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ILC TPC resolution studies with charge dispersion in MPGDs with a resistive anode Madhu Dixit Carleton University & TRIUMF IPNS KEK 14/05/2005 Slide 2 M. DixitIPNS KEK 14/10/052 The International Linear Collider (ILC) 2007 - LHC ready to explore new physics and to search for Higgs, supersymmetry, hidden new dimensions, etc. ILC critical to understanding LHC discoveries Detailed study of Higgs and SUSY particles Precision measurements Higgs e + e - -> Z H -> l l X M top 100 MeV, top 2% M Z & M W 5 MeV (from 30 MeV) (sin 2 ) 10 -5 (from 210 -4 ) Global Design Effort (GDE) timeline: 2006 accelerator CDR 2008 accelerator TDR, experimental collaborations, detector CDRs 2009 - Detector TDRs, construction 2015 - Physics at ILC ILC tracker resolution (1/p T ) ~ 5 x10 -5 (GeV/c) -1 (10 times better than at LEP!) Slide 3 M. DixitIPNS KEK 14/10/053 ILC tracker requirements Small cross sections 100 fb, low rates, no fast trigger. Higgs measurements & SUSY searches require: High granularity continuous tracking for good pattern recognition. Good energy flow measurement in tight high multiplicity jets. Excellent primary and secondary b, c, decay vertex reconstruction. TPC is an ideal tracker for ILC. Momentum resolution goal (1/p T ) ~ 5.10 -5 (GeV -1 ) achievable with vertex + Si inner tracker + TPC with (1/p T ) ~ 2 x 10 -4 (GeV -1 ) ILC TPC tracker goals: 200 track points with (r, ) = 100 m, (r, z) = 500 m 2 track resolution < 2mm in (r, ) and < 5 mm in (r, z) dE/dx resolution < 5% Slide 4 M. DixitIPNS KEK 14/10/054 ILC detector concepts Silicon tracker (B=5T) SiW ECAL SiD TPC (B=4T) SiW ECAL (medium) LDC TPC (B=3T) W/Scint ECAL (large) GLD Slide 5 M. DixitIPNS KEK 14/10/055 ILC TPC (TESLA design) cm E B = 4 T Slide 6 M. DixitIPNS KEK 14/10/056 ExB cancels track angle effect TPC wire/pad readout 100 m Average Aleph resolution ~ 150 m About 100 m best for all drift distances Limit from diffusion (10 cm drift) ~ 20 m; (2 m drift) ~ 90 m Conventional TPCs never achieve their potential! Example:Systematic effects in Aleph TPC at LEP Slide 7 M. DixitIPNS KEK 14/10/057 Transverse diffusion sets the ultimate limit on TPC resolution. ILC TPC resolution goals close to the diffusion limit. Wire/pad TPC resolution inherently limited by ExB & track angle systematic effects. A TPC read out with a MPGD endcap could meet the ILC resolution challenge if the precision of pad charge centroid determination could be improved. What is the best achievable resolution with conventional techniques? Micro Pattern Gas Detector (MPGD) Readout for ILC TPC Slide 8 M. DixitIPNS KEK 14/10/058 Micro Pattern Gas Detectors (MPGD) Unlike wires, MPGDs have no preferred direction - negligible ExB effect MPGDs achieve excellent 40 m resolution with 200 m wide pads. Conventional wire readout TPCs use cathode pads of width ~ a few mm. Proposed ILC TPC channel count ~ 1.5x10 6 with 2 mm wide pads. Narrower pads would lead to increased detector cost & complexity. The Gas Electron Multiplier (GEM)Micromegas ~ 50 m ~ mm Drift region Slide 9 America Carleton U Cornell/Purdue LBNL MIT U Montreal U VictoriaEurope RWTH Aachen DESY U Hamburg U Karlsruhe UMM Krakow MPI-MunichNIKHEF BINP Novosibirsk LAL Orsay IPN Orsay U Rostock CEA Saclay PNPI St. Petersburg Asian ILC gaseous- tracking groups Chiba U Hiroshima U Minadamo SU-IIT Kinki U U Osaka Saga U Tokyo UAT U Tokyo NRICP Tokyo Kogakuin U Tokyo KEK Tsukuba U Tsukuba Other USA MIT (LCRD) Temple/Wayne State (UCLC) Yale Worldwide R&D effort for ILC TPC Ron Settles main coordinator Large task list Resolution studies. Ion feedback studies. Gas studies for better resolution and low neutron background. Low mass field cage and endcap. High density low power electronics. Analysis and simulation software. Slide 10 ILC TPC R&D plans 1) Demonstration phase 1) Demonstration phase Continue work for ~1 year with small prototypes on mapping out parameter space, understanding resolution, etc, to prove feasibility of an MPGD TPC. For Si-based ideas this will include a basic proof-of-principle.Continue work for ~1 year with small prototypes on mapping out parameter space, understanding resolution, etc, to prove feasibility of an MPGD TPC. For Si-based ideas this will include a basic proof-of-principle. 2) Consolidation phase 2) Consolidation phase Build and operate large prototype ( 70cm, drift 50cm) which allows any MPGD technology, to test manufacturing techniques for MPGD endplates, fieldcage and electronics. Design work would start in ~1/2 year, building and testing another ~ 2 years.Build and operate large prototype ( 70cm, drift 50cm) which allows any MPGD technology, to test manufacturing techniques for MPGD endplates, fieldcage and electronics. Design work would start in ~1/2 year, building and testing another ~ 2 years. 3) Design phase 3) Design phase After phase 2, the decision as to which endplate technology to use for the LC TPC would be taken and final design started.After phase 2, the decision as to which endplate technology to use for the LC TPC would be taken and final design started. Slide 11 Point resolution, Micromegas Saclay/Orsay/Berkeley --Diffusion measurements (r, ) < 100 m possible --At moment only achieved for short drift (intrinsic ) for gain~5000 (350V mesh), noise~1000 e --ongoing effort B = 1T 1x10mm^2 pads mm^2, B = 1T Many groups are working on MPGD TPC R&D. Prototype results from Some Results -Summer 2005 (Ron Settles) Slide 12 Prototype results contd. Point resolution GEM DESY group. (r, ) measured for GEMs with 2x6mm 2 pads 30cm B=4T Gas:P5 Victoria group has achieved ~ 100 m resolution for short drift distances with narrower 1.2x7mm 2 pads. Slide 13 Prototype Results - cont. Point resolution, GEM --Example of (r, ) measured at Aachen GEMs with 2x6mm 2 pads by comparing track position with a Si hodoscope. --In general (also for Micromegas) the resolution is not as good as expected from diffusion. PRELIMINARY! Slide 14 Narrower pads leading to increased complexity & a larger number of readout channels. Disperse track charge after gas gain over a larger area to improve pad centroid with wide pads. For the GEM, large transverse diffusion in the high E- field field in transfer and induction gaps provides a natural mechanism to disperse the cluster charge. Measurements with prototype GEM-TPCs have shown that this increased diffusion is insufficient in a high B field for the ILC-TPC to achieve the resolution target with ~ 2 mm pads. Explore other concepts to disperse the charge Ideas to improve the MPGD TPC resolution Slide 15 M. DixitIPNS KEK 14/10/0515 Carleton setup for MPGD resolution studies with x rays Point source ~ 50 m collimated 4.5 keV x rays. Aleph TPC preamps. Rise = 40 ns, Fall = 2 s. DAQ - 500 MHz Tektronix digital scope. Slide 16 M. DixitIPNS KEK 14/10/0516 An idea - Measure the induced signals in a GEM GEMProportional wire Cathode padsAnode pads Short ~ 200 ns signal We measure x y 70 m But this technique requires expensive high frequency pulse shape sampling electronics Slide 17 M. DixitIPNS KEK 14/10/0517 Another idea- Position sensing from charge dispersion in MPGDs with a resistive anode Analogy:Analogy:Charge division is used to measure the avalanche position on a proportional wire. Deposit point charge at t=0 Solution for charge density on the wire(L ~ 0) Telegraph equation (1-D): Generalize the concept of 1-D charge division to 2-D Slide 18 M. DixitIPNS KEK 14/10/0518 Charge dispersion in a GEM with a resistive anode Slide 19 M. DixitIPNS KEK 14/10/0519 Equivalent circuit for currents in a GEM with an intermediate resistive anode Resistive anode foil Signal pickup pads Current generators Pad amplifier Slide 20 M. DixitIPNS KEK 14/10/0520 Charge cluster size ~ 1 mm ; signal detected by ~7 anodes (2 mm width) A photon event in the resistive anode GEM test cell Slide 21 M. DixitIPNS KEK 14/10/0521 Modified GEM anode with a high resistivity film bonded to a readout plane with an insulating spacer. 2-dimensional continuous RC network defined by material properties & geometry. Point charge at r = 0 & t = 0 disperses with time. Time dependent anode charge density sampled by readout pads. Equation for surface charge density function on the 2-dim. continuous RC network: (r,t) integral over pads (r) Q r / mm mmns Improving resolution with charge dispersion in a MPGD with a resistive anode M.S.Dixit et.al., Nucl. Instrum. Methods A518 (2004) 721. Slide 22 M. DixitIPNS KEK 14/10/0522 Charge dispersion signal for a GEM Simulation versus measurement (2 mm x 6 mm pads) Collimated ~ 50 m 4.5 keV x-ray spot on pad centre. Primary signal: Fast large amplitude main pulse on charge collecting pad. Simulated primary pulse is normalized to the data. Difference = induced signal (not included in simulation) studied previously: MPGD '99 (Orsay), LCWS '00 Secondary signal: The dispersion pulse on the neighboring pad is slower & smaller. Simulated secondary pulse normalization is the same as for the primary. Detailed simulation includes effects of, longitudinal & transverse diffusion, gas gain, detector pulse formation, charge dispersion & preamplifier rise and fall time effects. For tracks, include effects of unequal primary clusters. Slide 23 M. DixitIPNS KEK 14/10/0523 50 m pillars Drift Gap MESH Amplification Gap Al-Si Cermet on mylar Resistive anode Micromegas 530 k / Carbon loaded Kapton resistive anode was used with GEM. This was replaced with more uniform higher resistivity 1 M / Cermet for Micromegas. Slide 24 M. DixitIPNS KEK 14/10/0524 Charge dispersion signals in Micromegas Single event (2 mm wide pads) 2 x 4 channel Tektronix X-ray spot centred on pad 2 Primary signal Two 1 st neighbors 2 nd neighbor (note different scale) Ar/CO 2 90/10, Gain ~ 3000 1st neighbor peak ~ 100 ns after the primary pulse peak Slow rising 2 nd neighbor pulse ~ 25 MHz digitization could replace pulse shape sampling Slide 25 M. DixitIPNS KEK 14/10/0525 GEM pad response function for collimated x rays Simulation versus measurement Measured PRF deviates from simulation due to anode RC nonuniformities. (Solid line) Pad 23 Pad 24 Pad 22 Scan across width Ionization from 50 m collimated x-rays. 2x6 mm 2 pads Slide 26 M. DixitIPNS KEK 14/10/0526 GEM resolution ~ 70 m. Similar resolution measured for a Micromegas with a resistive anode readout using 2 mm x 6 mm pads Resistive anode double-GEM spatial resolution Collimated ~ 50 m x-ray spot 2x6 mm 2 pads Slide 27 M. DixitIPNS KEK 14/10/0527 Resistive anode suppresses sparking stabilizing Micromegas Extremely high gains without breakdown are possible A fringe benefit: High Micromegas gain with a resistive anode Argon/Isobutane 90/10 Slide 28 M. DixitIPNS KEK 14/10/0528 15 cm drift length with GEM or Micromegas readout B=0 (so far) Ar:CO 2 /90:10 chosen to simulate low transverse diffusion in a magnetic field. Aleph charge preamps. Rise = 40 ns, Fall = 2 s. 200 MHz FADCs rebinned to digitization effectively at 25 MHz. 60 tracking pads (2 x 6 mm 2 ) + 2 trigger pads (24 x 6 mm 2 ). The GEM-TPC resolution was first measured with conventional direct charge TPC readout. The resolution was next measured with a charge dispersion resistive anode readout with a double-GEM & with a Micromegas endcap. Carleton cosmic ray test MPGD-TPC Slide 29 M. DixitIPNS KEK 14/10/0529 Simulation - GEM TPC cosmic event with charge dispersion (track Z drift distance ~ 67 mm, Ar/CO 2 90/10 gas) Detailed model simulation including longitudinal & transverse diffusion, gas gain, detector pulse formation, charge dispersion & preamp rise & fall time effects. Centre pad amplitude used for normalization - no other free parameters. 2x6 mm 2 pads Simulation Data Slide 30 M. DixitIPNS KEK 14/10/0530 The pad response function (PRF) The PRF is a measure of signal size as a function of track position relative to the pad. For charge dispersion non charge collecting pads have signals in contrast to conventional direct charge readout. Unusual highly variable charge dispersion pulse shape; both the rise time & pulse amplitude depend on track position. We use pulse shape information to optimize the PRF. The PRF can, in principle, be determined from simulation. However, system RC nonuniformities & geometrical effects introduce bias in absolute position determination. The position bias can be corrected by calibration. PRF and bias determined empirically using a subset of data which was used for calibration. The remaining data was used for resolution studies. Slide 31 M. DixitIPNS KEK 14/10/0531 GEM & Micromegas PRFs for TPC track Ar:CO 2 (90:10) 2x6 mm 2 pads GEM PRFsMicromegas PRFs Micromegas PRF is narrower due to the use of higher resistivity anode & smaller diffusion after avalanche gain The pad response function maximum for longer drift distances is lower due to Z dependent normalization. Slide 32 M. DixitIPNS KEK 14/10/0532 a 2 a 4 b 2 & b 4 can be written down in terms of and & two scale parameters a & b. PRFs with the GEM & the Micromegas readout The PRFs are not Gaussian. The PRF depends on track position relative to the pad. PRF = PRF(x,z) PRF can be characterized by its FWHM (z) & base width (z). PRFs determined from the data have been fitted to a functional form consisting of a ratio of two symmetric 4th order polynomials. Slide 33 M. DixitIPNS KEK 14/10/0533 Track fit using the the PRF One parameter fit for x row (track position for a given row) using Bias = Mean of residuals (x row -x track ) as a function of x track Resolution = of track residuals for tracks with | | < 5 Determine x 0 & by minimizing for the entire event = rows i=pads 22 2 mm 6 mm Track at: x track = x 0 + tan y row Slide 34 M. DixitIPNS KEK 14/10/0534 Bias corrections with GEM & with Micromegas GEM Micromegas 2 mm pads Initial bias Remaining bias after correction Initial bias Remaining bias after correction Slide 35 M. DixitIPNS KEK 14/10/0535 What is the diffusion limit of resolution for a gaseous TPC? 0 includes noise & systematic effects. C d = diffusion constant; z = drift distance. Resolution depends on electron statistics. Electron number N fluctuates from event to event. N eff the average number of electrons = 1/ the inverse of average of 1/N Gain fluctuations also affect N eff Slide 36 M. DixitIPNS KEK 14/10/0536 Simulation for the effective number of electrons for resolution 2 mm x 6 mm pads - Ar/CO 2 90/10. dE/dx in Argon Statistics of primary ionization & cluster size distribution. dE/dx dependence on momentum. Account for track angle & detector acceptance effects. Use simulation to scale measured pulse heights to electron number. N eff = 1/ determined from pulse height distribution. N eff 38.9 10% (N average = 57) Cosmic ray momentum spectrum Total Ionization Measured pad pulse height distribution Mostly muons at sea level Slide 37 M. DixitIPNS KEK 14/10/0537 Measured TPC transverse resolution for Ar:CO 2 (90:10) R.K.Carnegie et.al., NIM A538 (2005) 372 R.K.Carnegie et.al., to be published [N eff = 38.9] Unpublished Compared to conventional readout, resistive readout gives better resolution for the GEM and the Micromegas readout. The z dependence follows the expectations from transverse diffusion & electron statistics. Slide 38 M. DixitIPNS KEK 14/10/0538 Summary TPC with MPGD readout is a very well suited technology for the ILC. Traditional readout has difficulty achieving TPC resolution goal, unless narrower pads are used. With charge dispersion, the cluster charge can be dispersed in a controlled way such that relatively wide pads can be used without sacrificing resolution. With such a readout system, it may be feasible to achieve ILC TPC resolution goal with relatively wide pads both for the GEM and the Micromegas readout. With R&D, ILC TPC resolution goals appear within reach. Beam test at KEK (next week) important step in developing the MPGD readout for the ILC TPC.