1 / 15

Ideal dE/dx measurement

2D(3D?) cluster counting with GEMs and small pads: the digital TPC? or how to measure dE/dx without measuring charges. Ideal dE/dx measurement. Count number of clusters along track cluster density should be proportional to dE/dx Obvious problem:

dezso
Download Presentation

Ideal dE/dx measurement

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. 2D(3D?) cluster counting with GEMs and small pads:the digital TPC? orhow to measure dE/dx without measuring charges

  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 % (TESLA-TPC, Ar/CH4/CO2, 93/5/2, calculation by HEED) how to avoid counting individual electrons of multi-electron clusters?

  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

  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

  5. GEM gain variations local gain variations: dynamic gain variations: COMPASS GEMs C. Altunbas et al., CERN-EP 2002-008 M. Hamann et al.(DESY/Univ. Hamburg) • 10% local gain variations • 20-30% dynamic gain variations

  6. Cluster Counting • Direct 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"

  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

  8. Generated Electrons HEED calculation Track (clusters) some delta-electron TPC frame (sideview) GEM plane

  9. Generated Electrons (close view) 65 keV delta-electron no diffusion with diffusion (ódiff,trans. = 1.1 mm, ódiff,long. = 4.4 mm for 250 cm drift and 4 T)

  10. Pad view (xy plane) multi-electron cluster pads (500 x 500 ìm2) above threshold (1500 e-) single electrons at GEM plane

  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

  12. Counts vs Pad Size m.i.p., 0.6 GeV pions high E tracks, 1000 GeV pions (Fermi plateau)

  13. Separation Power classic dE/dx by charge measurement + truncated mean, 2.1 ó separation simple pad counting, 2.1 ó separation

  14. Conclusions I • Separation 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.) pad size (noise  cost) diffusion spread (1.1 mm for 250 cm drift) • Doesn't fit together for TESLA-TPC, too much diffusion! • Electrons from multi-electron clusters dissolve and are individually counted (instead of clusters)

  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

More Related