1. GEM TPC Resolution from Charge Dispersion*
Madhu Dixit Carleton University & TRIUMF
Durham September 2004
R.K.Carnegie, J.-P.Martin, H.Mes, E.Neuheimer, A.Rankin, K.Sachs & A.Tomkins
Carleton University & University of Montreal
* A review of material presented at Victoria Linear Collider Workshop (July 2004)

2. Introduction
Transverse diffusion sets the ultimate limit on the TPC resolution.
Wire/pad TPCs do not reach this limit due to ExB & track angle effects.
MPGD TPC has the potential to reach the diffusion limit but is limited by imprecise pad centroid determination.
Options: a) Narrower pads, increased complexity & a much larger channel count; or b) Disperse avalanche charge to improve centroid determination.
1) GEM operated with large diffusion in transfer and induction gaps - R.K.Carnegie et.al., LCWS'02, physics/ (to be published in NIM).
2) Charge dispersion on a resistive anode: concept and 1st tests published: M.S.Dixit et.al., NIM A518 (2004) 721.
First results on TPC resolution with charge dispersion in a low diffusion gas (Ar/CO2:90/10, Tr= 230 m/cm) presented at LCWS 2004, Paris.
==> Resolution with charge dispersion in Ar/CO2, & in a large diffusion gas (P10,  Tr = 560 m/cm).
==> Progress in understanding the charge dispersion phenomenon.
3 September 2004
M.Dixit

3. Charge dispersion in a MPGD with a resistive anode
Modified GEM anode structure with a high resistivity film bonded to the readout plane with an insulating layer of glue.
2-dim RC network defined by material properties & geometry.
Point charge at r = 0 & t = 0 disperses with time.
Measure capacitively coupled charge signals on pads below.
Telegraph equation for the charge density function:
(r) Q
(r,t) integral over pads
r / mm
3 September 2004
M.Dixit

4. Cosmic ray track resolution of a GEM TPC using the charge dispersion signal
15 cm drift double GEM-TPC.
Ar/CO2 (90/10) & P10 gas mixtures.
60 readout pads (2 x 6 mm2).
Anode resistivity ~ 530 k/.
C ~ 0.22 pF/mm2.
Aleph TPC preamps. Rise= 40 ns, Decay= 2 s.
200 MHz custom 8 bit FADC readout.
3 September 2004
M.Dixit

5. A TPC track signal with charge dispersion
3 September 2004
M.Dixit

6. Tracking with the charge dispersion signal
Unusual highly variable charge pulse shape.
Pulses on charge collecting pads: Large pulses with fast fixed rise-time. The decay time depends on the system RC, the pad size & the initial charge cluster location.
Pulses on other pads: Smaller pulse heights & slow rise & decay times determined by the system RC & the pad location.
No unique recipe for the pad response function (PRF) as both the pulse height & rise-time depend on the initial charge position.
Present PRF recipe uses only the pulse height information independent of pulse rise-time.
We use part of cosmic ray data for PRF determination and for bias calibration. The remaining data is analyzed for resolution studies.
3 September 2004
M.Dixit

7. The pad response function for TPC tracks determined from the calibration data set
The PRF shape well described
by a generalized Lorentzian.
PRF(Ar/CO2 90/10) ~ mm
depends on the drift distance.
Up to 3 pads in a row contribute.
P10 has large diffusion.
PRF(P10) for long drift ~ 6 mm.
Up to 6 pads contribute.
Parameters: x, (z)
PRF for 150 ns integration window centered at peak amplitude
3 September 2004
M.Dixit

8. Bias in reconstructed positions from the PRF
Local non-uniformities in the anode RC lead to ~100 mm bias (systematic errors) in position determination. The bias can be removed by calibration.
The bias correction function is empirically determined from the calibration dataset.
Large transverse diffusion in P10 makes it difficult to evaluate effects of local RC non-uniformities and makes bias correction less precise.
Need a detector with more uniform RC to minimize bias.
Bias for row 4 in Ar/CO2 90/10
3 September 2004
M.Dixit

9. Transverse spatial resolution in Ar/CO2 (90/10)
Charge dispersion
Direct charge
R.K.Carnegie et.al., (accepted for publication in NIM).
Charge dispersion improves resolution in a low diffusion gas when there is insufficient charge sharing between pads.
3 September 2004
M.Dixit

10. Transverse spatial resolution in P10
Direct charge
Charge dispersion
Bias removal less effective due to large diffusion. Nevertheless, improved resolution close to the diffusion limit using charge dispersion even for P10 for which there is significant charge sharing between pads. Confirms effect first observed for ArCO2 (90/10).
3 September 2004
M.Dixit

11. The GEM charge dispersion signal Simulation versus measurement (2 mm x 6 mm pads) ~ 4.5 keV collimated x ray spot at the pad centre
Detailed simulation, includes effects of intrinsic detector charge pulse shape, diffusion in the gas, and the preamp (Aleph) rise and decay time effects.
Difference (induced signal not included in simulation) studied previously MPGD '99 (Orsay), LCWS '00
Neighboring pad dispersion signal peaks later (~150 ns) has slower decay time.
Fast rising direct signal on charge collecting pad.
3 September 2004
M.Dixit

12. GEM pad response function for a charge cluster Simulation versus measurement (2 mm x 6 mm pads)
Ionization from 50 m collimated x-rays.
Pad 23 24 22
Scan across pads
(Solid line)
Simulated PRF deviates from the data due to RC nonuniformities. The deviations are consistent with observed biases.
3 September 2004
M.Dixit

13. Cosmic ray track simulation - compared to the data (67 mm drift, Ar/CO2 90/10 gas)
An exact simulation requires specifying positions & sizes of all track clusters.
Equally spaced equal size charge clusters assumed here.
Single free parameter - normalization from the centre pad amplitude.
3 September 2004
M.Dixit

14. How to optimize the charge dispersion readout?
Keep the resistor noise small to maintain good S/N. (ENC ~ 200 e- for anode resistivity ~ 1000 K/ with 200 ns integration).
Efficient coupling of the readout plane with the anode. Canode-readout >> C anode-cathode will keep signal sharing with the cathode small & maximize the pad signal. (important for Micromegas).
Choose RC for to keep the intrinsic width of the PRF small for good 2-track resolution.
3 September 2004
M.Dixit

15. Diffusion effects & electronics shaping modify the charge dispersion measurement significantly
Intrinsic pulse shapes (40 ns/bin) and PRF (no diffusion & no electronics shaping effects).
 ~ 2.3 mm
Simulated pulses & PRF with electronics & diffusion in Tesla TPC TDR gas (Ar/CH4/CO2 4 T for 1 m drift assuming Aleph TPC preamp
 ~ 3.6 mm
3 September 2004
M.Dixit

16. Pad width and RC effects on charge dispersion
C = 0.22/mm2
C = 0.22/mm2
3 September 2004
M.Dixit

17. Conclusion & future plans
Better resolution with charge dispersion than with direct charge for large & small transverse diffusion gases - P10 & Ar/CO2.
The variation with drift distance of the TPC charge dispersion resolution is near the limit from the transverse diffusion in the gas.
Nonuniform anode RC leads to ~ 100 m bias corrections.
We understand the complexities of charge dispersion. The simulation agrees well with the data.
Spatial resolution near the diffusion limit (~ 70 m for Ar with CF4) within reach for all tracks for the ILC TPC.
Future plans:
Improved anode structure with more uniform RC properties.
Charge dispersion studies with the Micromegas.
TPC magnetic field & beam tests with charge dispersion.
Preamps better matched to the charge dispersion signal.
25 MHz digitizers to replace 200 MHz FADCs.
3 September 2004
M.Dixit

18. Effect of increasing the anode RC on pulse shapes (Vary R, keeping C constant)
3 September 2004
M.Dixit

19. Unshaped pad pulses for the wire chamber & for the charge dispersion TPC readout
Wire/pad gap 1 mm
100% MPGD signal collected in ~ 100 ns
The pads see close to 100% of the total avalanche charge.
Full wire chamber gain is not used due to ~ 200 ns shaping. Furthermore the pads see at best ~ 40% of the anode avalanche charge.
3 September 2004
M.Dixit

20. TPC wire chamber/pad readout system
The wire pulse is shaped before digitization to improve rate & 2-track TPC performance.
Alice TPC shaper (base width ~ 450 ns)
3 September 2004
M.Dixit