The GEM photomultiplier operated with noble gas mixtures

We present the results of detailed investigations of the Gas Electron Multiplier (GEM)-based photomultiplier, consisting of a solid CsI photocathode coupled to a cascade of GEM elements. The detector is "lled with non-ageing mixtures based on noble gases: Ar, Ne, Ar#Ne, Ar#Xe, Ar#CH 4 and Ar#N 2 . Very high gas gains, reaching 106, and rather fast anode pulses, of a width of 10 ns, were observed in some mixtures. Various phenomena and physical processes, found to a!ect the device operation, are discussed here: additional gain due to secondary scintillation; mixtures with enhanced ionization e$ciency; improvement of pulse-height resolution due to avalanche con"nement in the GEM holes; avalanche extension outside the GEM holes; gain limitation due to ion feedback and charging-up of GEM electrodes; photoelectron backscattering. ( 2000 Elsevier Science B.V. All rights reserved.