München 2019 – scientific programme
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HK: Fachverband Physik der Hadronen und Kerne
HK 7: Instrumentation I
HK 7.4: Talk
Monday, March 18, 2019, 15:00–15:15, HS 11
Ageing of GEMs in CH4-based gas mixtures compared to ageing processes in MWPCs — •Michael Jung for the ALICE collaboration — Institut für Kernphysik Frankfurt
The Time Projection Chamber (TPC) of ALICE was operated with Multi-Wire Proportional Chambers (MWPCs) until 2018. During these data taking periods, N2 and CO2 were used as quenching gas, even if Ar-CH4 can be considered as classical gas mixture for large-scale TPCs. Especially in high rate experiments like ALICE, it is well known that CH4 produces hydrocarbons in the plasma of the avalanche, which will cover the electrodes and lead to insulating depositions.
From 2021 on, the TPC will run with a quadruple stack of Gas Electron Multipliers (GEMs) to allow continuous data taking at 50 kHz in Pb-Pb collisions. Since GEMs are considered to have better ageing properties than MWPCs, the possibility to use CH4-based gas mixtures was evaluated with a GEM ageing test setup. Even if the degradation of the performance of the GEM detector was found to be less than in MWPCs, the usage of CH4 also leads to insulating deposits on the electrodes of the GEMs. Like in MWPCs, a gain drop as well as a degradation of the energy resolution was measured.
In this presentation the results of long-term irradiation tests with GEMs, operated in Ar-CH4 (95-5) will be presented together with a microscopic analysis of the irradiated GEMs and a comparison with ageing phenomena in MWPCs.
Supported by BMBF and the Helmholtz Association.