Study of the performance of an optically r eadout triple-GEM

Scintillation mechanisms in gases offer the possibility of an optical readout of micropattern gas detectors. This approach takes advantage of the large progress achieved in last years in the performance of the photosensors, opening the way to the realization of high granularity and very sensitive particle trackers. In this paper, the features of a triple-GEM structure filled with a He/CF4 (60/40) mixture and readout by a CMOS sensor are described. About 130 photons are detected per primary electron produced in the sensitive gap, making it possible to achieve a space resolution of about 35 μm. The device showed to be very sensitive even to small energy deposit: 4 photons/eV of released energy were detected.