Limits to proportionality in the double GEM detector

We have examined the operation of a GEM, double GEM and GEM+MGC combination using argon/DME 90/10, 60/40, 40/60, 20/80, 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. In Ar/DME 60/40 however, the double GEM detector operates non-proportionally at high gains, close to the Raether limit, also leading to a remarkable effect in the energy resolution: it reaches values as high as 70%. This is also tentatively shown in Ar/DME 40/60 and not at all in 90/10, leading to the conclusion that avoiding non-proportionality in cascaded GEMs requires quencher-poor gas mixtures such as Ar/DME 90/10.