GEM photomultiplier operation in CF4

The properties of a 3-GEM (Gas Electron Multiplier) element photomultiplier, with a semitransparent CsI photocathode and CF4 gas filling, are presented. Compared to other gas mixtures, such as CH4, Ar/CH4, Ar/N2 and He/Ar/N2, CF4 has superior performance: the highest gain, approaching 107, the fastest, 8 ns wide signal and the lowest photoelectron backscattering; the latter allows to reach photocathode quantum efficiency values approaching that in vacuum. The time resolution of the multi-GEM photomultiplier for single photoelectrons was measured to be 2 ns. These properties are of high relevance for applications in Cherenkov detectors and in tracking devices.