Ricerc@Sapienza

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 75?m 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.