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Linear Collider TPC R&D in Canada

Linear Collider TPC R&D in Canada. Bob Carnegie, Madhu Dixit , Dean Karlen, Steve Kennedy, Jean-Pierre Martin, Hans Mes, Ernie Neuheimer, Alasdair Rankin, & Kirsten Sachs Carleton University, University of Montreal, TRIUMF, & University of Victoria. ECFA/DESY Prague - November 16, 2002.

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Linear Collider TPC R&D in Canada

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  1. Linear Collider TPC R&D in Canada Bob Carnegie, Madhu Dixit, Dean Karlen, Steve Kennedy, Jean-Pierre Martin, Hans Mes, Ernie Neuheimer, Alasdair Rankin, & Kirsten Sachs Carleton University, University of Montreal, TRIUMF, & University of Victoria ECFA/DESY Prague - November 16, 2002

  2. Abstract In an ideal TPC, the limit to achievable spatial resolution comes only from diffusion. The traditional wire/pad TPC fails to meet this goal due to large ExB and track angle effects. While not hampered by such systematics, an MPGD readout TPC will achieve the ultimate diffusion limit of resolution if only the electron avalanche position could be accurately measured. We describe below our recent work on: i) measuring the limits to achievable GEM-TPC spatial resolution from gas diffusion by optimizing readout geometry; & ii) GEM test cell results on the possibility of improving position sensing in an MPGD from charge dispersion of avalanche charge on a resistive anode. M. Dixit

  3. ExB & track angle systematics in a TPC(Aleph TPC example) TPC wire/pad readout ExB cancels track angle effect 100 µm Average Aleph resolution ~ 150 µm About 100 µm best for all drift distances Limit from diffusion  (10 cm drift) ~ 15 µm;  (2 m drift) ~ 60 µm 100 µm limit for all drift distances comes from wide pad response M. Dixit

  4. An MPGD Readout TPC for the LC • ExB and track angle systematic effects cannot be avoided in a wire/pad TPC • Even when systematics cancel, the resolution is determined by the width of the pad response function (PRF) & not by physics of diffusion • Large (PRF) further limits the TPC 2 track resolving power • Positive ion space-charge effects - additional complication • A micro-pattern gas detector ( MPGD*) readout TPC has • Negligible ExB (no preferred angles in an MPGD) • Natural suppression of positive ion space charge effects • Resolution and 2-track resolving power approaching diffusion limit Such as Gas Electron Multiplier (GEM), Micromegas M. Dixit

  5. MPGD-TPC R&D in Canada • Study of limits to achievable diffusion limited resolution by optimizing readout geometry • 15 cm drift TPC with GEM readout • Cosmic ray tracking studies • The possibility of improving position sensing from charge dispersion in an MPGD with a resistive anode • GEM test cell results using collimated x-rays M. Dixit

  6. GEM-TPC studies 15 cm max. drift distance Gases: Ar CO2 ; P10 TPC wire preamplifiers salvaged from Aleph 64 channels of 200 MHz FADCs built by U of Montreal New readout pad layouts: • Trigger & veto + 174 pads with 3 fold multiplexing Tracking study: • diffusion effects • pad width effects on resolution • track angle effect M. Dixit

  7. Cosmic ray test GEM-TPC M. Dixit

  8. New TPC pad layout for increased acceptance Resolution study for central 2 & 3 mm wide pad rows amplitudes color coded: 100 80 60 40 25 15 2 trigger + 4 veto Track fit 3 top + 3 bottom 2.5 mm wide pad rows 3-fold multiplexed M. Dixit

  9. Track fit in x-y 3 parameter track fit:x0 (offset),f(angle),s(spread) Assume uniform line chargewith Gaussian spread s Integral over pad ~ pad charge Compare to observedcharge fractions in each row For now neglect ionization clustering along the track M. Dixit

  10. Determine Drift Velocity Drift-distance determined from drift-time: Adjust drift-velocity vdusing straight tracks TPC length: 15cm ForAr/CO2: vd ~ 8.3 µm/ns (Edrift ~ 137 V/cm) For P10: vd ~ 50 µm/ns M. Dixit

  11. Diffusion vs Drift Distance Fitted track width ~ charge cloud width from diffusion For ArCO2 For P10 M. Dixit

  12. Resolution From MC studies resolution depends only weakly on pad width if pad width is less than 4 times charge cloud size For short drift, charge cloud size ~ 500 µmGood resolution for 2 mm pad width Not so good for 3mm pad width Best resolution: ArCO2, narrow pads,short drift, small ||:135  4 µm M. Dixit

  13. Resolution vs Drift Distance Best resolution for short drift ArCO2: Small diffusion3 mm wide pads too wide P10: Large diffusionLess sensitive to pad width M. Dixit

  14. Resolution vs track angle Large track-angle effect possibly due to simplified track fit. Effect more pronounced for ArCO2 with narrow pads M. Dixit

  15. Resolution vs Amplitude Resolution improves with electron statistics For very large number of electronsresolution degraded because ofionization from -rays Effect is less pronouncedwith P10 gas M. Dixit

  16. Position sensing in a GEM from charge dispersion on a resistive anode(technique applicable to other MPGDs) Analogy: position sensing in 1-D in a proportional wire by charge division Telegraph equation (1-D): Deposit point charge at t=0 Solution for charge density (L ~ 0) Position sensing in 2-D in an MPGD with a resistive anode Telegraph equation in 2-D Solution for charge density in 2-D for simulation include finite charge cloud size + rise and fall time effects M. Dixit

  17. Resistive anode GEM test cell setup M. Dixit

  18. An event in the resistive anode GEM test cell Charge cluster size ~ 1 mm ; signal detected by ~7 anodes (2 mm width) M. Dixit

  19. Pad response function Simulation versus Measurement Width & shape of signal distributions on pads can be simulated The pad response function PRF depends on anode resistivity & the gap between anode and readout pad plane This PRF is too wide Require PRF ~ diffusion for optimum resolution M. Dixit

  20. Design simulation for PRF~ 700 µm M. Dixit

  21. Resolution tests with PRF~ 700 µm design adjacent strips with induced pulse + charge dispersion central strip: main pulse average single event 2.5 MΩ/ resistivity 100 µm gap 1.5 mm strips are too wide for PRF ~ 700 µm! M. Dixit

  22. GEM charge dispersion resolution study 50 µm collimated x-ray spot Scan across 1.5 mm wide strips Record 1000 events with Tektronix digitizing scope Single event produces measurable signal on 3 strips Early charge pulse, delayed charge dispersion pulse Use 500 events to define pulse shape polynomials Measure signal amplitudes for remaining 500 events Compute 3 pad centre of gravity for each event Correct for bias in CG determination M. Dixit

  23. Polynomial fits define pulse shapes Use 500 events to define standardized pulse shapes for earlycharge pulse (left), and delayed charge dispersion pulse (right) M. Dixit

  24. Bias correction to centre of gravity M. Dixit

  25. Resolution near a strip edge  = 78 µm M. Dixit

  26. Resolution near the centre of a strip  = 61 µm M. Dixit

  27. Resolution between edge & centre  = 67 µm M. Dixit

  28. Resolution scan - summary Spatial resolution Position residuals X-ray spot position (mm) M. Dixit

  29. Outlook & summary • Diffusion limited resolution ~ 140 µm in GEM readout TPC with 2mm wide pads (charge cloud width ~ 700 µm) for short distances • Promising preliminary results for localization from charge dispersion • further tests with GEM test cell planned • Optimize parameters • Incorporate in the mini-TPC readout • Cosmic tests - 128 FADC channels, no multiplexing • Beam tests http://www.physics.carleton.ca/~gmd/ M. Dixit

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