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 photo-sensors 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 o f a 4 mega-pixel CMOS sensor having a noise level less than two photons per pixel. The choice of a CF 4 rich gas mixture (He/CF 4 60/40) and a detailed optimization 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, about 700 photons per millimeter were collected and a space resolution of about 75μm was obtained. Moreover, the device showed to be very sensitive to the density of energy deposit: a relative uncertainty of 35% was found in the measurement of the dE/dx.