Properties of the GEM, double GEM and GEM+MGC combination

We have examined the operation of a GEM, double GEM and GEM+MGC combination in several counting gases, including argon/DME 60/40, 90/10, xenon/DME 90/10, xenon/CO2 90/10 as well as pure argon and pure xenon. In a cascaded GEM configuration, large gains are recorded, around 105 in argon/DME 90/10. Energy resolution is <25% FWHM for 5.9 keV photons. The collected electron cloud size is limited to a maximum of ∼107 electrons, depending somewhat on the voltage across the last amplifying GEM. For the MGC in combination with the GEM this maximum, the Raether limit, is found to be 2.7×107 electrons. This corresponds to a gain for the GEM+MGC of 1.5×105 for 5.9 keV photons in Ar/DME 60/40. In pure argon and xenon the maximum gain is, respectively, 4000 and 7000 for the double GEM, with energy resolutions of, respectively, 40% and 25% FWHM. Special attention is paid to behaviour of the gain with respect to the electric field strengths before and after the GEM, as well as between two GEMs, in view of a better understanding of the processes of charge collection into the hole and ejection out of the hole.