Study of GEM Structures for a TPC Readout M. Killenberg, S. Lotze, J. Mnich, A. Münnich, S. Roth, M. Weber RWTH Aachen October 2003

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

Charge Transfer Measurements in Magnetic Field 5 T magnet at DESY Charge transfer deduced From current measurements

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:

Langevin equation: Aleph: B = 1.5  = 9 Tesla: B = 4  = 24 Impact on electron collection ?  dE/dx resolution  = cyclotron frequency  = mean free time Calculation of drift lines (no diffusion) Collection of primary electrons

Simulation: Impact of Gas on Charge Transfer Low diffusion gas ArCO 2 High diffusion gas ArCH 4 Numerical simulation of diffusion with Garfield Illustrative example: Drift paths of electrons randomly distributed over a GEM hole

Simulation of Gain in GEM Structures Number of secondaries per primary electrons (single GEM)  Very broad distribution Creation of secondaries mostly at edges But there is no extraction at edges!

Gas amplication and electron extraction in a GEM x electron created x created & extracted Primary electrons Simulation of Gain in GEM Structures

 Simulations allow optimisation of GEM readout structure Comparison Measurements and Simulations

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 feedback Triple GEM setup Optimized for ion feedback

Width & Length of Signal Comparison of signals: GEM versus wire readout For optimal space resolution match pad size to cluster size

Measurement of Charge Width in Magnetic Field - Measurement of charge width after passing triple GEM stack - Reduction of diffusion in high magnetic fields Fe 55 source ArCH 4 C0 2 92/5/3 Charge width is governed by primary ionisation and diffussion between GEM foils Range of 2,68 keV electrons in Ar

Constrcution of a Triple GEM Readout Struture Large (1.4 m 3 ) TPC Triple GEM structure

TPC operating with Triple GEM Readout

 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