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. 16/11/02M. Dixit2 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.

3. 16/11/02M. Dixit3 ExB cancels track angle effect TPC wire/pad readout 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 ExB & track angle systematics in a TPC (Aleph TPC example)

4. 16/11/02M. Dixit4 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

5. 16/11/02M. Dixit5 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

6. 16/11/02M. Dixit6 GEM-TPC studies 15 cm max. drift distance Gases: Ar CO 2 ; 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

7. 16/11/02M. Dixit7 Cosmic ray test GEM-TPC

8. 16/11/02M. Dixit8 New TPC pad layout for increased acceptance amplitudes color coded: 2 trigger + 4 veto Resolution study for central 2 & 3 mm wide pad rows 1008060402515 Track fit 3 top + 3 bottom 2.5 mm wide pad rows 3-fold multiplexed

9. 16/11/02M. Dixit9 Track fit in x-y 3 parameter track fit: x 0 (offset),   (angle),   (spread) Assume uniform line charge with Gaussian spread  Integral over pad ~ pad charge Compare to observed charge fractions in each row For now neglect ionization clustering along the track

10. 16/11/02M. Dixit10 Determine Drift Velocity For Ar/CO 2 : v d ~ 8.3 µm/ns (E drift ~ 137 V/cm) For P10: v d ~ 50 µm/ns Drift-distance determined from drift-time: Adjust drift-velocity v d using straight tracks TPC length: 15cm

11. 16/11/02M. Dixit11 Diffusion vs Drift Distance Fitted track width ~ charge cloud width from diffusion For P10 For ArCO 2

12. 16/11/02M. Dixit12 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 µm Good resolution for 2 mm pad width Not so good for 3mm pad width Resolution Best resolution: ArCO 2, narrow pads, short drift, small |  |: 135  4 µm

13. 16/11/02M. Dixit13 Resolution vs Drift Distance Best resolution for short drift ArCO 2 : Small diffusion 3 mm wide pads too wide P10: Large diffusion Less sensitive to pad width

14. 16/11/02M. Dixit14 Resolution vs track angle Large track-angle effect possibly due to simplified track fit. Effect more pronounced for ArCO 2 with narrow pads

15. 16/11/02M. Dixit15 Resolution vs Amplitude Resolution improves with electron statistics For very large number of electrons resolution degraded because of ionization from  -rays Effect is less pronounced with P10 gas

16. 16/11/02M. Dixit16 Position sensing in a GEM from charge dispersion on a resistive anode (technique applicable to other MPGDs) Analogy:Analogy: position sensing in 1-D in a proportional wire by charge division Solution for charge density (L ~ 0) Telegraph equation (1-D): for simulation include finite charge cloud size + rise and fall time effects Deposit point charge at t=0 Telegraph equation in 2-D Position sensing in 2-D in an MPGD with a resistive anode Solution for charge density in 2-D

17. 16/11/02M. Dixit17 Resistive anode GEM test cell setup

18. 16/11/02M. Dixit18 Charge cluster size ~ 1 mm ; signal detected by ~7 anodes (2 mm width) An event in the resistive anode GEM test cell

19. 16/11/02M. Dixit19 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 Pad response function Simulation versus Measurement

20. 16/11/02M. Dixit20 Design simulation for  PRF ~ 700 µm

21. 16/11/02M. Dixit21 2.5 MΩ/  resistivity 100 µm gap 1.5 mm strips are too wide for  PRF ~ 700 µm! single event average central strip: main pulse adjacent strips with induced pulse + charge dispersion Resolution tests with  PRF ~ 700 µm design

22. 16/11/02M. Dixit22 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

23. 16/11/02M. Dixit23 Polynomial fits define pulse shapes Use 500 events to define standardized pulse shapes for early charge pulse (left), and delayed charge dispersion pulse (right)

24. 16/11/02M. Dixit24 Bias correction to centre of gravity

25. 16/11/02M. Dixit25 Resolution near a strip edge  = 78 µm

26. 16/11/02M. Dixit26 Resolution near the centre of a strip  = 61 µm

27. 16/11/02M. Dixit27 Resolution between edge & centre  = 67 µm

28. 16/11/02M. Dixit28 Resolution scan - summary Spatial resolution Position residuals X-ray spot position (mm)

29. 16/11/02M. Dixit29 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/