1 20-23 September 2010, Etretat, Rencontres QGP-France 2010, Adam Matyja Commissioning and...

120-23 September 2010, Etretat, Rencontres QGP-France 2010, Adam Matyja

Commissioning and performance of the ALICE TPC

Outline Components of TPC Calibration Performance results Summary

Adam Matyja

for the ALICE TPC collaboration

Subatech, Nantes & INP PAN Kraków


The ALICE detector

* ALICE TPC Collaboration, J. Alme et al., "The ALICE TPC, a large 3-dimensional tracking device with fast readout for ultra-high multiplicity events.", Physics. Ins-Det/10011950 (2010)

*


The principle of the Time Projection Chamber→ Based on multi-wire proportional chamber Charged particles ionize working gas atoms Ionization electrons drift with the constant

velocity (v) in the direction of readout chambers due to the electric field

Near the anode plane the strong electric field causes ionization avalanche

It induces the signal on the pad plane

Collected charge gives the information about the energy loss

It allows to obtain the information in two coordinates - (x,y)

The third coordinate is inferred from the measurement of the electrons drift time measurement (z=vt)

3 coordinates give the space point on the track

• Main tracking device

• Allows to distinguish the charged particle species


ALICE Time Projection Chamber General features:

Diameter Length : 5 m 5 m Azimuth angle coverage: 2 Pseudo-rapidity interval: ||<0.9 Readout chambers: 72 Drift field: 400 V/cm Maximum drift time: 94 s Central electrode HV: 100 kV

Gas: Active volume: 90 m3

Ne-CO2-N2: 85.7% - 9.5% - 4.8%

Cold gas - low diffusion Non-saturated drift velocity

temperature stability and homogeneity 0.1 K

Data readout: Pads (3 types): 557 568 Samples in time direction: 1000 Data taking rate:

~ 2.8 kHz for p-p ~ 300 Hz for Pb-Pb

5 m

2.5 m

2.5 m


ReadOut Chambers 2 sides with 18 sectors Sector consists of:

Inner chamber (IROC) Outer chamber (OROC)

72 readout chambers Pad readout

3 sizes

Components

Stable operation


Drift voltage systemComponents

Resistor rods

Voltage dividers' network

Provide constant electric field Water cooled voltage dividers

→ remove dissipated power Control of water conductivity Radiation length X / X0 of Field Cage

1.367 % - inner FC 0.607 % - gas 2.153 % - outer FC

Very stable operationFew well understood trips

(beam loss) Tomography of FC in good

agreement with MC


Recirculating gas system

Precise control of gas mixture O2 and H2O contamination

removed by Cu catalyser To minimize signal loss

(e- attachment) Contamination:

~ 1 ppm O2 (design < 5)

Humidity kept at fixed level

→ to avoid aging of components

In operation since 2006

Components


Cooling systemProvide temperature stability ~ 500 temperature sensors Leakless underpressure system with

~ 60 adjustable cooling circuits Thermal screening towards ITS and TRD Copper shields of service support wheel Cooling of ROC bodies Water cooling of FEE in copper envelope (~27 kW) Result: Temperature homogenity: T = 0.046 K

Components

FEC with its cooling envelope

Good agreement with design

specifications


Detector Control SystemEnsure a safe and correct operation of TPC Integrated into Experiment Control System Hardware architecture

Supervisory layer: user interface (PC) + databases

Control layer: hub - collect & process information from supervisory and field layers

Field layer: electronics to control equipment (power supplies, FEE, …)

TPC is fully controlled by ALICE shifter

Components


Noise measurements Noise level improved during commissioning Mean noise level:

0.7 ADC count (700 e) Designed - 1 ADC count (1000 e)

Data volume of empty event: non-zero suppressed (ZS): ~ 700MB ZS event: ~ 30kB

Typical size of the event with data: 0.1 - 0.2 MB (p - p)

360 kB TPC @ 7 TeV ~ 30 MB (Pb - Pb, dN/dy = 2000)

Calibration

Current noise


Gain calibration using 83KrCalibration

main peak (41.6 keV)

OROCResolution of main peak:

• 4.0 % for IROCs

• 4.3 % for OROCs

Gain variation

Result:

Relative gain variation C-side

Determine gain for each pad Procedure:

Inject radioactive 83Kr Characteristic decay spectrum Dedicated clusterizer Fit the main peak (41.6 keV) Parabolic fit Calibration constants

3 different HV settings (gains) High statistics: several 108 Kr events Accuracy of peak position: << 1%

(design: 1.5%) Repeated after electronic maintenance

or every year


Kr calibration - systematics

IROC

OROC

short mid long

Edge effect well visible

→ Parabolic fit applied to avoid it

Radial Azimuth

The shape reflects a mechanical deformation of the pad plane

Calibration

Sector-by-sector


Laser system 336 laser beams Used for:

E B effect Drift velocity measurements Alignment

Calibration

Reconstructed laser tracks

The principle of laser system for the TPC

Laser features: = 266 nm or E = h = 4.66 eV Energy: 100 mJ/pulse Duration of pulse: 5 ns

The ionization in the gas volume along the laser path occurs via two photon absorption by organic impurities.


E B effect

Correction map from laser tracks Measure r For each laser track For several magnetic field settings

Laser system Calibration

Caused by: Mechanical or electrical

imperfections Imperfect B field

r 7 mm

→ for longest drift and nominal field Corrected to ~ 0.3 mm Detailed studies ongoing


Drift velocity measurements d = d(E, B, (Tin, Patm), CCO2, CN2)

Crucial for track matching with other detectors How to obtain drift velocity correction factor:

Matching laser tracks and mirror positions Matching TPC and ITS tracks Matching tracks from two halves of TPC Drift velocity monitor

Required accuracy: 10-4 update every 1h

Calibration

Photo electrons from central electrode

monitor top-bottom arrival time offset caused by T and P gradients


Drift velocity measurements d = d(E, B, (Tin, Patm), CCO2, CN2)

Crucial for track matching with other detectors How to obtain drift velocity correction factor:

Matching laser tracks and mirror positions Matching TPC and ITS tracks Matching tracks from two halves of TPC Drift velocity monitor

Required accuracy: 10-4 update every 1h

Calibration

Photo electrons from central electrode

monitor top-bottom arrival time offset caused by T and P gradients

Corrected spectrum


Space point resolutionPerformance

Depends on: Drift length Inclination angle Charge deposited on the anode wire

In r direction: Y = 300 - 800 m for small inclination angles (high

momentum tracks)

In drift direction: Z = 300 - 800 m

Good agreement with simulations


Momentum resolution High momentum tracks

Cosmic muon tracks treated independently in two halves of TPC

Comparison of pT at vertex gives resolution

Statistics: ~ 5 106 events Low momentum tracks

Deduced from the width of K0S mass

peak Current status (w/o many corrections):

(pT/pT)2 = (0.01)2 + (0.007pT)2

Achieved: ~ 7 % @ 10 GeV/c

~ 1 % below 1 GeV/c Place for improvement:

~ 4 % @ 10 GeV/c

Performance


dE/dx resolution - cosmicsAllows particle identification up to 50 GeV/c Statistics: 8.3106 cosmic tracks in 2008 Design goal: 5.5 % Measured: < 5 %

Performance

TPC cosmic data

500 < p < 550 MeV

p d

e

e

p


dE/dx spectrum in dataPerformance


Material budgetVertices from conversion photonsRadial distribution

Agreement between MC and DATA: 5 ~ 15 %

Performance

TPC begins here


Life of TPC 2006 - first tests at the surface

No ZS Only 2 sectors powered at a time

2007 - first commissioning underground C-side successfully operated Online ZS

2008 - commissioning with cosmics Full TPC Different run types implemented Extensive calibration data taken

_ 06. 12. 2009 - first s = 900 GeV collisions _ 30. 03. 2010 - first s = 7 TeV collisions


Summary Physics results are well visible:

"Midrapidity antiproton-to-proton ratio in pp collisons at s = 0.9 and 7 TeV measured by the ALICE experiment." - Phys. Rev. Lett. 105.072002 (2010)

"Two-pion Bose-Einstein correlations in pp collisions at s = 900 GeV." - Phys. Rev. D 82, 052001 (2010)

"Transverse momentum spectra of charged particles in proton-proton collisions at s = 900 GeV with ALICE at the LHC." - submitted to PLB (arXiv:1007.0719)

Other papers to be submitted soon: J/ production Particle spectra Strangeness D(*)-mesons

ALICE TPC works stably during p-p data taking Calibration done → working on improvements Very good performance, close to specifications We are waiting for Heavy Ions


The ALICE TPC collaborationHarald Appelshaeuser6, Peter Braun-Munzinger7, Peter Christiansen9, Panagiota

Foka7, Ulrich Frankenfeld7, Chilo Garabatos7, Peter Glassel8, Hans-Ake Gustafsson9, Haavard Helstrup1, Marian Ivanov7, Rudolf Janik2, Alexander Kalweit5, Ralf Keidel11, Marek Kowalski10, Dag Toppe Larsen1, Christian Lippmann3, Magnus

Mager3, Adam Matyja10,12 , Luciano Musa3, Borge Svane Nielsen4, Helmut Oeschler5, Miro Pikna2, Attiq Ur Rehman3, Rainer Renfordt6, Stefan Rossegger3,

Dieter Roehrich1, Hans-Rudolf Schmidt7, Martin Siska2, Brano Sitar2, Carsten Soegaard4, Johanna Stachel8, Peter Strmen2, Imrich Szarka2, Danilo Vranic7, Jens

Wiechula8

1. Department of Physics and Technology, University of Bergen, Bergen, Norway.2. Faculty of Mathematics, Physics and Informatics, Comenius University, Bratislava, Slovakia.3. European Organization for Nuclear Research (CERN), Geneva.4. Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark.5. Institut für Kernphysik, Technische Universität Darmstadt, Darmstadt, Germany.6. Institut für Kernphysik, Johann-Wolfgang-Goethe Universität Frankfurt, Frankfurt, Germany.7. Gesellschaft für Schwerionenforschung mbH (GSI), Darmstadt, Germany.8. Physikalisches Institut, Ruprecht-Katls-Universität Heidelberg, Heidelberg, Germany.9. Division of Experimental High Energy Physics, University of Lund, Lund, Sweden.10. The Henryk Niewodniczański Institute of Nuclear Physics, Polish Academy of Sciences, Cracow, Poland.11. Zentrum fur Technologietransfer und Telekommunikation (ZTT), Fachhochschule Worms, Worms, Germany.12. Now at SUBATECH, Ecole des Mines de Nantes, Universite de Nantes, CNRS-IN2P3, Nantes, France.


BACKUP


PID


Cluster finderTrack

Pad

Pad

-row

Track

Fired pad

Cluster

Pad

Pad

-row

Krypton

Fired pad

Cluster

Decay point

1 20-23 September 2010, Etretat, Rencontres QGP-France 2010, Adam Matyja Commissioning and...

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