE U


z(cm)_0`

50a

100 150 200b

250b

occ

up

ancy

0`

0.05`

0.1`

0.15`

z(cm)_0`

50a

100 150 200b

250b

occ

up

ancy

0`

0.1`

0.2`

z(cm)_0`

50a

100 150 200b

250b

occ

up

ancy

0`

0.1`

0.2`

0.3`

0.4`

Figure 7.8: Occupancy in differentTPCregionsasa functionof z-coordinate.Thelowerplot wasobtainedfor apad-row radiusof 90 cm,themiddleonefor 130cmandthetop onefor 250cm. Valuesof theparametersusedinthesesimulationsarepresentedin Section7.2.1.3.


Positive chargepile-up

Therearetwo sourcesof thepositive chargein thedrift volumeof theTPC:theionizationcausedby theionizing particlesandthepositive-ion feedbackfrom thereadoutchamber. The latter is reducedby theimplementationof thegatinggrid,while thefirst leadsto abuild-upof positivechargein thedrift volume.This is becausethe ion mobility is smallerby 3–4ordersof magnitudethanthatof electrons,which, inpractice,resultsin the permanentpresenceof a positive charge in the TPC drift volume,affecting thedrift field. Theelectricfield dueto thespace-chargehasbeencalculatedthroughthefollowing steps.

^ Theprimarychargedensityρprim A r B zC , wherer is theradialcoordinateandz is alongthedrift, hasbeencalculatedusingtheHIJING eventgeneratortunedto dNch/dy D 8000atmidrapidityfor cen-tral Pb–Pbevents.Themultiplicity of minimum-biaseventswhichproducethemajorcontributionto thespace-chargecreationhasbeenscaledby anappropriatefactor(1/5).

^ Thebuild-up of thepositive charge in thedrift volume,resultingin thechargedenstiyρ A r B zC , hasbeencalculatedfor

– luminosity cdD 1 H 1027 cmW 2sW 1,

– ion mobility µion D 4 cmVW 1sW 1.

^ The potentialΦA r B zC , andthusthe extra electricfield, hasbeenobtainedby solving the Poissonequation

∇ 2ΦA rB zCLDeJ 1ε0

ρ A r B zCYBε0 beingtheelectricalpermittivity of vacuum.

Results

The numericalvaluesfor the distortionshave beenobtainedby integrating the equationfor the driftvelocity duringthedrift timeandcomparingtheobtainedelectronpositionwith theinitial one.

Initial z-position of electron (cm)0 50 100 150 200 250

dR

(cm

)

-0.03

-0.02

-0.01

0

0.01

0.02

0.03

0.04

0.05 R = 90 cm

R = 250 cm


dφ

(mra

d)

-0.1

0

0.1

0.2

0.3

0.4

R = 90 cm

R = 250 cm

Figure 7.9: Radial and azimuthaldistortionsdue to space-charge in 90% Ne, 10% CO2. Magneticfield isuniform andparallelto z-axis.


Distortions due to the space-charge In Fig. 7.9 theradialandazimuthaldistortionsdueto thespace-charge in thedrift volumeareshown. Themagneticfield is assumedto beuniform andparallelto thez-axis. Note that the distortionsof this type for Ar-basedgasmixtureswould be larger by a factorofabout5 with respectto Ne-basedones.This is becauseof thehigherionizationin argonandthe lowerAr-ion mobility.

Distortions due to the nonuniformity of the magnetic field The uniformity of the magneticfieldplaysa crucialrole in thetrackreconstruction.It determinesthevalidity of thetrackmodelandthusthepossibilityof onlinetracking.

Wehave studiedtwo configurationsof themagneticfield:

(oo) L3 magnet+ theMuonArm dipole’

(cc) L3 magnet+ theMuonArm dipole+ extra iron plugsin theL3 magnetdoor.

Oneshouldkeepin mind that theALICE referencesystemis placed30 cm above theL3 magnetaxis,which resultsin a ϕ-asymmetryof the magneticfield inside the TPC volume. The electricfield wasassumedto be uniform andparallelto thez-axis. The resultsareshown in Figs.7.10and7.11 for the(oo)-configuration,andin Figs. 7.12and7.13 for the (cc)-configuration. Onecanseethat the ironplugsimprove, in general,thequality of themagneticfield, reducingthedistortionsby up to a factorof2. Furtherin thisSectionthe(cc)-configurationof themagneticfield hasbeenused.

Overall distortions Therealisticcase,wherethedrift field E is distortedby thespace-chargeandB istherealmagneticfield of theL3 magnetwith theMuonArm dipole,is shown in Figs.7.14and7.15.

It shouldbenotedthat,althoughradialdistortionscanexceed1 mm,theeffective shift of theclusteralongthepadrow is smallerby a factorof tanα, whereα is ananglebetweenthetrackandthepadaxis.Azimuthaldistortionsreaching1 mradleadto a largeshift alongthepadrow of about1 mmat thelower(90 cm) and2 mm at theupper(250cm) radiusof theTPC.This necessitatesof corrections.However,as the experienceof the NA49 and CERESexperimentsshows, distortionsof this magnitudecanbecorrected.Weareawarethatdistortionsdueto theimperfectionof themagneticfield (themostcommonin present-dayfixed-targetexperiments)andthosedueto thespace-chargeareof a differentnature:The


dR

(cm

)

-0.01

0

0.01

0.02

0.03

0.04

0.05 φf

= 0o

φf

= 90o

φf

= 180o

φf

= 270o

φf

= 0o

φf

= 90o

φf

= 180o

φf

= 270o


dφ

(mra

d)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

φf

= 0o

φf

= 90o

φf

= 180o

φf

= 270o

Figure 7.10: Radialandazimuthaldistortionsat lower TPC radius(90 cm) dueto nonuniformityof magneticfield ((oo)-configuration).Electricfield is uniformandparallelto z-axis.



dR

(cm

)

-0.05

0

0.05

0.1

0.15 φf

= 0o

φf

= 90o

φf

= 180o

φf

= 270o


dφ

(mra

d)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

φf

= 0o

φf

= 90o

φf

= 180o

φf

= 270o

Figure 7.11: Radialandazimuthaldistortionsat upperTPCradius(250cm) dueto nonuniformityof magneticfield ((oo)-configuration).Electricfield is uniformandparallelto z-axis.


dR

(cm

)

0

0.005

0.01

0.015

0.02

0.025

0.03 φf

= 0o

φf

= 90o

φf

= 180o

φf

= 270o


dφ

(mra

d)

0

0.2

0.4

0.6

0.8

1 φf

= 0o

φf

= 90o

φf

= 180o

φf

= 270o

Figure 7.12: Radialandazimuthaldistortionsat lower TPC radius(90 cm) dueto nonuniformityof magneticfield ((cc)-configuration).Electricfield is uniform andparallelto z-axis.



dR

(cm

)

-0.1

-0.05

0

0.05

0.1

0.15 φf

= 0o

φf

= 90o

φf

= 180o

φf

= 270o


dφ

(mra

d)

0

0.2

0.4

0.6

0.8

φf

= 0o

φf

= 90o

φf

= 180o

φf

= 270o

Figure 7.13: Radialandazimuthaldistortionsat upperTPCradius(250cm) dueto nonuniformityof magneticfield ((cc)-configuration).Electricfield is uniform andparallelto z-axis.


dR

(cm

)

-0.02

0

0.02

0.04

φf

= 0o

φf

= 90o

φf

= 180o

φf

= 270o


dφ

(mra

d)

0

0.2

0.4

0.6

0.8

1

1.2 φf

= 0o

φf

= 90o

φf

= 180o

φf

= 270o

Figure 7.14: Radialandazimuthaldistortionsat lower TPCradius(90 cm) dueto space-chargeandnonunifor-mity of magneticfield.



dR

(cm

)

-0.1

-0.05

0

0.05

0.1

0.15

φf

= 0o

φf

= 90o

φf

= 180o

φf

= 270o


dφ

(mra

d)

0

0.2

0.4

0.6

0.8

φf

= 0o

φf

= 90o

φf

= 180o

φf

= 270o

Figure 7.15: RadialandazimuthaldistortionsatupperTPCradius(250cm) dueto space-chargeandnonunifor-mity of magneticfield.

first arestaticwhile thelatterare,in thebestcaseof astableluminosity, quasi-static.Thiswouldprobablyrequireamoresophisticatedapproachto thecorrectionsthanin theexperimentsmentionedabove.

Distortionsin thedrift (z) directionaresignificantlysmalleranddonotexceedfew tensof microns.

7.3.2 Cluster finding

We have chosena classicalapproachfor the trackingin the ALICE TPC.This meansthat, beforethetrackingitself, we have to find two-dimensionalclustersin pad-row–time planes.Thenwe reconstructthepositionsof thecorrespondingspacepoints,whichareinterpretedasthecrossingpointsbetweenthetracksandthecentresof thepadrows.

Up to now we have useda simplevariantof the clusterfinder. First we look for ‘preclusters’,i.e.thegroupsof adjacentcells in a pad-row–timeplanewhich have, afterdigitization,thesignalabove thezero-supressionthreshold. The two distinct preclustersarehenceseparatedfrom eachotherby a gapwherethesignalis lessthanthezero-suppressionthreshold.For eachpreclusterwe thenfind all its localmaxima.If thereis only onelocal maximumwe assumethatsucha preclusterhasbeeninitiatedby onetrackonly. In this casewe storeasthespace-pointcoordinatesthecentreof gravity of theprecluster. Ifthereareseveral local maximawe split this preclusterinto the correspondingclustersin the followingway: For eachlocal maximumwe searchfor a groupof adjacentcells with thesignalgreaterthanthesignalof thecell at thenearestsaddlepoint. In otherwords,we cut thepeaksat thesignallevel of thenearestsaddlepoint. Thenwe take the centresof gravity of thesegroupsof cells asthe reconstructedpositionsof thecorrespondingspacepoints.

Anotherimportantpieceof information,whichhasto beprovidedby theclusterfinder, is theestimateof theerrorsof thereconstructedspacepointsin boththepad-row andthedrift directions.For trackingwewill need,however, two typesof error(for detailsseethedescriptionof thetrackingalgorithmbelow).After extrapolatingatrackfrom onepadrow to thenext wehaveto defineawindow aroundthepredictedpoint in termsof the standarddeviations, which comprisethe extrapolationerrorsand the expectedspace-pointerrors. Thesetypesof space-pointerror, which dependon the trackparameters,arecalled‘potential’ space-pointerrors.When,duringthefiltering step,we try to assigna definitespacepoint tothetrack,it is betterto useanactualestimatefor theerrorsof thisparticularspacepoint,whichdependson the parametersof the correspondingcluster. Thesetypesof error are called ‘actual’ space-point


errors. Becauseof thecomplicatedway of clusterforming in a TPC,it is impracticalto calculateboththeseerrorstheoretically. Instead,we rely on theresultsof ourmicroscopicsimulations.

In addition to the raw data, the microscopicsimulatorgives the exact coordinatesof the tracks’crossingpointswith thecentresof thepad-row planes.Comparingthe reconstructedspace-pointposi-tionswith thegeneratedonesandparametrizingthecorrespondingdeviationsasa functionof thetrackparametersandthepad-row number, wegettheparametrizationfor thepotentialspace-pointerrors.Thiswasdoneusingseveralsimulatedeventswith a very low trackdensity, whenthenumberof overlappingclusterswasnegligible.

Concerningtheactualspace-pointerrors,we found that they areapproximatelyproportionalto thedispersions(secondcentralmoments)of thecorrespondingclusters.Thisis nottruein general.However,at leastfor trackswhichcrosstheentireTPC(i.e. for pionswith pt g 80MeV/c and45hiV θ V 135h ) it isagoodapproximation.Theclusterdispersionscanbecalculatedfor eachclusterand,aftermultiplicationby constants(which have beenfound againusing the microscopicsimulator, separatelyfor the twodirections),areusedastheerrorsof thereconstructionof this cluster.

Theprecisionof the reconstructionof theclusterpositionsdependson the trackparametersandonthepad-row number;in otherwords,it is afunctionof thedrift lengthandtheanglesatwhichtrackscrosspad-row–drift planes.This precisionis alsoa functionof the propertiesof the drift gas,the particulardesignof thereadoutchambers,andparametersof thefront-endelectronics.In Table7.5thespace-pointresolutionsfor well-separatedtracksaresummarized.Theseresultsareaveragedover padrows in theinnerandouterTPCsectors,aswell asoverall trackswhichcrosstheentireTPC.Duringthesimulationsall theTPCparametersweresetto thevaluesdiscussedabove. Thevaluesreportedhereareabout10%betterthe thoseshown in Fig. 4.13on page55 for MIPs, becauseherewe alsoaveragedover particlemomenta.

Table7.5: Averagedspace-pointresolutions.

Paddirection[mm] Drift direction[mm]

Innersectors 0.87 1.28Outersectors 0.72 1.13

An additionalquantity, whichwehaveto know aftertheclusterfinding,is thechargedepositedin theclusters.We needthis informationfor particleidentificationby dE/dx measurements.Thereareat leasttwo waysto determinethischarge.As ameasureof thechargein acluster, wecanuseeitherthesum(inADC counts)of all digitswhichareassociatedwith thisclusteror just thelargestdigit in thecluster. Wehave foundthatin ourcasethelatteroptiongivesbetterresults(seebelow).

In orderto betterunfold theoverlappingclustersonehasto fit themto agivenshape,whichwehavenot yet done.Thefirst reasonfor this is that thefitting is a ratherslow procedure.Theotherdifficultywith thefitting is thefollowing: Beforestartingto fit, we would like to know, at least,how many trackscontribute to thegiven cluster. It would alsobevery usefulto have an estimateof theanglesat whichthesetrackscrossthe particularpadrow. Otherwise,we have to fit too many parameters,andso thefitting procedurebecomesvery unstable.

Unfortunately, we have no informationabouttracksduring the clusterfinding. Nevertheless,wethink thatthisstraightforwardcluster‘unfolding’ couldbeverypromising,if onemanagedsimultaneoustrackfindingandclusterfinding. Thisapproachis currentlyunderstudy.

7.3.3 Track finding

Trackfinding for thepredictedparticledensitiesis oneof themostchallengingtasksin theALICE ex-periment.It is still underdevelopmentandherewereportthecurrentstatus.Trackfindingis basedonthe


Kalman-filteringapproach.Kalman-like algorithmsarewidely usedin high-energy physicsexperimentsandtheir advantagesandshortcomingsarewell known [12].

Therearetwo maindisadvantagesof theKalmanfilter, whichaffect thetrackingin theALICE TPC.The first is that we have to reconstructclustersbeforeapplying the Kalman-filterprocedure.As hasalreadybeennoted,this is in thepresentsituationnot a trivial task. We have about40%occupancy inthe innersectorsof theTPC; thereforea certainnumberof theclustersget lost, andtheothersmaybesignificantlydisplaced.Thesedisplacementsareratherhardto take into account. The otherproblemwith theKalman-filtertrackingis that it reliesessentiallyon thedeterminationof good‘seeds’to starta stablefiltering procedure.Unfortunately, for the tracking in the ALICE TPC we have to constructtheseedsusingtheTPCdatathemselves. TheTPCis a key startingpoint for the trackingin theentireALICE set-up.Practicallynoneof theotherdetectorscanprovide the initial informationabouttracks.Therefore,wehaveto maketheseedsin astraightforwardcombinatorialway, andthismorethandoublesthecomputingtime.

On theotherhand,thereis awholelist of very attractive propertiesof theKalman-filterapproach.

^ It is amethodfor simultaneoustrackrecognitionandfitting.

^ Within thismethodthereis apossibilityto rejectincorrectspacepoints‘on thefly’, duringtheonlytrackingpassover a track. Suchincorrectpointscanappearasa consequenceof theimperfectionof theclusterfinder. They canbedueto noiseor they canbepointsfrom othertracksaccidentallycapturedin the list of pointsto beassociatedwith thetrackunderconsideration.In thecaseof aglobal trackingapproachoneusuallyneedsanadditionalfitting passto get rid of theseincorrectpoints.

^ In thecaseof substantialmultiplescattering,trackmeasurementsarecorrelatedandthereforelargematrices(of thesizeof thenumberof measuredpoints)needto beinvertedduringa globalfit. IntheKalman-filterprocedureweonly haveto manipulateupto 5 H 5 matrices(althoughmany times,equalto thenumberof measuredpoints),which is muchfaster.

^ Usingthis approachonecanhandlemultiple scatteringandenergy lossesmoreeasilythanin thecaseof globalmethods.

^ Kalmanfiltering is a naturalway to find theextrapolationof a trackfrom onedetectorto another(for examplefrom theTPCto theITS or TRD).

In thefollowing webriefly describeour implementationof theKalman-filteralgorithm.Thefirst andmosttime-consumingstepis seedfinding. It beginswith asearchfor all pairsof points

in the outermostpad row and in a pad row n rows closerto the interactionpoint (n D 20 at present)whichareprojectingto theprimaryvertex. Thepositionof theprimaryvertex is reconstructed,with highprecision,from hits in theITS pixel layers,independentlyof thetrackdeterminationin theTPC.Whenareasonablepairof suchpointsis found,wecalculatetheparametersof ahelix goingthroughthesepointsandtheprimaryvertex, andtaketheparametersof thishelix asaninitial approximationof theparametersof thepotentialtrack. Thecorrespondingcovariancematrix canbeevaluatedby takingthepoint errors,which aregivenby theclusterfinder, andapplyinganartificially largeuncertaintyto theprimary-vertexposition.At this stepwe assignanuncertaintyof thesizeof thebeampipe,in orderto take into accountmultiple scatteringandnot to losethe tracksfrom decayscloseto theprimary vertex. This is theonlyplacewherewe introducea certain(not too strong)vertex constraint,andlaterwe allow tracksto haveany impactparametersin boththez-directionandr-ϕ plane.Usingthecalculatedhelix parametersandtheir covariancematrix we start the Kalmanfilter from the outerpoint of the pair to the inner one. Ifat leasthalf of thepossiblepointsbetweenthe initial onesweresuccessfullyassociatedwith this trackcandidate,we save it asaseedandcontinueto look for anotherpair of initial points.

7.4 Tracking performance 165

To avoid abiasof theseedthroughclusterdistortions(owing to overlapsandδ-rays)andsubsequentlossof thetrackcandidate,asecondseed-findingis performed(usinganotherpairof padrows,atpresentthe10thand(n I 10)th).

At the endof the seedfinding we sort the seedsaccordingto increasingtrack curvature. Thenweproceedwith theKalmanfilter throughtheentireTPC,startingwith thestiffesttracks,removing assignedclustersandcontinuingwith softerones.

For eachseedwe calculatethetrackparameters,aswell astheir covariancematrix, at thenext padrow. Duringthisextrapolationstepwetakeinto accountmultiplescattering(by addingthecorrespondingmatrix to the track covariancematrix) andmeanenergy loss(by meansof the Bethe–Blochformula),assumingthat the actualparticleis a pion. Thenon this padrow we definea ‘window’ alongthe paddirectioninsidewhich we look for a clusterto be associatedwith the track. To calculatethis windowwe do thefollowing steps.First, theformulawhich parametrizestheerrorsof theclusterreconstructionasa functionof the track parametersandpad-row numbergivesus an expectedvaluefor theerrorsofthe clusterreconstruction— the potentialspace-pointerrors. Then we take the elementof the trackcovariancematrix,whichdescribestheuncertanityof thetrackpositionin thepaddirection,computethesquarerootof thesumof thiselementandthesquareof thepotentialspace-pointerror, andmultiply theresultby a constant,which is a parameterof the trackingprogram.Sincetheresidualdistributionsare,unfortunately, essentiallynot Gaussian,we useasa constanta factorof 5 (insteadof 3, which would begoodenoughin thecaseof Gaussianerrors).

After the window is definedwe checkall the clusterswhich appearwithin it. Herewe have threepossibilities.

^ Thereareno clustersin thewindow. In this casewe try to find necessaryclusterson thenext padrow. If therewerealreadyseveralconsecutive padrowswithoutclustersassociatedwith this track,we terminatethe trackingfor this candidateandremove it from the correspondinglist, but keepall of its clustersin theevent. Consequentlywe allow thetrackto missasmany ashalf of all padrows in aparticularTPCsector. This is becausewehave ratherlargedeadzonesbetweentheTPCsectors.

^ Thereis oneclusterinsidethewindow. If theχ2 valuecalculatedfor this clusteris smallenough(at presentwe useasanupperlimit 12 per two degreesof freedom),we attachthis clusterto thetrackandupdatethetrackparametersaccordingto thestandardfiltering procedure.If theχ2 valueis too large,wecontinueasin thefirst case.

^ Thereis morethanonepossiblecluster. In thiscasewechoosetheclusterwhichgivesthesmallestχ2 valueandgo to thepreviouscase.

From time to time it happensthat the track leaves a TPC sectorandentersanother. In this casewerecalculatethetrackparametersandthecovariancematrix sothatthey arealwaysexpressedin thelocalcoordinatesystemof thesectorwithin which thetrackis at thatmoment.

Whenacurrenttrackcandidatereachestheinnerboundaryof theTPC,we checkif thereareat least40% of all possibleclustersattachedto it. If so, we considerthe track candidateasa found track andremove its clustersfrom the event. Otherwise,we remove the track candidateand the correspondingclustersareleft in theevent.

7.4 Tracking performance

In this sectionwe presentthe trackingefficiency, momentumanddE/dx resolutions.The angularandimpact-parameterresolution,involving thetrackingin theITS arediscussedelsewhere[13].


7.4.1 Tracking efficiency

In orderto determinethetrackingefficiency within theTPCacceptancewe definethefollowing quanti-ties.

1. ‘Generatedgoodtrack’ — atrackwhichcrossesat least40%of all padrows andproducesat leastonehit on thepadrows chosenfor theseed-findingprocedure.

2. ‘Foundgoodtrack’ — a trackfor which thenumberof assignedclustersis larger than40%of thetotalnumberof padrows. In additionwerequirefor suchtracksthatnomorethan10%of clustersare incorrectly assignedand that at leasthalf of the innermost10% of clusterswere assignedcorrectly.

3. ‘Foundfake track’ — a trackwith thesufficient number, but incorrectassignment,of clusters.

Thetrackingefficiency is thendefinedastheratioof thenumberof ‘found goodtracks’to thenumberof‘generatedgoodtracks’,while theprobabilityto find a ‘f ake’ trackis expressedby thenumberof ‘foundfaketracks’normalizedin thesameway. Thetrackingefficiency wasdeterminedonly for primarytracks.

No significantdependenceof thetrackingefficiency on thetransversemomentumof theparticlesoron theirdip anglewasfound(seeFig. 7.16).Thisconfirmsearlierestimatesgivenin Ref. [1].

In orderto studythedependenceof thetrackingefficiency ontheparticledensitywehaveperformedsimulationsfor threedifferentvaluesof dNch/dy. WeusedtheHIJING generatorwith thetotalmultiplic-

λ (deg)-40 -30 -20 -10 0 10 20 30

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Figure 7.16: Trackingefficiency asa functionof particletransversemomentumanddip angle.

7.4 Tracking performance 167

ity scaledby appropriatefactorsto obtaintheright rapidity densityin thecentralregion. Theresultsarepresentedin Table7.6.

Table7.6: Trackingefficiency for differenttrackdensities.

dNch/dy Efficiency for realtracks Probabilityof fake tracks

1300 0.98 04300 0.94 08300 0.88 0.02

7.4.2 Two-track efficiency

In orderto evaluatetwo-trackefficiency we modifiedthe standardHIJING event generator. For everysecondgeneratedparticlethemomentumof thenext particlewasgeneratedwith asmalldifferencewithrespectto themomentumof thepreviousone.This differencewasuniformly distributedwithin asphereof radius0.03GeV/c (in momentumspace).In thiswaywe increasedartificially thenumberof particleswith closemomenta.

After thereconstructionof suchanevent,thetwo-trackefficiency canbecalculatedasa ratio of thenumberof foundparticlepairsto thenumberof generatedones.This ratio,asa functionof theabsolutevalueof the(generated)momentumdifferenceof thetwo particles,is shown in Fig. 7.17.This resulthasbeenobtainedfor theparticledensitydNch/dy s 8300.

|∆p| (GeV/c)0 0.02 0.04

t0.06 0.08

t0.1

T

wo

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1

dNu

chv /dy = 8300w

Figure7.17: Two-trackefficiency for particledensitydNch/dy x 8300asafunctionof absolutevalueof generatedmomentumdifferenceof two particles.

Onecanseethatwe losepairsof particlesif their momentadiffer by lessthan0U 015GeV/c. How-ever, thetwo-trackefficiency never falls to zeroevenwhenmomentumdifferenceis closeto zero.This isbecausemultiplescatteringin thematerialcrossedby particleswith closemomentabetweentheinterac-tion pointandtheTPCsensitivevolume(beampipe,ITS, innervesselof theTPC)tendsto separatethem.This effect increasestheprobability to detectsuchpairsof particles,but at thesametime it deterioratesthetwo-trackmomentumresolution.Nevertheless,if apairof particleswassuccessfullyregisteredin theTPC,theactualmomentumdifferenceat thevertex canthenbereconstructedusingtheITS information.

We have estimatedthat for thepresentITS andTPCdesignthe two-trackmomentumresolutionisonly slightly worsethanthatpresentedin theALICE TechnicalProposal[1]. Theresultsaresummarized


Table7.7: Two-trackmomentumresolution.

dNch/dy δqlong [MeV/c] δqside [MeV/c] δqout [MeV/c]pt s 0U 2 MeV/c pt s 0U 5 MeV/c pt s 1U 0 MeV/c

1000 1.0 0.5 3.7 7.4 136000 1.1 0.5 3.9 8.5 16

in Table7.7 wherethetwo-trackmomentumresolutionsaregivenseparately, for the threecomponentsof the momentumdifference,and for low andhigh charged-particledensities.The resolutionsof thelongitudinal(in thez-direction)componentandof the ‘side’ (thedirectionin the transverseplane,per-pendicularto the total momentumof thepair) componentarepracticallymomentumindependent.Theresolutionsof thesetwo componentsaredeterminedby theangularresolutionattheinteractionpoint. Ontheotherhand,theresolutionof the‘out’ (thedirectionin thetransverseplane,parallelto thetotal mo-mentumof thepair)componentdeteriorateswith increasingpt, becausein thiscasethemaincontributionis dueto themomentumresolutionitself.

7.4.3 Momentum resolution

Themomentumresolutionsfor thethreetrackdensitiesareshown in theTable7.8.Becauseof particularinterestin high-momentumelectronsin ALICE, 10% of electronswith pt s 5 GeV/c wereartificiallyaddedto theHIJING generatedevents.

Table 7.8: Momentumresolutionsaveragedover the HIJING spectrumandfor high-momentumelectronsonly(pt x 5 GeV/c).

∆pt y pt [%]dNch/dy averaged pt s 5 GeV/c

1300 1.6 5.14300 2.0 7.68300 2.1 8.5

It canbe seenthat for lower track densitiesthe obtainedresolutionsareconsistentwith thosepre-dicted in the ALICE TechnicalProposal. However, the resolutionsdeteriorate(especiallyfor high-momentumtracks)whenthetrackdensityincreases.Thisis mainlydueto theclusteroverlappingswhicharemoreprobablefor higherparticlemultiplicities. Nevertheless,we hopeto improve themomentumresolutionsby refittingclustersusinginformationabouttracksalreadyreconstructed.

From thesenumbersit is obvious that (for electronphysics)the requiredmomentumresolutionof2.5% at 4 GeV/c can only be obtainedin conjunctionwith the ITS and TRD and for running withincreasedmagneticfield. The improvementin ∆pt y pt which is expectedwhencombininginformationfrom theTPCandITS for trackingandwhenrunningat B s 0U 5 T is demonstratedin Fig. 11.13of theALICE TechnicalProposal[1].

7.4.4 dE/dx resolution

As mentionedabove,wecomparedtwo methodsfor obtainingdE/dx information.Themostnaturalwayis to usethetotal chargedepositedin clusters.However, it hasalsobeenfoundthat,for thetrackswhicharewithin theTPCacceptence,onecansuccessfullyusethemaximaldigits in aclusterfor theevaluationof dE/dx. We divide thecharge,or themaximaldigit, by thelengthof thecorrespondingtracksegment;thenthedE/dx valueis calculatedusingthetruncatedmeanmethod.Thebestresolutionin ourconditions

7.5 Track matching 169

canbeachievedif wediscard5%of thesmallestsignalsand25%of thelargestones.Thecorrespondingresultsfor minimum-ionizingpionsarepresentedin Table7.9.

Table7.9: dE/dx resolutionfor total-chargeandmaximal-digitmethods.

dN/dy dE/dx (total charge)[%] dE/dx (maximaldigit) [%]

1300 7.8 7.04300 12.6 8.68300 17.3 10.0

In theconditionsof low multiplicity, bothmethodsgiveapproximatelythesameresult.However, forhighertrackdensitiesthedifferencebecomesmoreobvious. As we do not, in fact,unfold overlappingclusters,the total charge is often moredistortedthanthe signalat the peak,which is why in this casethemaximal-digitmethodyieldsbetterresults.We would alsolike to emphasizethatfor well-separatedtracks,having assignedat least90% of all possibleclusters,the dE/dx resolutionis 5.5%,which is inagreementwith theesimategivenin Ref. [1].

Weobservedaslightdependenceof thedE/dx resolutiononthepolartrackangle.Theresolutionforthemostinclinedtracksis worsethanthat for thetrackswhich areperpendicularto thebeamdirection.Thedifference,however, doesnotexceed1%. Thiseffect is dueto thevariationof theclustershapewithpolarangleof thetrack.

Therefore,despitethebetterpeformanceof themaximal-digitmethod,we would considerthetotal-charge methodto be lessdependenton peculiaritiesof thesimulations,andthusmorereliable. Takingthis into account,we areplanningto repeatthetotal-charge methodcalculationswhenwe implementamorereliableclusterseparation.

Theresultsof thesimulationindicatethattherequirementsin dE/dx resolutioncanbefulfilled evenat thehighestmultiplicity densitiesexpectedin Pb–Pbcollisions

7.5 Track matching

The charged-particletracksfound in the TPC have to be connectedto otherALICE detectorsin orderto improve themomentumresolutionandparticleidentification.We describeheretheconnectionto theITS andto theTRD.

7.5.1 Connectionto ITS

Themethodusedfor theTPC–ITStrackmatchingis desribedin Ref. [13]. The tracks,after the track-finding stepin theTPC,aremoreor lessorderedaccordingto trackcurvature(becausewe have startedfrom theseedsorderedaccordingto stiffness).Nevertheless,beforeactualmatchingwecheckandwhennecessaryrepairtheordering.

During trackingin theITS (aswell asin theTPC)wedonotallow for hit/clustersharingamongdif-ferenttracks(wealwaysremoveassignedhits). Thereforeit is betterto startthetrackfinding with moreaccuratetracks. The dominantcontribution to the extrapolationerrorscomesfrom multiple scatteringin theTPCfield cage,HV degraderandvessel.Themultiple-scatteringcontribution of courseincreaseswith decreasingmomentum.Thereforewe startfrom trackswith the lowestcurvature(i.e. thehighesttransversemomentum),becausetheextrapolationto theouterITS layersfor thesetrackswill be moreaccuratethanfor thosewith smallermomenta.

ThedistancebetweentheTPCandthe ITS sensitive volumesis ratherlarge (about45 cm) andthetrackdensityinsidetheITS is sohigh(theoccupancy in theouterITS layerscanreachupto 3%–4%)thatsimplecontinuationof thetrackingprocedureusedfor theTPCwouldbeineffective. Thehit multiplicityin a z 3σ window aroundtheextrapolatedpositionof thetrackis on average3–4,andtherearecasesin


which it reaches20. If we usedonly thecriterionof minimal χ2 for thehit assignmenttherewould beahigh probabilityof incorrectassignments.Therefore,we have implementedsomeimprovementsto theKalman-filterprocedureusedfor theTPCtracking.

First, we do not decidewhethera hit insidethe z 3σ window belongsto thetrackor not aftereachtrackstep. Instead,we assignto the track,oneby one,all thehits within thepredictedwindow havingreasonableχ2 (not only theonewith minimal χ2). In this way we build for eachtrackfoundin theTPCthetreeof candidatesthroughall of theITS. Thedecisionis made,andthusthematchingis done,onlyafterwe canusetheinformationfrom all of theITS layers.Wechoosethecandidate(i.e. thepathalongthe tree),andcorrespondinghits, which for themaximalnumberof assignedhits hastheminimal sumof theχ2.

Anotherchangein thealgorithm,with respectto theTPCKalman-filterprocedure,is thatweuseex-plicitly thevertex constraint.Theprimary-vertex positionis known, prior to theactualtracking,from theITS pixel detectormeasurementswith anaccuracy betterof than20µm (seeRef. [13]) for centralPb–Pbcollisions. During tracking,at eachITS layer, we propagatethetrackfrom thenominalvertex positiontowardstheITS layerconcerned,usingits currentparameters.In thiswayweprojectthevertex positionon themeasurementplane,obtainingtwo additional‘measurements’(two coordinates).Effectively wethenhave a four-componentmeasurementvector, whichweusein theKalmanprocedure.

0

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%)

ITS matching

ITS efficiency limit

fake tracks

Figure7.18: Efficiency andfake-trackprobabilityfor thetrackmatchingbetweentheTPCandtheITS.

In Fig. 7.18theefficiency of thealgorithmdescribedabove is shown. Thepresentedvaluesincludethe track-matchingefficiency aswell astrack-findingefficiency in theITS (they cannotbeeasilysepa-rated).For a trackto bereconstructedin theITS we requirethatat leastfive of thesix pointsbefound.Becausewe assumea certaindetectionefficiency for the ITS layers(98%for thepixel layersand95%for theotherfour layers),thereis somemaximalachievabletrack-findingefficiency in theITS (94.5%).Thereconstructedtrackis consideredgoodif all assignedpointsarecorrect,otherwisethetrackfalls inthefake-trackcategory. For trackswith transversemomentaabove 400MeV/c we achieve anefficiencyof only a few per centbelow the maximumachievable. For lower pt the efficiency is sligthly worsebecauseof thelargerinfluenceof multiplescattering.

7.5.2 Connectionto TRD

BecausetheTRD design,andalsotheTRD detailedsimulation,areunderdevelopment,wehaveusedthefastsimulationapproach(similar to theoneusedin theTechnicalProposalfor theTPC–ITSmatching),

7.5 Track matching 171

to estimatetheefficiency of thematchingof theTPCtrackswith theTRD trackinginformation.In fact,if wecomparetheresultsobtainedwith thissimplifiedmethodandthoseobtainedwith theKalmanfilterin the caseof the ITS, we will find very goodagreement.Therefore,we areconfidentthat whenweincludethedetailedTDR description,theresultswill not changesubstantially.

In order to estimatethe track position and angularresolutionat the TRD inner surfacewe haveusedtheestimateof thetrack-parametererrorsaftertheTPCtrackingandwe take into accountmultiplescatteringin thematerialbetweentheTPCsensitive volumeandtheTRD (outerfield cage,HV-degraderand outer TPC vessel). Knowing the averageTRD occupancy and its point resolution,we can by asimpleMonteCarloprogramcalculatetheprobabilityto correctly(or incorrectly)matchtheTRD hits tothetrackextrapolation.Theresults,asa functionof transversemomentum,areshown in Fig. 7.19.

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Pro

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fake tracks

Figure 7.19: Estimatedefficiency andfake-trackprobability for the track matchingbetweenthe TPC andtheTRD.

Again, for the high-pt tracks,the estimatedmatchingefficiency is excellent, about98%. On theotherhand,for trackswith transversemomentumbelow 700MeV/c the efficiency startsto deteriorateeven fasterthanin thecaseof theTPC–ITSconnection.This is theconsequenceof the largerbendinganglesfor thesamept in theplacefurtherfrom theinteractionpointandthehigherTRD occupancy.

173

8 R&D for alternative readoutchambers

Theconditionsin theALICE experiment[1] aresuchthatthechambershave to beoperatedat ratherlarge gains( g 104) and track densities. In the courseof evaluatingall optionsfor readoutchamberswe have, consequently, alsoinvestigateda numberof differenttechnologies.Amongthealternativestowire-basedreadoutchambersareRing-CathodeChambers(RCC)andGasElectronMultipliers (GEM).Theresultsobtainedin theseR&D studiesarediscussedbelow.

8.1 Ring-CathodeChamber

In 1992theresearchprojectRD32[2] wasstartedatCERNto investigatelow-diffusiongasmixturesandnovel designs,bothfor readoutchambersandanalogfront-endelectronics.Furthermorenew techniquesfor high-volumedatareadoutanddatareductionwereexplored.Oneof theresultsof theseinvestigationswasthedevelopmentof areadoutschemefor wire chambersincorporatingring-shapedcathodeelements.This readoutarchitecturewouldallow easyconstructiondueto only onewire plane.Owing to thebettercouplingof thechargesignalto thereadout,thesignalsobservedonthepadelementsarelargerby abouta factorfour comparedto theclassicalTPCreadout(NA49 geometry, seeRef. [3]) for a givengasgain.A new four-channelpreamplifier/shaperin bipolartechnologyoptimizedfor low powerconsumptionandhigh baselinestability completedthisdevelopment.

In the courseof further studiesit wasdemonstratedon small prototypesthat the original conceptusingsmall closedrings could be replacedby openC-shapedstructuresmaking it easierto insert theanode-wireplanewithout losingtheadvantagesof thering design.In addition,aschemewasdevelopedto producea sufficientnumberof ringsto equipa reasonablylargeprototype.

preamplifier-shaperT

AB-bonding

ring cathode

sense wire

gating strip

ground strip

drifting e -

equipotential line

multi-layer board

open gateclosed gate

6.25 mm

Figure 8.1: Schematiccross-sectionof the readoutboardwith ring-cathodeelementsandpreamplifier/shaperchips.In additionequipotentiallinesanddrift linesfor electronsareshown: left, gateclosed;right, gateopen.

174 8 R&D for alternativereadoutchambers

In 1996work startedto incorporatethenew developmentsinto a fully operationaldetectorprototypeandtestit underrealisticconditions.An agreementwasreachedwith NA49 to setupthedetectorin frontof the Main TPCsof NA49 with the leadbeampassingthroughthe detector. Part of the phasespacenot coveredby NA49 would becomeaccessible.As a consequencethereadoutof theprototypehadtobe matchedto NA49 requirements.In 1998the RCC prototypewith an active areaof 40 23 cm2

and 1200active electronicchannelswasoperatedsuccessfullywithin theNA49 environment. In thefollowing text only theprinciple functionalityof theRCCis explainedanddocumented.More detailedinformation,especiallyon theanaloganddigital electronics,canbefoundelsewhere[4].

In Fig.8.1aschematiccross-sectionthroughtheRCCALICE prototypeis shown. Thering elementsare positionedon a multilayer board. Four rings are coupledelectrically to one preamplifier/shaperchannelwhich is bondeddirectly to thebacksideof theboard.Thusa singlepadelementhasa sizeof25 0 2 65 mm2. Conductingstripsareinsertedbetweentheopensidesof therings. Variationof theirpotentialallows to gatetheRCC.

8.1.1 Preamplifier/Shaper

Thepreamplifier/shaperis optimizedfor thecharacteristicsignalwhich is inducedby thepositive ionscreatedduringtheamplificationprocesscloseto thewire of theRCC.Thebasicdesignof thechip wasdevelopedwithin the RD32 projectstartingin 1992. The aim wasto designa circuit with a baselinerestorationbetterthan1 permille after 2 µs. Furthermorea very low power consumptionthat allowsthe mountingof the chip very closeto the detectorreadoutplane(padplane)wasenvisaged. For theproductionof thechip theradiationhardbipolarprocessfrom HARRISwaschosen.In November1997thefirst four-channelchipswerecut from thewafersof a multichip projectandtestedat CERN.Fromthis run approximately350chipswerefinally selectedaccordingto specificationandlaterusedfor theRCCprototype.Thetechnicalspecificationsof thechip aresummarisedin Table8.1.

Thecircuit diagramof the chip is shown in Fig. 8.2. The responseof thepreamplifier/shaperchipwhenconnectedto asmallprototypereadoutchamberis shown in Fig. 8.3.

Table8.1: Parametersof thepreamplifier/shaperchip.

Process UHF1 Nunberof channels 4Die size 2.6mm 2.7mm Package S024/TABSupplyvoltage 2 5 V/ 2 5 V Nunberof Adjustpot 4Powerconsumption(total) 30 mW Powerconsumption/channel 7.5mWInput impedance(DC) 150Ω Input impedance(at 1 MHz) 300ΩConversiongain 5 mV/fC Outputdynamicrange 2 VNoise(ondetector) 2600e PSRRon VEE 10dBLinearity 1% Crosstalk 50 dBShapingtime 225ns Shapingcapacitor/channel 1 externalTail cancellation(1 to 4 us) 2 10 3 Tail cancellation( 4 us) 0 5 10 3

8.1.2 TAB bonding

Thetapeautomatedbonding(TAB), seeRef. [4], is a technologyusuallyappliedfor productionof bigserieswherehigh reliability is required.In principletheTAB processin our applicationcanbedividedin thefollowing steps:

8.1 Ring-CathodeChamber 175

exterieur

POT exterieur

0

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+

+

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00

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Figure8.2: Circuit diagramof thepreamplifier/shaperchip.

C

B

A

A 50 ns 22 mV

B 5 µs 1.0 mV

C 0.5 µs 1.0 mV

Figure 8.3: Signalfrom a preamplifier/shaperconnectedto a smallprototypereadoutchamber.


a) Delivery of dicefrom thefoundryto thecompany providing TAB.b) ILB (innerleadbondingi.e.mountingof diceontape)whichallows to performafull performance

test(AC + DC).c) Selectionof dicewhichperformwithin certainqualitylimits (possibilityto selectdifferentclasses).d) Transferof selecteddicefrom ILB tapeto thesubstrate(detectorbackplaneor PCB).

Stepa) to d) would be doneby the samecompany which delivers finally a fully functional andguaranteeddevice accordingto specifications.

Application of TAB for the production of the RCC prototype

For theRCCprototypewe decidedto bondthechipsdirectly onto thebacksideof thereadoutplaneoftheRCC.As this wasdoneby thecompany it hadtheadvantagethatfully testedreadoutsegmentsweredelivered.Furthermore,theelectricalconnectionfrom thepreamplifier/shaperto thebuffer amplifierwasdonewith flexible cables(FLEX TAB).

8.1.3 Pad responsefunction

Severalhigh-precisionmeasurementshave beenperformedto determinethewidth of thepadresponsefunction (PRF)for the specificwire padgeometryof the RCC. It hasto be mentionedthat the naturalPRF is only determinedby the charge coupling of the electrons/ionsto the cathodeplanewhile thereconstructedPRFis dependenton the sampling,i.e. the chosenpadwidth. For the measurementsanalphasourcewasusedwhichensuresahigh ionizationdepositwith aminimumof fluctuations.Angularwire/padeffectswereminimizedby usinga triggercounterto selectparticlesthatwereemittedfrom thesourceparallelto thereadoutplane.Figure8.4shows thefit of thePRFwith themodelsof Gatti [5] andEndo[6]. Applying a Gaussianfit to thedatameasuredwith theRCCstructure(ring radiusof 3 mm) aσPRF of 2.0mmwasobtained.

Thesmallestresidualwereobtainedwith theGatti functionwhich might beexplainedby thehighernumberof degreesof freedomcomparedto GaussandEndo. The measurementsshow that the widthof the measuredPRFis approximately66% of the radiusof the cathodering. More detailsaboutthemeasurementwith thesmallprototypeTPCcanbefoundin Ref. [7].

Figure 8.4: Padresponsefunctionof the ring-cathodechamber. ThecurvesareGatti (dash–dot),Gauss(dots)andEndo(dash)estimates.


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Figure 8.5: Transparency for electronscomparedto measurement(full dots)asa functionof the offsetvoltageVG (a) andasa functionof biasvoltage∆V with offsetvoltageVG 30 V (b). Solid line: electronsarriving ontheanodewire; dashedline: electronsarriving on thegatestructure.

8.1.4 Gating

The transmissionfor electronsof thegatestructuredependson theoffset voltagethat is appliedto thegatingstrips. Thevoltagesettingfor a given transparency, which is definedasthe fractionof electronsthatarrive on theamplificationwires,dependson thedrift field strength.Thehigherthedrift field, thehighermustbetheoffsetvoltage.

For thegatetransmissionmeasurementsof theRCCabetasourcewasgluedto theMylar window ofthefield cage.Thecurrentin thesense-wirecircuit wasmeasuredasa functionof thegate-offsetvoltagefor differentdrift fields. It hasto be mentionedthat thedrift field strengthwaskept for safetyreasonsfar below the400V/cm necessaryfor theoperationof theALICE TPC.Theclosingcharacteristicof thegatingstructurefor electronswasmeasuredwith abetasourceandwith theleadbeam.

A detailedsimulationof the behaviour of the RCC hasbeenperformedusing GARFIELD (seeRef. [4]). For the simulationthe RCC hasbeenmodelledascloselyaspossibleto the actualdesign.Electronsapproachthe wire throughthe opening. The anodewires have a diameterof 20 µm, anda90%Ne,10%CO2 gasmixtureis assumed.

Transparencyfor electrons

Assumingawire potentialof 850V andadrift field of 125V/cm, thetransparency in theopengatestatevariesfrom 50%at anoffsetpotentialof 0 V to nearly100%for anoffsetof 100V. This agreeswellwith measurementsof thisquantityasshown in Fig. 8.5a.Goodagreementwith themeasurementis alsoobtainedfor thebiasvoltagevariationwith anoffsetvoltageVG of 30V, seeFig. 8.5b.

Transparencyfor ions

Ionsgo,undermostangles,to thering cathode.Thesearetheionsproducedunderanglesthatdonotfacethe ring-cathodeexit. Whenthegateis open,the ionsproducedin a 30 sectorfacingthe ring-cathodeexit will enterthedrift zone.Whenthegateis closed,thissectorvanishes.Thegatingstructureis reachedby someions irrespective of thestateof thegate:whenthegateis open,slightly under20 give access


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a b

Figure 8.6: Angularregion aroundthesensewire from wherepositive ionsendup on thegatinggrid, the ring-cathodeor thedrift volume,respectively, asa functionof biasvoltage∆V (a)andoffsetvoltageVG (b).

to thegatingelectrodes,anangularrangethatincreasesto nearly45 whenthegateis closed(Fig. 8.6).Sincethegatingefficiency for ionsdependsstronglyontheassumptionsabouttheinitial distribution

of the ion cloudaroundthesensewires,no predictionson thepermille level canbe made.In additiondiffusioneffectsmayplay an importantrole whenthegateis closed.Theseeffectsarealsodifficult tocalculatewith therequiredprecision.

8.1.5 Isochrony

Neon(Ne) ionsneed45–55µs to reachthering cathode,while CO2 ionsneed25–35µs. Oncepastthegate,ions enterthedrift region wherethefield is 180V/cm, with drift velocitiesof only 800cm/s. InFig. 8.7 linesof isochrony areshown for positive Ne ions.

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]

Figure 8.7: Isochrones(10 µs) for drift of positive ionsaway from thesensewire (offsetvoltageVG 100V,biasvoltage∆V 0 V, drift field 185V/cm).


8.1.6 Lead-beamtests

For the lead-beamtesttheprototypeTPC,shown in ColourFig. XII, wasmounteddownstreamof theVertex II magnetof NA49 directly in front of oneof the Main TPCs. The sizeof the prototypewas40 35 40 cm3.

Pedestals

After completeinstallationof the systemin the experimentalareaandconnectionto the DAQ systempedestaldata(emptyevents,gatingpulseron) weretaken. The result is shown in Fig. 8.8. The noisebehaviour wasasexpectedandmeasuredto bearound1.1 ADC countswhich correspondsto 2600electrons.Thebaselineshows a very smallsystematictendency to dropover thereadouttime by about0.1ADC counts.

Baselinestability

A qualitative impressionof the baselinestability canbe obtainedfrom inspectionof the signalsof asinglepadwith theonlinemonitor. An exampleis shown in Fig. 8.9. However, for a morequantitative

time bins ¡

(¢100 ns)

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mea

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(AD

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ount

s)

Figure 8.8: Meanvalueandr.m.s.of pedestalsasa functionof timebin (averageover128padsfrom onerow).

Figure 8.9: Digitized signals from one ring-cathodeelement recordedfrom a central Pb–Pbinteraction(40 GeV/nucleon).Thefull rangecorrespondsto 45 µs.


evaluationof thebaselinestability with sub-permilleresolutionradioactive 83Kr hasto beused.Pb–Pbinteractionscould not be usedfor this purposebecauseof the ratherhigh occupancy which resultsinmorethanonehit duringthereadouttime.

8.1.7 Conclusionson RCC

Thebenchtestsandin particularthelead-beamtestshave shown that thesystemin generalworksquitewell. In thefollowing theweakandstrongaspectsof every componentarebriefly summarized.

« Thering cathodeswork accordingto expectation:thecouplingof thesignalis foundto bearound90%,i.e. abouta factor4 betterthanclassicalTPCswith field wires(NA49) andabouta factor2betterthanTPCswithoutfield wires(CERES,STAR).

« Thebaselineshift is muchsmallerthanin classicalTPCswhenoperatedwith Ne/CO2 (NA49).Simulationssuggestthat a readoutwithout field wires (CERES,STAR) thoughwould be rathersimilar to theRCC.Unfortunatelyits baselinebehaviour couldnotbemeasuredover thefull timerangerelevantfor ALICE (100µs) becauseof a limitation in thereadout.

« Thewidth of thepad responsefunction (PRF) is asexpectedfrom thering diameterandaboutappropriatefor anapplicationin ALICE. Ideally onewould usea somewhat largerPRFof about3 mm matchingtheexpectedclustersizegivenby diffusion.Thepositionresolutioncouldunfor-tunatelynotbedeterminedin thetests.

« For the massproduction of rings, promisingtechniqueshave beendeveloped. However, morestudiesareneededto evaluateafinal productionscheme.

« Thenew gatingschemeusinggating strips insteadof a wire planeworksin principlequitewell.Its limits in termsof electrontransmissioncould only be determineddown to theper-cent level.The behaviour at the 10 4 level hasto be known thoughfor ALICE applications.The closingpropertiesfor positive ions have not beenmeasured.From simulationsit is concludedthat forNe/CO2 gasit is not possibleto closethe gatewhile the positive ions are still inside the ringcathode.Owing to thehighdrift velocity thepositive Ne ionspasstheopeningafteraround70µs.Thegateopeningtime in ALICE will be100 µs, thereforethepresentgatingschemeneedsto beoptimizedto ensurethatnopositive ionsenterthedrift volume.

8.2 GEM-basedreadoutchambers

8.2.1 Principle of operation of GEMs

Essentially, theGasElectronMultiplier (GEM) is adouble-sidedmetallizedKaptonfoil perforatedwithholestypically of 70 µm diameterand140µm pitch [8]. Theapplicationof a potentialdifferenceacrossthe foil createsa high field region in theholesandenablesthecollection,amplificationandtransferofionization electronsas illustratedin Fig. 8.10. A cascadedstructureof several foils producesa higheffectivegainatarelatively low amplificationof asinglefoil. Differentlyfrom wire chambers,thesignalis producedby themotionandcollectionof electrons,whichareproducedin thelastfoil abovetheanodepads.Ionsdo not contribute to thesignalthusavoiding thetail cancellationproblem.Furthermore,thelateralsizeof the charge clustercollectedon the pads(the PRF) is predominantlydeterminedby thediffusionin thegasandnotby thereadoutgeometry. Thefeedbackof positive ionsinto thedrift volumeis suppressedto 10 1–10 2 by theGEM structureitself [9].

8.2 GEM-basedreadoutchambers 181

Figure 8.10: Schematicview of theamplificationprinciplein a GEM hole.

8.2.2 Basicpropertiesof GEMs relevant for TPC readoutchambers

8.2.2.1 Gain and electron-collectionefficiency

Electronamplificationof a factorof 103 for asingleGEM foil hasbeenobserved.Henceagainof 104

is possiblefor moderateamplificationvoltages∆U in adoubleGEMset-up.Ourmeasurement(Fig.8.11)shows, in agreementwith Ref. [10], thatagain 2 104 is reachedat avoltage∆U ¬ 425V in eachoftheGEM foils. Anotherimportantentity, whichdependsonthevalueof thedrift field, is thetransparencyof theGEM, in otherwordsthecollectionefficiency for primaryelectrons.At TPCoperatingconditions(Edrift 200–400V/cm) thecollectionefficiency wasmeasuredto bearound95%[9,11].

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CERN/µ

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Effe

ctiv

e ga

in

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DOUBLE GEMAr-CO¸

2 70-30

Figure 8.11: Comparisonof the effective gainsachieved in variousGEM configurationsat CERN (opensym-bols[10]) andat GSI (closedsymbols[12]).


0¹

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E¿

= 5 kV cm -1

Fe55 5.9 keV

SINGLE GEM+PCB

4À

10¹

2Á

Figure8.12: Dependenceof theclustersizeonthedrift field for asingleGEM detector, operatedatconstantgainandinductionfield.

8.2.2.2 Pad responsefunction

Differently from conventionalwire chambers,the signal for a GEM stemsmainly from the electronscollectedon thepad(assumingtheappropriateintegrationtime of theamplifier). This suggeststhat theclustersize,andhencethePRF, is dominantlygivenby diffusionof theelectroncloudduring thedrift.Themeasuredclustersize(at thebaseabove thresholdandFWHM) asa functionof thedrift field for agiveninductionfield (5 kV/cm) andgap(2 mm)is shown in Fig.8.12.Thetypicalbehaviour of diffusionin argon-DME is found, with a minimum clustersize of 0.35 mm (FWHM) around500 V/cm [13],indicatingthat thespreadof theelectroncloudby theGEM intrinsic propertiesis negligible relative tothediffusioncontribution in theALICE TPC.This, however, meansthat the rϕ-resolutionis worseforshortdrift distances( 50 cm), i.e. whenthe lateralsizeof theelectroncloudbecomescomparabletothepadwidth.

8.2.2.3 Energy resolution

A pulse-heightspectrumfrom a 55Fe sourcemeasuredin a doubleGEM structurein conjunctionwitha field cageis shown in Fig. 8.13. The detectorwasoperatedin this casewith an 90% Ar, 10% CO2

mixture andwasilluminatedwith 5.9 keV photonsperpendicularto the drift direction. The spectrumexhibitsanenergy resolutionof Â 23%FWHM. Thisresolutionis comparableto standardwire-chamberreadout.

8.2.2.4 Ageing

Ageing measurementshave beenperformedfor GEM–MSGCstructuresanddid not show any deteri-orationof detectorgain up to 15 mC/mm2 wire accumulatedcharge [14]. A priori, however, it is notclearwhetherthis behaviour holdsaswell for doubleGEM structuresoperatedfor high gains,wherehigh charge densitiesoccurin particularat thesecondGEM foil. In orderto investigatetheageingbe-haviour athighgainsadoubleGEM [gas:70%Ar, 30%CO2 ] wasirradiatedwith anintense55Fesourcefor aboutfour weekscorrespondingto an integratedcharge of 0.7 mC/mm2 [15]. This correspondstoabout102 yearsof ALICE heavy-ion operation.In summary, we canstatethat,within theaccuracy ofthemeasurement,no significantdropin gainhasbeenobserved. Inspectionof theGEM foils underanelectronmicroscopeshows no depositsfrom polymerization;theoccurrenceof Newton rings,however,


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Figure 8.13: Pulse-heightdistribution for 5.9keV photonsin an90%Ar, 10%CO2 gasmixtureobtainedwith adoubleGEM detectorin conjunctionwith a field cage.

indicatesthe build-up of a thin ( Â µm) polymerfilm. The conclusion,which canbe drawn from thisrelatively simple test, is that the observed ageingbehaviour would not precludethe useof GEM foilsfrom anapplicationsuchasALICE-TPCreadoutchambers.

8.2.2.5 Ion feedback

Becauseof the asymmetricelectricfields above andbelow a GEM foil the transmissionfor ions andelectronsis different, i.e. ions createdin the amplificationzonearepreferentiallydirectedtowardstheupper layer of a GEM foil (Fig. 8.10 on page181). This featurecould, in casethe ion feedbackissuppressedto a level of 10 4, avoid agatinggrid.

Theion feedbackrelative to theelectronsignalhasbeenmeasuredby comparingthecurrentson thereadoutpadandtheGEM cathode[9]. It wasfoundthatfor symmetricoperationof adoubleGEM (bothGEMsat thesame∆U of 450V) theion feedbackis of theorderof 5%–10%,dependingon thedetailsof theoperatingconditions,i.e. thedrift, transferandinductionfield settingsasshown in Fig. 8.14.Thisvaluecould be improved for a — lessdesirable— asymmetricoperationof the two GEM foils (e.g.∆UGEM1 ¬ 350V and∆UGEM2 ¬ 550V) to a level of Â 1%[10].

8.2.2.6 Sparking

High-gainoperationinvolvesalsofor GEMshighchargedensities(in particularfrom slow, highly ioniz-ing particles)whichcouldleadto sparks.Thesesparkscouldbeharmfulasthey mightdestroy theGEMfoil. In fact,discharging thelargecapacityof a10 Ã 10 cm2 GEM foil leadsto amplitudes(measuredat1 MΩ directlyat thepad)of morethen60 V.

Figure8.15 shows the sparkratewithout sourcefor differentvoltages. The sparkrate is a rathersteepfunction of the appliedvoltageand is of the order of one sparkper hour at a total voltageof∆UGEM1 Ä ∆UGEM2 ¬ 900 V. It seemsto saturateat above 960 V at a rateof about3 Hz. A randomsparkrateof the orderof several sparksper hour is alsoreportedfrom CMS [16], Hera-B[17] andinRef. [18]. In orderto simulatea worst-casescenariowe irradiateda doubleGEM with anintensealphasource(241Am, 5.4 MeV). We find a sparkprobabilityper incidentalphaparticleof theorderof 10 3

(seeFig. 8.16)whenthealphaparticleis stoppedcloseto theGEM foil, i.e. whentheionizationdensitycorrespondsto theBraggpeak.Thesparkprobabilitydecreasesby two ordersof magnitudeif thesourceis positionedsuchthattheenergy losscloseto thefoils is smaller.


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= 3900 V

Figure 8.14: Ratioof ion currentto electroncurrentasa functionof drift (top), transfer(middle),andinduction(bottom)fields.


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Figure 8.15: Sparkingratewithoutsourceasa functionof highvoltage.

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Figure8.16: Sparkprobabilityperincidentalphaparticleasafunctionof distanceof thesourcefrom thecathodeat a voltageacrosseachGEM foil of U É 457V.


8.2.3 Conclusionson GEMs

Themeasurementsclearly indicatethatadditionalgatingis mandatoryfor theapplicationof a GEM asTPC readoutchamberin order to reachthe requiredsuppressionof ion feedbackinto the drift region.Boththegainachievedandtheprimary-electroncollectionefficiency donotindicateany severeproblemswith respectto a potentialapplicationasTPCreadoutchamber. An integratedcharge correspondingtoabout102 yearsof ALICE heavy-ion operationhasbeenaccumulatedwithout any observabledeterio-rationof thechamberperformance.Theobserved ageingbehaviour would thusnot precludetheuseofGEM foils asTPCreadoutchambers.Wemeasureamoderatebackgroundsparkrateof 10Ê 4 Ë 10Ê 3 Hz(per 100 cm2 foil) at TPC operatingconditions. The irradiationwith slow alphaparticlesyields 10Ê 3

sparksper alphaparticle. The large capacityof the GEM foil gives rise to very large amplitudesincaseof a spark.This might have a destructive impacton theGEM foil itself. Moreover, theseenergeticsparkscouldproduceunfavourableconditionsfor thefront-endelectronicsor otherdetectorsin adenselypacked experiment.Generally, theGEM holdsasa potentialalternative to conventionalreadoutcham-bers.However, for a large-scaleapplicationmoreexperience,in particularaboutthelong-termeffectsofsparks,is mandatory.

187

9 Installation, slow control and safety

9.1 Implementation and infrastructur e

9.1.1 ALICE experimental area

TheALICE detectorwill beinstalledat Point2 of theLHC accelerator:theexperimentalareadesignedfor the L3 experiment. The main accessshaft, 23 m in diameter, provides a 15 Ì 7 m2 installationpassageandspacefor countingrooms. The countingroomsareseparatedfrom the experimentalareaby a concreteshieldingplug (seeFig. 9.1). Theexperimentalcavern is 21.4m in diameterandwill bere-equippedwith a2 Ì 20 t cranehaving aclearanceof about3 m over theL3 magnet.

TheL3 magnetprovidesa11.6m longand11.2m diametersolenoidalfield of up to 0.5T. Theend-capshave a door-like construction.Thedoor-frameswill supportlargebeamstraversingtheL3 magnet,from which theALICE centraldetectorswill besupported.

Figure 9.1: Generallayout of the basicundergroundstructuresat Point 2, showing the L3 magnetand thecountingrooms.

188 9 Installation,slowcontrol andsafety

9.1.2 Implementation of the TPC detector

9.1.2.1 General integration considerations

TheTPCdetectoris supportedby acylindrical spaceframeconstruction,whichalsoservesasasupportfor all thecentraldetectorunits. Thespaceframeis placedon large supportbeamsstraddlingthecoilsectionof theL3 magnet.(seeFig. 9.2). This allows thecompleteassemblyof thecentraldetectorunitsoutsidetheL3 magnet.

The overall ALICE installationscenariorequiresthat the TPC canbe displacedindependentlyofthespaceframe,in orderto gainaccessto the ITS detectorandthecentralvacuumchamber. TheTPCdetectorwill, therefore,be supportedfrom two rails fixed to the spaceframe,which canbe prolongedoutsidethespaceframe.

It is conceivable that the completespaceframe,with the TPC detectorinstalled,is lowereddownasoneunit into the experimentalarea. However, the presentinstallationscenarioforeseesa separateinstallationof theTPCdetector.

Figure 9.2: Generalview of theTPCdetectorandthespaceframeinsidetheL3 magnet.

9.1 Implementationandinfrastructure 189

9.1.2.2 The spaceframe

Thespaceframeis dividedinto 18sectorsof 20Í following theagreedsectorizationof thecentraldetec-tors. All materialhasbeenconcentratedat thesectorboundariesandtwo concentricallyplacedsupportringsasindicatedin Fig. 9.3.

Figure 9.3: Generallayoutof thespaceframeshowing thepositionof theTPCsupportrails.

The frameis supportedon two supportbeams.Therearetwo supportpointson eachbeam,whichassuresthesameverticaldisplacementof theframeandthebeamsat all supportpoints.Thehorizontaldisplacementof theframeis blockedon onesideandfreeto moveon theotherside.Thesupportbeamsare12.1m long andsupportedat their extremitiesby theL3 doorstructure.

Thecombinedspaceframeandsupportbeamstructurehasbeencalculatedfor atotal loadof 50t [1].Thecalculationswerebasedon reducingthedeformationof any two pointson thespaceframeto a fewmm andlimiting the overall vertical displacementto Î 5 mm. Furthermore,the supportmembersofthe frame in front of the PHOSandHMPID detectorswere removed 1 in ordernot to introduceanyadditionalmaterialin theacceptanceof thesedetectors.

The calculationsshow that the movementsof theTPC supportrails canbe limited to 4 mm in thevertical direction and 5 mm in the horizontaldirection, with a correspondinghigheststresslevel of140MPa. Thetotalweightof thespaceframeis 14 t andeachsupportbeamhasaweightof 11 t.

9.1.2.3 Pre-assemblyphase

Thepresentsurfacezoneat Point2 includessufficient assemblyhall spaceto meettheALICE require-mentsandno new hall constructionwill be necessaryfor the detectorassembly. The overall ALICEscheduleforeseesa pre-assemblyphasefor thecompleteTPCdetectorin theSXL2 assemblyhall priorto theinstallationin theundergroundarea,asindicatedin Fig. 9.4. Thedetectorwill befully assembledtogetherwith thespaceframestructure.This will allow anearlypreparationof thevariousdetectorser-vicesandpermit the installationandaccessscenariosto beanalysedandcorrectedbeforelowering theTPCinto theexperimentalcavern. All handlingof theTPCoutsidethespaceframewill bedoneusinga transportframe. Thetransportframewill allow theTPCto betransportedto Point2 andwill alsobeusedduringtheinstallationof theTPCinto thespaceframe.

1A decisionabouttheexistanceof holesin thespaceframewill betakenin spring2000.


Figure 9.4: Pre-assemblyof theTPCin theSXL2 assemblyhall at Point2. Theleft partof thefigureshows theTPCbeingmovedfrom thetransportframeto thespaceframe.

9.1.2.4 Installation in the underground cavern

TheITSdetectoris locatedinsidetheinnerapertureof theTPCandheldin placeby theinnercontainmentvessel. This will allow stablealignmentbetweenthe two detectorsand avoids introducinga specialsupportstructurefor theITS detector.

Beforeproceedingwith theinstallationof theTPCit is necessaryto install thespaceframeinto theL3 magnetvolumeandcompletethe installationof theDipole magnetandtheabsorberstructures.Atthis stagethe TPC,supportedby the transportframe,canbe lowereddown into theexperimentalareaandplacedon theextensionrailsusedfor theinstallationof thespaceframe,asindicatedin Fig. 9.5.

TheTPCwill bepositionedabout4.5 m away from the intersectionpoint anda setof rails will befixed to the inner containmentvesselFig. 9.6. This will allow the ITS andthe vacuumchamberto beinstalled.A detaileddescriptionof theinstallationof theITS detectorandthefixing to theTRD canbefoundin theITS TDR [2].

Oncethe installationof the ITS andvacuumchamberis completetheTPCwill be movedover theITS into its final position,wheretheITS will befixedto theTPC.This is achievedby a setof eccentriclevers,accessiblefrom theoutside,whichtransferthesupportpointsof theITS from therails to theTPCinnervessel.

9.1.3 Access,maintenanceand services

9.1.3.1 Accessfor maintenanceand repair

Accessfor maintenanceto the variouspartsof the TPC detectoris relatively straightforward. Bothreadoutplanesandsupportwheelsareeasilyaccessiblefrom the platformsplacedat several levels onbothsidesof thespaceframe.Theaccessplatformswill beequippedwith supportpointsfor thechamberextractiontools.Theinstallationor removal of areadoutchamber, particularlyontheabsorberside,musthoweverberegardedasrelatively complicatedandtimeconsuming.All accessto theITS detectoror theregion betweentheITS andthefront absorberwill necessitateadisplacementof theTPC.


Figure 9.5: LoweringtheTPCdetectorinto theexperimentalarea.It shouldbenotedthattheaccessshafthasamaximumopeningof 15.0m Ï 7.2m.

Figure 9.6: Positionof theTPCdetectorduringinstallationof theITS detectorandthevacuumchamber.


9.1.3.2 Services

TheTPCservicesaredescribedin Chapter3. All serviceswill have to passthroughthenarrow chicane-shapedclearance(100mm) betweenthemagnetdoorsandthedoorframes(Fig. 9.7). In orderto installthe servicesthe door will have to be opened,which prohibitsany further serviceinstallationson theabsorberside,oncetheMuonspectrometeris installed.

Figure9.7: Routingof cablesfrom thecentraldetectorsthroughthegapin theL3 magnetdoors.

Thegassupplywill comefrom theexistingsurfacebuilding, andthedistributionunitswill belocatedon theshieldingplug in PX24.

To keepthelossesandcostof cableinstallationto a minimumtheracksfor thepower supplieswillbeinstalledascloseaspossibleto theL3 magnet.They will belocatedatbothsidesof theL3 magnetatfloor level, asindicatedin Fig. 9.8. Monitoring of thepower supplieswill becarriedout remotelyfromthecontrolroom,sinceaccessto thepowersupplieswill notbepossibleduringLHC operation.

In the event of a displacementof the TPC detectorall serviceswill have to be disconnected.Thisis facilitatedby installing ‘patch-panels’on thesupportwheels.An alternative possibility would be toarrangeall servicesin ‘garlands’,which would avoid any disconnection.This possibilityhas,however,not yet beenstudied.

9.1.4 Assemblyand installation schedule

Theinstallationof theTPCdetectoris integratedwith thegeneralschedulefor theALICE project.Fig-ure 9.9 shows the overall schedulefor the TPC project. The TPC will be pre-assembledin the SXL2assemblyhall at Point 2 betweenAugust2003andJune2004,andthe installationin theexperimentalareawill take placein September2004.


Figure 9.8: Principalroutingof services,showing thepositionof theracksin front of theL3 magnet.TheMuonplatformandL3 magnetdoorstructuresarenotshown.

Task Name

TPC DETECTORTechnical Design ReportFIELD CAGE AND END PLATESCHAMBERSFRONT-END ELECTRONICSPRE-ASSEMBLY SXL2

Space Frame Ready for TPC Pre-assemblyField Cage Trial Fit into Space FrameRemove TPC From Space FrameInstall Chambers in Field CageFit Electronics disc to Field CagePre-assembly Complete

INSTALLATIONInstall Extension RailsLower TPCInstall TPCCheck AlignmentRoll-out For ITS InstallationFinal ConnectionRoll-in After ITS InstallationConnect ServicesRemove RailsClose L3 Magnet DoorsITSVACUUM SYSTEMMACHINE INTERFACE

Build Beam Line Support StructureInstall Low B Quads (RB24 Side)Installation completeInstall Low B Quads RB26 Side)Installation complete

SHIELDINGComplete Beam shielding (RB24Side)Close Tremi (Plug)Shielding On Plug

LHC BEAM

Qtr 1 Qtr 2 Qtr 3 Qtr 4 Qtr 1 Qtr 2 Qtr 3 Qtr 4 Qtr 1 Qtr 2 Qtr 3 Qtr 42003 2004 2005

Figure 9.9: Overall schedulefor theTPCassemblyandinstallation.


9.2 Slow control

TheALICE DetectorControlSystem(DCS)is responsiblefor themonitoringandcontrolof thecorrectoperationalconditionsof thesubdetectors.As this taskalsoinvolvessafetyaspects,thehardwarelinksusedare independentof the DAQ. The ALICE DCS is presentedin Ref. [3] andwill be describedindetail laterin theALICE ComputingTechnicalProposal.Its functionalitiesinclude(seealsoRef. [4]):

Ð startingor shuttingoff adetector, or componentsof a detector, in acontrolledway;

Ð monitoringof characteristics(analogand/orstatusvalues)whicharenecessaryfor detectoropera-tion and/orthephysicsdataanalysis;

Ð reportingof alarmconditions;

Ð loggingandarchiving of characteristics,alarmsandoperatorinteractions;

Ð retrieving archiveddatafor trenddisplaysor detectoranalysis;

Ð uploadinganddownloadingcompleteeventsto andfrom thefront-end(seeSection5.1.6).

In addition,interactionsarerequiredwith a numberof externalsystemslike theareasafetysystem,cooling andventilationsystem,electricity mainssupply, LHC, andmagnets.However, thesesystems,with theexceptionof themagnets,will only provide informative links to theDCS.Suchfunctionalitieshave to beimplementedin two operatingconditions.

Ð During normalphysicsdata-takingtheDCSwill controlstartingandoperationof all theALICEsubdetectors.For thispurposestandardoperatorcommandswill beavailable.Malfunctioningwillbesignalledthroughcentralizedalarmsandto thedetector-dedicatedcontrolstation.

Ð During installationand/ormaintenanceperiodsit will be necessaryto run differentdetectors,orpartitionsof them, separatelybut simultaneously. In this caseinterferenceamongdetectorsorbetweenthemandexternalservicesmustbescreened.

To satisfytheabove requirementstheDCSarchitecturewill have two essentialfeatures–scalabilityandmodularity–andwill bebasedon distributedintelligence.Theslow controlsystemwill bedesignedandorganizedin layers,correspondingto differentlevelsof visibility andaccessrights.Thehigherlevelswill have amoreglobalview, andwill only beallowedto make a limited setof macroscopicactions.Atthe otherend, lower layerswill have accessto moredetailedinformationandcontrol. At the highestlevel of the experimenta SupervisoryControl layer will provide thecommunicationsamongthe mainALICE subsystemssuchas: theDataAcquisitionControl (DAQC), theTriggerControl (TRC) andtheDCS.TheDCSwill beaccessedthroughtheSupervisoryControl layerandno peer-to-peerconnectionbetweenDCSandDAQ is envisaged.TheSupervisoryControlwill have thefollowing features.

Ð It will provide aglobalview of thewholeexperimentto theoperator.

Ð It will allow thecontrolof theexperimentthroughcommandsto theDCS,theDAQCandtheTRC.It will be capableof generatingthesequenceof operationsin orderto bring theexperimentto agivenworkingcondition.However, detailedactionswill betheresponsibilityof thesubsystems.

Ð It will collectanddispatchall thecommunicationsbetweenthesubsystems.

Ð It will monitor theoperationof thesubsystems,generatealarms,andprovide the interlock logicwherenecessary.

9.2 Slowcontrol 195

Ð It will allow thedynamicsplittingof thedetectorinto independentpartitionsandthepossibilityofconcurrentdata-takingfrom thepartitions.

Hardwareprotectionof TPCcomponentswill be implementedwherever possible.This is thecase,for example,for theramp-down of sensewire highvoltagesin thepresenceof sustainedover-currents.

9.2.1 Hardware

As for thegeneralALICE DCS,thehardwarestructureof theTPCDCSwill includethreelayers.

Ð Processlayer. This is thelayerof field instrumentationsuchassensorheads,actuators,etc. Thefield instrumentationhasto complywith therequirementsof thedetectorhardware.However, theinterfacesto the control equipmentwill follow well-establishedelectricalstandardslike 0–10Vfor voltageinterfacesor 4–20mA for current-loopinterfaces.Thesignalsto bemonitoredfor theTPCdetectorarelistedin Tables9.1and9.2.

Ð Control layer. This correspondsto multipurpose-controlcomputerequipmentof the PLC (Pro-grammableLogic Controller) type, in compliancewith the relevant recommendation[5]. How-ever, wherever convenientin the caseof a large numberof field-instrumentationchannelsto becontrolled,VME-basedcontrollersmaybeused.Thishardwarelayeralsoincludesself-containedintelligentinstrumentslike high- andlow-voltagepowersupplies.

Ð Supervisory level. Theequipmenton this layerconsistsof general-purposeworkstationswhichwill be linked to the control layer throughthe TCP/IP. The workstationswill provide the Man–MachineInterface(MMI) to the DCS andwill behave asserver stationsfor detectormonitoringanddatalogging, or as client stationsfor detectorcontrol. At the level of generalsupervisorycontrol, the workstationswill be dedicatedto the managementof configurationdatafor all thedetectorsandequipment,partitioning,alarms,loggingandarchiving, anddatacommunication.

9.2.2 Communication

Thedatatransmissionlinks canbecategorizedin layersequivalentto thehardwarearchitecture.At thefield-instrumentationlevel, point-to-pointlinks for voltageor currentsignalswill bethegeneralcase.Anexceptionwill bethelargenumberof FEEtemperaturesensors(two perboard,i.e.9360in total),whicharereadby dedicatedon-boardASICs. Similarly, the voltagefor the FEE is regulatedon-board.TheASICswill beconnectedto thecontrollerlevel via oneof theproposedstandardfieldbuses.This doesnot changethehardwarearchitecturesincethebussystemwill beseenasanextensionof thecontrollerstation. In addition is it foreseento allow the up- anddownloadof completeeventsto and from theFEEvia theDCSinterface(seeSection5.1.6). This providestheability to testtheentireTPCwithoutinvokingthegeneralDAQ.Theconnectionbetweendifferentsubsystemswill beestablishedby afieldbusor a dedicatedLAN which is alsoconnectedto thesupervisorylevel. Accessto theequipmentwill beallowed from remotelocations.However, accessrestrictionsareplanneddependingon the locationsinorderto avoid conflicts.

9.2.3 Software

Thecontroller-level software,which will residein thecontrol computersthataredirectly linked to theprocess,will beconfiguredindividually for eachsubdetector. For developmentandmaintenanceof thedetectorseachgroupwill alsoconfigurea personalizedMMI. This softwarewill bebasedon thesameproduct(s)asfor theALICE DCSsystemandwill thereforeallow integrationin theoverallsystemduringoperationof theexperimentand,separateaccessandcontrolof eachsubsystemduringotherperiods.It


Table

9.1:Main

parametersofthe

DetectorC

ontrolSystem

forthe

TP

C.

System

s/sub-system

sLocation

Controlledparam

etersN

umber

Linktype

Parameters

Control

FE

Ecooling

UX

25inletand

outletliquid-coolanttempera-

ture36

analogtem

peratureR

/W

UX

25inletand

outletliquid-coolantpressure4

analogpressure

Rend-cap

liquid-coolantvalvecontrol

4analog

voltageR

/Wpad

planegastem

perature72

analogtem

peratureR

UX

25tem

peraturethreshold

forcooling

alarm2

analogvoltage

R/W

UX

25pressurethresholdfor

coolingalarm

2analog

voltageR

/WP

X24

safetyswitch

1binary

voltageon/off

FE

Econtrol

detectorF

EE

temperature

2Ñ

130Ñ

36bus

temperature

Rdetector

FE

Evoltageregulation

3Ñ

130Ñ

36bus

currentR

/Wdetector

interface(status,exceptions,pedestals,

events)130Ñ

36bus

complex

R/W

detectorboardon/off

1bus

bitpatternon/off

FE

Elow

voltageU

X25

FE

Epow

ersupply2Ñ

36serial

currentR

/WU

X25

FE

Epow

ersupplytemperature

2Ñ36

analogtem

peratureR

UX

25F

EE

powersupplystatus/enablew

ord2Ñ

36serial

bitpatternR

/Wdetector

FE

Evoltageregulation

3Ñ

130Ñ36

buscurrent

R/W

PX

24safetysw

itch1

binaryvoltage

on/off

9.2 Slowcontrol 197

Tab

le9.

2:M

ain

para

met

erso

fthe

Det

ecto

rCon

trol

Sys

tem

for

the

TP

C,c

ont’d

Sys

tem

s/su

b-sy

stem

sLo

catio

nC

ontr

olle

dpar

amet

ers

Num

ber

Link

type

Para

met

ers

Con

trol

The

rmal

scre

ende

tect

orth

erm

alsc

reen

plat

ete

mpe

ratu

re72

+36

anal

ogte

mpe

ratu

reR

dete

ctor

inne

r-cyl

inde

rtem

pera

ture

18an

alog

tem

pera

ture

RU

X25

inle

tand

outle

tliq

uid-

cool

antte

mpe

ra-

ture

2

Ò (72+

36)

anal

ogte

mpe

ratu

reR

/W

UX

25liq

uid-

cool

antv

alve

cont

rol

72+

36an

alog

volta

geR

/Wdr

iftvo

lum

ega

stem

pera

ture

72an

alog

tem

pera

ture

RP

X24

safe

tysw

itch

1bi

nary

volta

geon

/off

Fie

ldca

geH

VU

X25

HV

supp

lyon

/off

1se

rial

volta

geR

/WU

X25

HV

settt

ings

and

read

ings

1se

rial

com

plex

R/W

UX

25ex

tern

alre

sist

ors

4an

alog

volta

geR

UX

25sa

fety

switc

h1

bina

ryvo

ltage

on/o

ffR

eado

utch

ambe

rsP

X24

HV

supp

lyon

/off

1se

rial

volta

geR

/WP

X24

HV

settt

ings

and

read

ings

1se

rial

com

plex

R/W

PX

24sa

fety

switc

h1

seria

lvo

ltage

on/o

ffG

assy

stem

PX

24pr

imar

yin

let

and

outle

tgas

tem

pera

-tu

re2

anal

ogte

mpe

ratu

reR

PX

24pr

imar

yin

leta

ndou

tletp

ress

ure

4an

alog

pres

sure

RP

X24

prim

ary

inle

tand

outle

tgas

flow

2an

alog

flow

RP

X24

regu

latio

n5

seria

lco

mpl

exR

PX

24sa

fety

switc

h1

seria

lvo

ltage

on/o

ffP

X24

purit

yco

ntro

l2

seria

lco

mpl

exbi

tpat

tern

dete

ctor

prim

ary

inle

tan

dou

tletg

aste

mpe

ra-

ture

2

Ò 18an

alog

tem

pera

ture

R

dete

ctor

prim

ary

inle

tand

outle

tpre

ssur

e2

Ò 18an

alog

pres

sure

Rde

tect

orpr

imar

yin

leta

ndou

tletg

asflo

w2

Ò 18an

alog

flow

R


is plannedthat thedriver softwarefor thecontrollerstationsto interfacethefield instrumentationto theALICE DCSarchitecturewill bebasedon theOPC [6] standard.OPC,i.e. ObjectLinking andEmbed-ding (OLE) [7] for ProcessControlallows Windows applicationsto accesscontroldatain a controlledway. It meansthathardwareandapplicationsfrom differentmanufacturerscanbeconnectedmoreeasily.OPCis currentlybeingevaluatedin thecontext of theCERNJCOPproject.It is basedon theMicrosofttechnologyDCOM (DistributedComponentObjectModel) andprovidesa standardizedaccessmethodandunifiedinterfacebetweenthefield level anda SCADA (SupervisoryControlAnd DataAcquisition)systemor office applicationsrunningunderWindows. It is thereforepossible,for example,for anExcelmacroto readdatafrom a PLC via theOPCInterfaceandto displayit graphically. TheOPCinterfacestandardis definedanddevelopedby theOPCFoundationwhichincludesthemajorcompaniesin theau-tomationsector(Siemens,Fisher-Rosemount,NationalInstruments,RockwellSoftware,et al.). A widerangeof OPCserversandapplicationsis alreadyavailableandadditionalcompanieshave announcedtheir adherence.

9.3 Safetyand quality management

9.3.1 Mechanical

In thebiddingprocessa detailedsetof specificationsfor theTPCcylindersandend-plateswill bepro-posedto the manufacturer. This will containappropriateconditionsin ordernot to increasethe pricesinordinatelywith respectto choiceof materialandvery extensive testingby the manufacturers.As amatterof productionfollow-up,dimensionalcheckswill becarriedoutatany point duringtheconstruc-tion andprior to shipmentanddelivery to CERN.Thesupplieris expectedto becapableof working toquality assurancestandardISO9001or anequivalentnationalstandard.

AlthoughtheTPCdetectorwill beoperatedat a pressureof 1 mbarabove atmospheric,thevesselsaredesignedfor amaximumover-pressureof 5 mbar.

9.3.2 Gas

In addition to adherenceto mechanicaltolerances,the fabrication,finishing, andchoiceof materialsmustensureanadequategaspurity in orderto run thedetectorwith thedesiredperformanceandwithinoperationalcost.Excessiveleaksleadto intolerablegasflowsandtheinjectionof freshgas.It is thereforeforeseenthatdetailedleaktestswill beperformedat theconstructor’s sitewith acompletetrial assemblyof thedetector. Theprocedureof thesetestsis describedin Section3.1.2.6.

All the gasesusedin the TPC arenonflammable.However, becauseof its large volume, the driftvesselwill have to be flushedwith air prior to any intervention- suchasreplacinga readoutmodule-which involve openingthedetectorin orderto avert anoxygendeficiency situationin theenvironment.The useof a moderatedoseof a short-lived gaseousisotope(200 MBq, half-life 1.9 h) complieswiththe existing rulesof radiationprotection. As far as the containmentvesselsareconcerned,redundantandstand-alonesafetymechanismshave beenimplementedin orderto protecttheTPCfrom under- orover-pressures.

9.3.3 Radiation protection

The two mainmechanismsthatmay induceradioactivity in theTPCarelow-energy neutronactivationandinelastichadronicinteractionsat high energy. Themaximumneutronfluencesover a periodof tenyearsat theinnerandouterradii of theTPCare3.5 Ì 1011/cm2 and1.1 Ì 1011/cm2, respectively. Scalingfrom theequivalentdoseratesinducedby thehigh-luminositypp interactionregions[8] to thoseof theALICE experimentalconditions(approximatelya factorof 100 lower), we do not expectany radiationhazardsto becausedby theaccumulationof radionuclidesin theTPC.

9.3 Safetyandqualitymanagement 199

9.3.4 RF shielding

Both surfacesof the inner andouterTPC containmentvesselsarecoatedwith a Ó 50 µm thick layerof aluminium foil. Thicker metallic shieldingis incompatiblewith our requirementsfor a low-massstructurefor theTPC.

9.3.5 Electrical systemprotection

9.3.5.1 High voltagefor the field cage

Whenassessingtherisksof operatingthevery high-voltagecircuitry of theTPCfield cage,it becomesevidentthatany componentfailurein thesystemmustautomaticallyleadto high-voltageshut-off. Ontheotherhand,thepossibilitiesto recognizecomponentfailurearevery limited, usuallyrestrictedto mea-suringthecurrentthroughthevoltagedivider resistorchain.For example,shortedpathsandconnectionsarerecognizedwhenthereis anincreasein thedivider current,whilst openconnectionsareindicatedbyadecreaseof current.In bothscenariostheHV supplysystemwill beshutdown beforecorrective actioncanbetaken.

Given the limited accessandthe mechanicalfragility of the system,it is alsoclearthat correctivemeasuresto thevoltagedivider network itself areextremelydifficult, if not impossible,to make. Sincein situ interventionson the resistorchain arebasicallyexcluded,the voltagedividing network insidethe resistorrod must be very robust and undergo stringentquality control and test procedurespriorto its installation. Therefore,every single component(e.g. the resistors),and eachstageduring theassemblyof therod arecarefully inspectedandtested.Uponcompletionof thework theentireresistorrodwill beoperatedabovenominalrating( Ô 100kV) andburnt in by thermalcycling for severalweeks.Furthermore,asoutlinedin Section3.1.2,thevoltagedividing network will betestedat regularintervalsduringassembly.

9.3.5.2 High voltagefor readoutchambers

Thereadoutchambersrequireanoperatingvoltageof around1700V. For theoutersectorstwo separatevoltagesareforeseen,whereasthe innersectorsrequireonly onevoltage. In total, 108supplylinesareneeded.The installationis basedon standardcoaxialhigh-voltagecablesratedfor 3 kV, togetherwithstandardhigh-voltageconnectors.

Standard,remotely-controlledpowersupplieswith voltageandcurrentmonitoringwill beused.If anover-currentis detected,thecorrespondingvoltagewill berampeddown at apresetrate.No partsof thereadoutchambersunderhigh voltageareaccessibleoncethechambershave beeninstalled.Therefore,no specialprovisionssuchasinterlocksareforeseen.

9.3.5.3 Low voltage

Thefront-endelectronicsof theTPCis a typical low-voltagehigh-currentsystem(about21kA in total),which mayrun therisk of fire in caseof uncontrolledcurrents.To avoid any dangerto theTPCanditsreadoutsystem,thefollowing strategy hasbeenadopted.

Firstly, thepower suppliesthemselvesareground-free.Thegroundreferenceis obtainedonly at thedetectorside.This avoidsany accidentalparasiticcurrentsin theconductingpaths(not adaptedto suchlargecurrents)flowing backto thepower supplyif oneof thegroundlinesis broken.

Secondly, thepoweringof thesystemwill bemonitoredby theDCS.Eachfront-endcardprovidesameasurementof all the incomingvoltages.If thereis a voltagedrop, thesystemcanbepowereddownonatime-scaleof milliseconds.By monitoringalsothetemperatureof eachfront-endcard,theDCScanreactto temperatureexcursions,andshutoff therelevantsectionof thesystem.

Furthermore,the designof the front-endcardsandtheir connectionsto the groundof the readoutchamberis suchthatthecoppercross-sectionis sufficiently largeto accommodatehighcurrentdensities


(seeSection5.1.8). This would be thecaseif thegroundreturnline wasaccidentallyconnectedto thegeneralground,whichwould leadto aparasiticcurrentthroughtheTPCsupportstructure.

9.3.6 Laser

TheTPClasercalibrationsystemusestwo pulsedNd-YAG lasersbothhaving anenergy of about40 mJperpulseat a wavelengthof 266nm. Thelaserswill beplacedascloseaspossibleto theTPCoutsidethe L3 magnet. The laserlight will be guidedby a mirror systemto the end-platesof the TPC. Thelight path will be totally enclosedby light-tight tubes. A similar light-tight systemis also foreseenfor thedistribution of thelaserlight at theperimeterof thesupportwheel(seeSection4.3.1).Thelightenclosureandlight pathswill only beaccessibleto authorizedpersonnel.Oncethelaserlight hasenteredtheTPCdrift volumethroughthequartzwindows no furtherprotectionis needed.All partsof thelasersystemwill belabelledwith properwarningsymbols.

9.3.7 Safetyaspects

TheTPCdetectorhasbeenthesubjectof a recentInitial SafetyDiscussion[9]. Theoutcomeof this ISDwasthatthedesignof theTPCdetectordid not includeany majorsafetyrisks.

TheTPCdetectorusesnonflammablegasmixturesandtheabsenceof toxic, corrosive,or flammablecomponentsmakestheTPCan intrinsically safedetector. Theoperationof the two Nd-Yag laserswillbe inhibited duringaccessperiodsandthe laserbeamswill be guidedthroughsteeltubes.Apart fromtheinitial constructionperiodthehandlingof theTPCwill alwaysrely onthemechanicalstabilityof thespaceframeor thetransportframe,whichwill reducetheprobabilityof any mechanicalfailure.

Theclosedvolumeinsidethedipolemagnetandthepartof theMuon spectrometerthatpenetratesinto the L3 magnetwill be separatelymonitoredfor both flammablegasandoxygendeficiency. Theaccessto theinsideof theL3 magnetwill berestrictedandregardedasaconfinedvolume.

Thehigh-voltage( Ô 100kV) appliedto thedetectorwill not beaccessibleonceit is installedin itsfinal position,but couldconstituteasafetyhazardduringtestsandstand-aloneoperations.An appropriateouterprotectionlayerfor thehigh-voltagecablewill beincludedin thefinal design.

All constructionmaterialsandelectronicprinted circuit boardswill conform to the CERN safetyinstructionTIS IS41andIS23 concerningtheuseof plasticandothernon-metallicmaterialsat CERNwith respectto fire safetyandradiationresistance.

201

10 Organization

TPC organization

The ALICE TPCorganizationis constitutedby a projectleader, a deputyprojectleaderandeight sec-tions: Field Cage,Read-OutChambers,Front-EndElectronics,GasSystem,Slow Control,ReadoutandLevel-3,TestBeamsandSimulation.

Field Cage

Test BeamsÕ Gas

SystemÖ Slow

Control× Readout

Level-3Read-Out Chambers

Front-EndElectronics

SimulationÖ

Project LeaderPeter Braun-Munzinger

GSIØ

Deputy Project Leader Peter Glaessel

HeidelbergÙ

ALICE TPCÚ

TPC task force

Thefollowing personshave contributedto thework presentedin thisTechnicalDesignReport.

H. Appelshauser, J.Bachler, J.A.Belikov, C.Blume,P. Braun-Munzinger, H.W. Daues,R.EsteveBosch,Z. Fodor, P. Foka,C.Garabatos,C.Gregory, H.A. Gustafsson,J.Hehner, M. Hoch,M. Ivanov, M. Kowal-ski,L. Leistam,V. Lindenstruth,D. Mi skowiec,T. Meyer, L. Musa,R.Renfordt,M.J.Richter, D. Rohrich,K. Safarık, A. Sandoval, H. Sann,E. Schafer, H.R.Schmidt,S.Sedykh,A. Sharma,B. Skaali,J.Stachel,H. Stelzer, R.Stock,D. Swoboda,P. VandeVyvre,R.Veenhof,D. Vranic,B. WindelbandandtheALICETA2 engineeringgroup.

TPC TDR editorial committee

TheTPCTDR editorialcommitteewascomposedof thefollowing persons:

P. Foka(Chair),H. Appelshauser, P. Braun-Munzinger, H.A. Gustafsson,M. Kowalski,T. Meyer, A. San-doval, H.R. Schmidt,J.Stachel,R. Stock

Participating institutions

Thefollowing institutionswill participatein theconstructionof theTPCdetector.

Ð Bergen,Norway, Departmentof Physics,Universityof Bergen.Ð Bratislava,Slovakia,Facultyof MathematicsandPhysics,ComeniusUniversity.Ð Budapest,Hungary, KFKI ResearchInstitutefor ParticleandNuclearPhysics,HungarianAcademyof Sciences.Ð CERN,Switzerland,EuropeanLaboratoryfor ParticlePhysics.Ð Cracow, Poland,HenrykNiewodniczanskiInstituteof NuclearPhysics,High Energy PhysicsDe-partment.

202 10 Organization

Ð Darmstadt,Germany, Gesellschaftfur Schwerionenforschung(GSI).Ð Frankfurt,Germany, Institut fur Kernphysik,Johann-WolfgangGoetheUniversitat.Ð Heidelberg, Germany, Kirchhoff Institut fur Physik,Ruprecht-KarlsUniversitat.Ð Heidelberg, Germany, PhysikalischesInstitut,Ruprecht-KarlsUniversitat.Ð Lund,Sweden,Divisionof CosmicandSubatomicPhysics,Universityof Lund.Ð Marburg, Germany, FachbereichPhysik,PhilippsUniversitat.Ð Copenhagen,NielsBohr Institute.Ð Oslo,Norway, Departmentof Physics,Universityof Oslo.

Responsibilities

Table10.1presentsthesharingof responsibilitiesfor theconstructionof theTPCdetector.

Table10.1: Sharingof responsibilitiesfor theconstructionandinstallationof theTPCdetector.

Item Institution

Fieldcage CERNReadoutchambers Bratislava,Budapest,Frankfurt,GSI,HD (Stachel)FEEandreadout Bergen,Budapest,CERN,Frankfurt,GSI,

HD (Herrmann),HD (Lindenstruth),HD (Stachel),Lund,Marburg, Oslo

Gassystem GSISlow control CERN,GSI,MarburgHigh voltageandlow voltage Crakow, Frankfurt,HD (Stachel)Lasersystem Copenhagen,GSI

Construction programme

The designof the field cagewill be finalizedat the endof 2000. It will be constructedduring 2001,assembledandtestedin 2002andby May 2003it will be readyfor installation. TheR&D of thefieldcageandgasenvelopewill extenduntil theendof 2001to develop thebestassemblyandcertificationprocedures.

A full-size prototypereadoutchamberwill be constructedandtestedby the third quarterof 2000.Theproductionandtestingof thefull setof readoutchamberswill commenceduringthefirst quarterof2001andwill continueuntil March2003.

Regardingthegassystem,theR&D onarecoveryplantwill continueuntil theendof 2000,atwhichtime it will bedecidedwhetheror not it is needed.

Thecomponentsof thefront-endelectronicswill bedesignedandprototypeduntil thefourthquarterof 2001. Their production,startingin the fourth quarterof 2001,will be finishedby the endof 2002.Testingof the componentswill run in parallel with the construction,and the systemintegration andsystemtestwill bedonefrom thefourthquarterof 2002until May 2003.

The design,construction,testand installationscheduleof the TPC componentsis summarizedinFig. 10.1.

Cost estimateand resources

Thetotal costof theTPChasbeenestimatedtakinginto accountrealisticyieldsfor theelectronicchips.Industrialquoteswereusedasmuchaspossible.Table10.2givestheglobalcostof theTPCin kCHF.

203

Task Name

TPC DETECTORTechnical Design ReportFIELD CAGE AND END PLATES

DesignIssue of contractFabrication & ConstructionDelivery at CERNInspection & CertificationAssembly and EquipmentField Cage Ready

CHAMBERSDesign & prototypingOrdering materialProductionGluing padplane and Alu barsTesting vacuum, planarity, accuracyWiringCablingHV testsReady for pre-assembly

FRONT-END ELECTRONICSPROTOTYPING

PASA - prototypeALTRO - prototypeADC - EVALUATIONFEE CABLE - PROTOTYPEFEE BUS - PROTOTYPEFEC - prototypeRCU - prototype

PRODUCTIONADC - productionPASA - productionALTRO - productionFEC - productionFEE cable - productionFEE bus - productionRCU - production

TESTPASA - testALTRO - testFEC - testFEE cable - testFEE bus - testRCU - testSystem test

Assembly of service support wheelMounting on service support wheelLV and cooling connectionsTestingReady for pre-assembly

PRE-ASSEMBLY SXL2INSTALLATION

Qtr 4 Qtr 1 Qtr 2 Qtr 3 Qtr 4 Qtr 1 Qtr 2 Qtr 3 Qtr 4 Qtr 1 Qtr 2 Qtr 3 Qtr 4 Qtr 1 Qtr 2 Qtr 3 Qtr 4 Qtr 1 Qtr 2 Qtr 3 Qtr 4 Qtr 1 Qtr 2 Qtr 3 Qtr 42000 2001 2002 2003 2004 2005

Figure 10.1: Chartof thetime-linefor theconstructionof theTPC.

Table10.2: Globalcostof theTPCin kCHF.

Item Cost

Fieldcage 1660Readoutchambers 1665High/low voltage 950Frontendelectronics 9000Readout 2040General 1531

Total 17046

TheLevel-3 farm,doingthelocalpatternrecognitionanddatacompressionduringthePbperiodandusedfor offline reconstructionoutsideof it, hasbeencostedat 1980 kCHF andis budgetedunderthecommonprojects.

The resourcesof the participatinginstitutionscover the costsof the construction,installationandcommissioningof theTPC.

205

References

Chapter 1

[1] ALICE Collaboration,TechnicalProposal,CERN/LHCC/95–71.[2] A. Mueller, in Proc.of theINPC 98;Nucl. Phys.A654 (1999)37;hep-ph/9902302.[3] ALICE Collaboration,TechnicalProposal,Addendum2, CERN/LHCC/99-13.[4] ALICE Collaboration,TechnicalProposal,Addendum1, CERN/LHCC/96–32.

Chapter 2

[1] H.G.Fischer, PrivateCommunication.[2] ALICE Collaboration,ALICE TDR 4, CERN/LHCC99-12.[3] ALICE Collaboration,TechnicalProposal,Addendum2, CERN/LHCC99-13.

Chapter 3

[1] S.Afanasiev etal. (NA49 Collaboration),Nucl. Instrum.MethodsA430 (1999)210.[2] R. Bouclieretal., TechnicalNoteCMS–TN/96–038.[3] J.Bachler, InternalNoteALICE/99–25.[4] D. Vranic, InternalNoteALICE/97–22.[5] J.Harrisetal., Nucl. Instrum.MethodsA315 (1992)33.[6] F. Hahn,PrivateCommunication.[7] I. LehrausandW. Tejessy, InternalNoteALEPH 97–009,TPCGEN97–001.[8] A. Marin (CERES/NA45 Collaboration),First Resultsfrom theCERESRadialTPC,in Proc.of

theQuarkMatter’99Conference,Torino, Italy (to bepublishedin Nucl. Phys.A).[9] S.Wenig,Nucl. Instrum.MethodsA409 (1998)100.

Chapter 4

[1] H. Wiemanetal. (STAR Collaboration),Nucl. Phys.A638 (1998)559c.[2] L. Rolandi,InternalNoteCERN-ALEPH-84-004.[3] C.Brandt,in Proc.of theAIP Conference1984,10894-109.[4] F.Sauliet al., Nucl. Instrum.MethodsA386 (1997)531.[5] ALICE Collaboration,TechnicalProposal,Addendum2, CERN/LHCC/99-13.[6] S.Afanasiev etal. (NA49 Collaboration),Nucl. Instrum.MethodsA430 (1999)210.[7] A. Marin (CERES/NA45 Collaboration),First Resultsfrom theCERESRadialTPC,in Proc.of

theQuarkMatter’99Conference,Torino, Italy (to bepublishedin Nucl. Phys.A).[8] W. Blum andL. Rolandi,ParticleDetectionwith Drift Chambers,SpringerVerlag,Berlin, Hei-

delberg, New York, 1993,ISBN 3-540-56425-X.[9] http://www.cern.ch/Alice/transparencies/rjd/Welcome.html.

[10] E. Mathieson,Nucl. Instrum.MethodsA270 (1988)602.[11] B. Yu etal., IEEETrans.Nucl. Sci.NS-38(1991)454.[12] T. LohseandW. Witzeling, InternalNoteCERN-ALEPH-91-156.[13] R.L. Gluckstern,Nucl. Instrum.Methods24 (1963)381.[14] ALICE Collaboration,TechnicalProposal,CERN/LHCC/95-71.

206

[15] I. Lehraus,Nucl. Instrum.Methods217(1983)43.[16] W.W.M. Allison andJ.H.Cobb,Ann. Rev. Nucl. Part.Sci.30 (1980)253.[17] J.A.Hornbeck,Phys.Rev. 84 (1951)615.[18] E.C.BeatyandP.L. Patterson,Phys.Rev. 170(1968)116.[19] A. Barret al.,Nucl. Phys.B61(1998)264.[20] P. BonneauandM. Bosteels,CERN/LHCC/99-33.[21] CMS Collaboration,CMS TDR 5, CERN/LHCC/98-6.[22] J.Bachleretal., InternalNoteALICE INT-99-48.[23] D.Decampetal., Nucl. Instrum.MethodsA294 (1990)121.[24] P. Nemethyet al.,Nucl. Instrum.Methods212(1983)273.[25] S.R.Amendoliaetal., Nucl. Instrum.Methods234(1985)47.[26] A. Lebedev, PrivateCommunication;

M. Alyushin et al., IEEE NuclearScienceSymposium1996,Anaheim,CA, ConferenceRecord,N27–33,p 499–503.

[27] L.V. Griffith etal., Rev. Sci. Instrum.61 (1990)2138.[28] F. Bieseretal., Nucl. Instrum.MethodsA385 (1997)535.[29] S.Bauer, Diplomarbeit,Universitat Frankfurt,1997.[30] F. Gabler, Diplomarbeit,Universitat Frankfurt,1995.[31] F. Gabler, B. LasiukandC.A.Whitten,InternalNoteSTAR 360,1998.

Chapter 5

[1] J.C.Berset,L. Musa,M. Richter, InternalNoteALICE99-49(in preparation).[2] L. Musa,InternalNoteALICE99-50.[3] ALICE Collaboration,TechnicalProposal,CERN/LHCC/95-71.[4] QSE-SAMTEC,MatchedImpedanceConnector.[5] R. Baur et al., Nucl. Instrum.MethodsA409 (1998)278 and in Proc.of the 3rd Workshopon

Electronicsfor LHC Experiments,CERN-LHCC-97-60.[6] D.A. Landiset al., IEEETrans.Nucl. Sci.NS-29, No. 1, 1982.[7] G. Gramegnaetal., Nucl. Instrum.MethodsA390 (1997)241.[8] J.C.Bersetetal., InternalNoteALICE99-51(in preparation).[9] P. Szymanski,InternalNoteALICE INT-98-13.

[10] J.C.Bersetetal., InternalNoteALICE99-52(in preparation).[11] L. Musaetal., Front-EndElectronicsfor theALICE TPC-Detector, in Proc.of the4thWorkshop

onElectronicsfor LHC ExperimentsRome,September21-25,1998.[12] F. Anghinolfi et al., IEEE Trans.Nucl. Science42, No. 4, 1995,p.772.[13] R.A. Boie,A.T. Hrisoho,andP. Rehak,Nucl. Instrum.MethodsA192 (1982)365.[14] High PerformanceBackplaneDesignwith GTL+, SCEA011TexasInstruments,June1999.[15] G. RubinandP. VandeVyvre, InternalNoteALICE 96-43.[16] G. Rubin,InternalNoteALICE 98-21.[17] http://www.cern.ch/TTC/intro.html.[18] STAR collaboration,STAR ConceptualDesignReport,LawrenceBerkeley Laboratory, University

of California,PUB-5347,June1992.[19] A. Ljubicic, Jr. etal.,DesignandImplementationof theSTAR Experiment’s DAQ, in Proc.of the

10thIEEE RealTimeConference,9/22-26/97,Beaune,France.[20] J.Bergeretal., IKF-report,IKF-HENPG/1-98,Universityof Frankfurt.[21] ALICE Collaboration,TechnicalProposal,Addendum1, CERN/LHCC/96-32.[22] ALICE Collaboration,TechnicalProposal,Addendum2, CERN/LHCC/99-13.[23] ALICE Collaboration,ALICE TDR 2, CERN/LHCC/99-4.[24] J.Berger, Diplomathesis,Universityof Frankfurt,1998.

207

[25] M. NelsonandJ.L.Gailly, TheDataCompressionBook,M&T Books,New York (1996).[26] K. Sulimma,PrivateCommunication.[27] STAR Collaboration,DAQ Review, BNL, 1997.[28] C.E.Shannon,Bell SystemTechnicalJournal,27 (1948)379-423.[29] H. Beker and M. Schindler, Data compressionon zero suppressedHigh Energy PhysicsData,

Instituteof ComputerGraphics,TechnicalUniversityVienna(1997).[30] Gray, R. M., IEEE ASSPMagazineApril (1984)4.[31] Linde,Y., Buzo,A, andGray, R. M., IEEE Transactionson Communications28 (1980)84.[32] M. Mattavelli, PrivateCommunication.[33] H. Appelshauser, PhDthesis,Universityof Frankfurt,1996.[34] J.Gunther, PhDthesis,Universityof Frankfurt,1998.[35] http://www.rhic.bnl.gov/STAR/html/daql/public/software/cluster.html.[36] C. Adler et al., TheproposedLevel-3 TriggerSystemfor STAR, subm.to IEEE Transactionsin

NuclearScience(1999).[37] http://ikf1.star.bnl.gov/l3/l3.html/l3clstatus.ps.[38] P. Yepes,InternalNoteSTAR 246(1995).[39] P. Yepes,InternalNoteSTAR 248(1996);Nucl. Instrum.MethodsA380 (1996)582.[40] F. Puhlhofer, D. Rohrich,R. Keidel,Nucl. Instrum.MethodsA263 (1988)360.[41] D. Brinkmannetal., Nucl. Instrum.MethodsA354 (1995)419.[42] http://www.pcisig.org.[43] CERNALICE DAQ group,InternalNoteALICE 99–03.[44] CERNALICE DAQ group,InternalNoteALICE 99–04.[45] G. RubinandJ.Sulyan,InternalNoteALICE 97–14.[46] Reportby ElectronicDepartmentof the Departamentode FisicaFacultadde CienciasExactas

UniversidadNacionaldeLa Plata.[47] TheProgrammableLogic DataBook 1994,Xilinx Inc. (1995)SanJose,California.

Chapter 7

[1] ALICE Collaboration,TechnicalProposal,CERN/LHCC/95–71.[2] M. Kowalski, InternalNoteALICE/96-36.[3] http://www1.cern.ch/ALICE/Projects/offline/AliceOffLineHomePage.html.[4] G.F. Knoll, RadiationDetectionandMeasurements,Wiley, New York, 1979.[5] I. Lehrhausetal., IEEETrans.Nucl. Sci.NS-30(1983)50.[6] W. Blum andL. Rolandi,ParticleDetectionwith Drift Chambers,SpringerVerlag,Berlin, Hei-

delberg, New York, 1993,ISBN 3-540-56425-X.[7] J.E.Moyal, Phil. Mag.46 (1955)263;

W.W. M. AlissonandJ.H. Cobb,Ann. Rev. Nucl. Part.Sci.,30 (1980)253;S.K .Kotelnikov etal., Nucl. Instrum.MethodsA307 (1991)273.

[8] K. Lassila-Perini,L. Urban,Nucl. Instrum.MethodsA362 (1995)416.[9] H.-G. Fischer, PrivateCommunication.

[10] W. Bettset al., InternalNoteSTAR SN0263(1996).[11] J.S.GordonandE. Mathieson,Nucl. Instrum.Methods227(1984)267;

E. MathiesonandJ.S.Gordon,Nucl. Instrum.Methods227(1984)277;J.R.Thompsonetal., Nucl. Instrum.MethodsA234 (1985)505;E. Mathieson,Nucl. Instrum.MethodsA270 (1988)602.

[12] P. Billior , Nucl. Instrum.Methods225(1984)352;M. Regler andR. Fruhwirth, Reconstructionof ChargedTracks,in Proc.of theAdvancedStudyInstituteonTechniquesandConceptsin High Energy Physics,PlenumPubl.Corp.,1989;R.Fruhwirth,Applicationof Filter Methodsto theReconstructionof Tracksandverticesin Events

of ExperimentalHigh Energy Physics,HEPHY-PUB516/88,Vienna1988;B. Batyunya etal., InternalNoteALICE INT-97-24.

[13] ALICE Collaboration,ALICE TDR 4, CERN/LHCC99-12.

Chapter 8

[1] ALICE Collaboration,TechnicalProposal,CERN/LHCC/95-71(1995).[2] RD-32Collaboration,R&D Proposal,CERN/DRDC92-32,DRDC/P-43;

RD-32Collaboration,Final Report,CERNLHCC 96-16.[3] S.Afanasiev etal., Nucl. Instrum.MethodsA430 (1999)210.[4] J.Bachleretal., InternalNoteALICE INT-99-48.[5] E. Gatti et al., Nucl. Instrum.Methods163(1979)83-92.[6] I. Endoet al.,Nucl. Instrum.Methods188(1981)51-58.[7] J.Bachleretal., InternalNoteALICE INT-97-19.[8] F. Sauliet al.,Nucl. Instrum.MethodsA386 (1997)531.[9] H.R.SchmidtandH. Stelzer, InternalNoteALICE INT-99-54.

[10] S. Bachmannet al, Charge Amplification andTransferProcessesin theGasElectronMultiplier,CERN-EP/99-48(to bepublishedin Nucl. Instrum.Methods).

[11] M. Hoch,DoctoralThesis,TU Vienna(1998).[12] A. Mykulyak et al., InternalNoteALICE INT-99-55.[13] A. Bressanet al., Nucl. Instrum.MethodsA425 (1999)262; CERN preprintCERN-EP-98-163

(1998).[14] J.Benllochetal., Nucl. Instrum.MethodsA419 (1998)410.[15] H.R.Schmidt,H. SannandH. Stelzer, InternalNoteALICE INT-99-56.[16] P. Vanlaeretal, InternalNoteCMS IN-1999/024.[17] M. Hildebrandt,PhD.Thesis,Heidelberg (1999).[18] A. Bressanet al., Nucl. Instrum.MethodsA424 (1999)321.

Chapter 9

[1] J.Bielski, InternalNoteALICE/97-32(updatedwith anannex in July1999).[2] ALICE Collaboration,ALICE TDR 4, CERN/LHCC99-12.[3] O. Villalobosetal., InternalNoteALICE 98-23(new updatedversionin preparation).[4] ALICE/LHCb Teams,InternalNoteALICE/98-03.[5] D. Blancetal., CERN-EP-071(EOS)May 1998.[6] http://www.opceurope.org; http://www.opcfoundation.org/opc-public-tech.htm.[7] http://www.microsoft.com/activex/default.asp.[8] CMS Collaboration,CMS TDR 5, CERN/LHCC98-6.[9] Initial SafetyDiscussionfor theALICE TPCDetector, TIS/GS/WW/1999-054.

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