study of gem structures for a tpc readout m. killenberg, s. lotze, j. mnich, a. münnich, s. roth,...
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Study of GEM Structures for a TPC Readout
M. Killenberg, S. Lotze, J. Mnich, A. Münnich, S. Roth, M. Weber
RWTH AachenOctober 2003
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Experiments & numerical simulation
R&D on GEM for a TPC at Aachen
o Charge transfer in high magnetic fields: Collection of primary electrons ( dE/dx) Effective gain Ion feedback
o Gas studies: TPC gas: high drift velocity at low field High neutron background low H content Impact of gas on charge transfer in GEM
o Length & width of electron signal resolution
o Mechanics of GEM readout structure and long term TPC operation
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Charge Transfer Measurements in Magnetic Field
5 T magnet at DESY
Charge transfer deduced From current measurements
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o Triple GEM structure works in a large magnetic field
o B field improves some parameter (signal
height)
o Only small effect on collection of primary
electrons
Result of Measurements:
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Langevin equation:
Aleph: B = 1.5 T @ = 9 Tesla: B = 4 T @ = 24
Impact on electron collection ?
dE/dx resolution
= cyclotron frequency = mean free time
Calculation of drift lines (no diffusion)
Collection of primary electrons
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Simulation: Impact of Gas on Charge Transfer
Low diffusion gas ArCO2High diffusion gas ArCH4
Numerical simulation of diffusion with Garfield
Illustrative example:Drift paths of electrons randomly distributed over a GEM hole
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Simulation of Gain in GEM Structures
Number of secondaries per primary electrons(single GEM)
Very broad distribution
Creation of secondariesmostly at edges
But there is no extractionat edges!
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Gas amplication and electron extraction in a GEM
x electron createdx created & extracted
Primary electrons
Simulation of Gain in GEM Structures
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Simulations allow optimisation of GEM readout structure
Comparison Measurements and Simulations
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Measurement of Ion Feedback in Magnetic Field
TPC: Ion feedback into drift volume would distort electric field naturally suppressed in GEM structures
improves with magnetic field
Magnetic field [T]
Ion
fee
db
ack
Triple GEM setupOptimized for ion feedback
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Width & Length of Signal
Comparison of signals: GEM versus wire readout
For optimal space resolutionmatch pad size to cluster size
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Measurement of Charge Width in Magnetic Field
- Measurement of charge width after passing triple GEM stack- Reduction of diffusion in high magnetic fields
Fe55 source
ArCH4C02 92/5/3
2
20B2
0 cB1
σσ
Charge width is governed by primary ionisationand diffussion between GEM foils
Range of 2,68 keV electrons in Ar
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Constrcution of a Triple GEM Readout Struture
Large (1.4 m3) TPC
Triple GEM structure
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TPC operating with Triple GEM Readout
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GEM readout is promising candidate to build a TPC with 100 m single point resolution in high B field
Conclusion & Outlook
o GEM readout structures successfully operated in 5 T field ion feedback & signal height improve no big loss of primary electrons cluster widths reduces as expected
o Long term stable operation of TPC with triple GEM structure
o Simulation tools in hand to optimize layout & operation parameter ion feedback <1% achieved
Next steps:
o Simulation of conditions for a TPC at the LC determine neutron backgound to choose gas and required ion feedback
o Build prototype TPC to demonstrate performance in test beam and magnetic field