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

•16/11/02 •M. Dixit •2

AbstractIn 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.

•16/11/02 •M. Dixit •3

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)

•16/11/02 •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

•16/11/02 •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

•16/11/02 •M. Dixit •6

GEM-TPC studies

15 cm max. drift distanceGases: Ar CO2 ; P10TPC wire preamplifiers salvaged from Aleph64 channels of 200 MHz FADCs built by U of Montreal

New readout pad layouts:•Trigger & veto + 174 pads with 3 fold multiplexingTracking study:• diffusion effects• pad width effects on resolution •track angle effect

•16/11/02 •M. Dixit •7

Cosmic ray test GEM-TPC

•16/11/02 •M. Dixit •8

New TPC pad layout for increased acceptance

amplitudes color coded:

2 trigger + 4 veto

Resolution study for central 2 & 3 mm wide pad rows

100 80 60 40 25 15

Track fit 3 top + 3 bottom 2.5 mm wide pad rows 3-fold multiplexed

•16/11/02 •M. Dixit •9

Track fit in x-y

3 parameter track fit: x0 (offset), (angle), (spread)

Assume uniform line chargewith Gaussian spread Integral over pad ~ pad chargeCompare to observedcharge fractions in each row

For now neglect ionization clustering along the track

•16/11/02 •M. Dixit •10

Determine Drift Velocity

For Ar/CO2: vd ~ 8.3 µm/ns

(Edrift ~ 137 V/cm)

For P10: vd ~ 50 µm/ns

Drift-distance determined from drift-time:

Adjust drift-velocity vd using straight tracks

TPC length: 15cm

•16/11/02 •M. Dixit •11

Diffusion vs Drift Distance

Fitted track width ~ charge cloud width from diffusion

For P10

cmm 212

Tr

cmm 449

Tr

For ArCO2

•16/11/02 •M. Dixit •12

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

Resolution

Best resolution:

ArCO2, narrow pads,short drift, small ||:135 4 µm

•16/11/02 •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

•16/11/02 •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

•16/11/02 •M. Dixit •15

Resolution vs AmplitudeResolution improves with electron statistics

For very large number of electronsresolution degraded because ofionization from -rays

Effect is less pronouncedwith P10 gas

•16/11/02 •M. Dixit •16

Position sensing in a GEM from charge dispersion on a resistive anode

(technique applicable to other MPGDs)

AnalogyAnalogy::AnalogyAnalogy::

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

2

2

2

2 1xQ

RCtQ

tQ

RL

Q(x, t) RC

4 t

x 2RC

4 te

Qt

1

RC

2Q

r2

1

r

Qr

Q(r, t) RC

2t

r 2RC

4 te

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

•16/11/02 •M. Dixit •17

Resistive anode GEM test cell setup

•16/11/02 •M. Dixit •18

Charge cluster size ~ 1 mm ; signal detected by ~7 anodes (2 mm width)

An event in the resistive anode GEM test cell

•16/11/02 •M. Dixit •19

•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

•16/11/02 •M. Dixit •20

Design simulation for PRF ~ 700 µm

•16/11/02 •M. Dixit •21

2.5 MΩ/ resistivity 100 µm gap1.5 mm strips are too wide for PRF ~ 700 µm!

single event

average

central strip: main pulse

adjacent strips withinduced pulse

+ charge dispersion

Resolution tests with PRF ~ 700 µm design

•16/11/02 •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

•16/11/02 •M. Dixit •23

Polynomial fits define pulse shapes

Use 500 events to define standardized pulse shapes for early charge pulse (left), and delayed charge dispersion pulse (right)

•16/11/02 •M. Dixit •24

Bias correction to centre of gravity

•16/11/02 •M. Dixit •25

Resolution near a strip edge

= 78 µm

•16/11/02 •M. Dixit •26

Resolution near the centre of a strip

= 61 µm

•16/11/02 •M. Dixit •27

Resolution between edge & centre

= 67 µm

•16/11/02 •M. Dixit •28

Resolution scan - summary

Spatial resolution

Position residuals

X-ray spot position (mm)

•16/11/02 •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/