Michael Hauschild (CERN), LCWS02, 27-Aug-2002, page 1

2D(3D?) cluster counting with GEMs and small

pads:the digital TPC?

or

how to measure dE/dx without measuring

charges

Michael Hauschild (CERN), LCWS02, 27-Aug-2002, page 2

Ideal dE/dx measurement

Count number of clusters along track

cluster density should be proportional to dE/dx

Obvious problem:

cluster density is high (20 - 30 clusters/cm in Ar mixtures for m.i.p.) = 1 cluster per 300 - 500 ìm

need device with high granularity to resolve them in space

Other problem:

Clusters sometimes have more than one electron: 1 el. 82.4 %

2 el. 6.9 %

3 el 2.0 %

10 el. 0.64 %

100 el. 0.0014 %

how to avoid counting individualelectrons of multi-electron clusters?

(TESLA-TPC, Ar/CH4/CO2, 93/5/2, calculation by

HEED)

Michael Hauschild (CERN), LCWS02, 27-Aug-2002, page 3

Classic dE/dx measurement

Widely used (because counting is difficult):

measure charge over some track length (sampling length)

"average" charge of many samples = dE/dx

Charge measurement requires:

electronics with good charge resolution, e.g. 8 bit or more

stable gain!!!

physics needs ÄG/G < 1/10 ó(dE/dx)/dE/dx (better < 1/20)

some algorithm to remove unwanted multi-electron clusters (delta electrons)

commonly used: truncated mean, remove a fixed fraction of highest charge measurements, typically 20-30%, robust

< 0.5% (better < 0.2%) overall gain stability

Michael Hauschild (CERN), LCWS02, 27-Aug-2002, page 4

dE/dx with GEMs

GEMs (Gas Electron Multiplier) + pads (typical size 5-10 mm) are suggested as possible TPC-readout device

Nice detector for tracking, what about dE/dx?

Problem: GEMs show gas gain variations

local variations over the surface (static)

could make calibrations more complicated, not a problem in principle

time dependent (dynamic) variations due to charge-up effects

difficult to control and to calibrate, depend on background, might vary within a bunch train

Michael Hauschild (CERN), LCWS02, 27-Aug-2002, page 5

GEM gain variations

local gain variations: dynamic gain variations:

M. Hamann et al.(DESY/Univ. Hamburg) C. Altunbas et al., CERN-EP 2002-008

COMPASS GEMs

➔ 10% local gain variations ➔ 20-30% dynamic gain variations

Michael Hauschild (CERN), LCWS02, 27-Aug-2002, page 6

Cluster CountingDirect cluster counting avoids any problems with gas gain instabilities

In theory ultimate way to get dE/dx

30 clusters/cm * 120 cm track length = 3600 clusters = 1.7% dE/dx resolution (TESLA-TDR: 4-4.5%)

Not a brand new idea:

previous attempts tried to resolve clusters in time:

slow gas / drift velocity (e.g. CO2) + good time and multi-hit

resolution, worked in lab + prototype detectors, never used in real big detectors for physics

Now (that's new):

micro-pattern devices + small pads = high granularity could make it possible to resolve them in space (2D), if time could be added even 3D(?)

need only 1-bit "ADC"

Michael Hauschild (CERN), LCWS02, 27-Aug-2002, page 7

Could it work?

Simulation study made:

generate clusters/electrons (also long range delta electrons using HEED (by I. Smirnov), take gas parameters (diffusion etc.) from MAGBOLTZ (by S. Biagi)

track electrons through TPC volume, squeeze them through GEM holes, apply gas gain (use Polya distribution for fluctuations)

track all electrons created in gas amplification to a pad plane (including diffusion, ódiffusion = 135 ìm over 2 mm)

collect electrons on pads, allow 5% losses in gaps, add noise (200 el. R.M.S. per pad, optimistic?)

apply threshold (1500 el.) and simply count number of pads above threshold = clusters(?)

➔ very CPU time consuming, need 50 Mill. electrons per full TPC track

Michael Hauschild (CERN), LCWS02, 27-Aug-2002, page 8

Generated Electrons

HEED calculation

GEM plane

Track(clusters)

some delta-electron

TPC frame(sideview)

Michael Hauschild (CERN), LCWS02, 27-Aug-2002, page 9

Generated Electrons (close view)

no diffusion

with diffusion

(ódiff,trans. = 1.1 mm, ódiff,long. = 4.4 mm for 250 cm drift and 4 T)

65 keVdelta-electron

Michael Hauschild (CERN), LCWS02, 27-Aug-2002, page 10

Pad view (xy plane)

multi-electron cluster

single electrons at GEM plane

pads (500 x 500 ìm2) above threshold (1500

e-)

Michael Hauschild (CERN), LCWS02, 27-Aug-2002, page 11

Questions

Counting with small pads seems to work

Some questions to answer:

optimal pad size?

large pads: clusters can't be resolved

small pads: (too) many channels (cost!), bad signal/noise ratio

noise?

need low threshold to count a single electron after gas amplification on a single small pad

diffusion?

at large drift length (up to 250 cm at TESLA-TPC), multi-electron clusters are spread by diffusion, individual electrons appear and are counted again, not clusters

Michael Hauschild (CERN), LCWS02, 27-Aug-2002, page 12

Counts vs Pad Size

m.i.p., 0.6 GeV pions

high E tracks, 1000 GeV pions

(Fermi plateau)

Michael Hauschild (CERN), LCWS02, 27-Aug-2002, page 13

Separation Power

simple pad counting,2.1 ó separation

classic dE/dx by charge measurement + truncated mean, 2.1 ó separation

Michael Hauschild (CERN), LCWS02, 27-Aug-2002, page 14

Conclusions ISeparation power with cluster counting as good as classic dE/dx by charge + truncated mean...

...but not better! (Factor 2 improvement expected)

Need to match: average distance between clusters

(375 ìm for m.i.p.)

diffusion spread(1.1 mm for 250 cm

drift)

pad size(noise cost)

➔ Doesn't fit together for TESLA-TPC, too much diffusion!

➔ Electrons from multi-electron clusters dissolve and are individually counted (instead of clusters)

Michael Hauschild (CERN), LCWS02, 27-Aug-2002, page 15

Conclusions II

Cluster counting better than classic dE/dx by charge needs:

gas with low ionisation

large distance between clusters

low probability for multi-electron clusters

low diffusion

either low diffusion gas or

short drift length

pad sizes

Helium(Neon?) mixtures could be a possible candidate for successful cluster counting