ORANGE: A high sensitivity particle tracker based on optically read out GEM

GEM-based detectors had a noticeable development in last years and have successfully been employed in different fields from High Energy Physics to imaging applications. Light production associated to the electron multiplication allows to perform an optical readout of these devices. The big progress achieved in CMOS-based photosensors makes possible to develop a high sensitivity, high granularity and low noise readout. In this paper we present the results obtained by reading out the light produced by a triple-GEM structure by means of a 4 mega-pixel CMOS sensor having a noise level less than two photons per pixel. The choice of a CF4 rich gas mixture (He/CF4 60/40) and a detailed optimisation of the electric fields allowed to reach a light-yield high enough to obtain very visible signals from minimum ionizing particles. In a test performed with 450 MeV electron beam, 800 photons per millimeter were collected and a space resolution of about 75μm was obtained.