High- and low-pressure operation of the gas electron multiplier

We have studied the operation of the Gas Electron Multiplier (GEM) in gas mixtures Xe–CO2, Ar–CO2 and CH4 at different pressures varying from 0.1 to 5 atm. In Ar- and Xe-based mixtures, the maximum GEM gain considerably decreases with pressure, from a few hundreds at 1 atm to below 10 at 5 atm. Combined gain of GEM and the Micro-Strip Gas Chamber (MSGC) can exceed values of 10000 at 1 atm and 100 at 5 atm. High GEM gains, of above 1000, were obtained in CH4 at low pressures. We have observed the effect of the avalanche confinement in GEM micro-holes, resulting in violation of the pressure scaling and in the possibility of GEM operation in pure noble gases.