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 filled 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 10 ⁶, and rather fast anode pulses, of a width of 10 ns, were observed in some mixtures. Various phenomena and physical processes, found to affect the device operation, are discussed here: additional gain due to secondary scintillation; mixtures with enhanced ionization efficiency; improvement of pulse-height resolution due to avalanche confinement in the GEM holes; avalanche extension outside the GEM holes; gain limitation due to ion feedback and charging-up of GEM electrodes; photoelectron backscattering.