High granularity tracker based on a Triple-GEM optically read by a CMOS-based camera

The detection of photons produced during the avalanche development in gas chambers has been the subject of detailed studies in the past. The great progresses achieved in last years in the performance of micro-pattern gas detectors on one side and of photo-sensors on the other provide the possibility of making high granularity and very sensitive particle trackers. In this paper, the results obtained with a triple-GEM structure read-out by a CMOS based sensor are described. The use of an He/CF$_4$ (60/40) gas mixture and a detailed optimization of the electric fields made possible to obtain and very high GEM light yield. About 80 photons per primary electron were detected by the sensor resulting in a very good capability of tracking both muons from cosmic rays and electrons from natural radioactivity.