Combined readout of a triple-GEM detector

Optical readout of GEM based devices by means of high granularity and low noise CMOS sensors allows to obtain very interesting tracking performance. Space resolution of the order of tens of $μ$m were measured on the GEM plane along with an energy resolution of 20%$÷$30%. The main limitation of CMOS sensors is represented by their poor information about time structure of the event. In this paper, the use of a concurrent light readout by means of a suitable photomultiplier and the acquisition of the electric signal induced on the GEM electrode are exploited to provide the necessary timing informations. The analysis of the PMT waveform allows a 3D reconstruction of each single clusters with a resolution on z of 100 $μ$m. Moreover, from the PMT signals it is possible to obtain a fast reconstruction of the energy released within the detector with a resolution of the order of 25% even in the tens of keV range useful, for example, for triggering purpose.