A position sensitive gamma-ray detector based on silicon drift detectors coupled to scintillators for application in the MEGA Compton telescope

An array of gamma-ray detectors based on silicon drift detectors (SDD) coupled to thallium activated cesium iodide (CsI(Tl)) scintillating crystals is presented. Because of SDDs low output capacitance and the possibility to have the first amplifying stage directly integrated on chip, SDDs show better noise performance than traditional p-i-n photodiodes. The array is read out with custom made ASICs built to match with the SDDs electrical properties. Shaped signals are digitally converted by a digital front-end electronic board which also takes care of ASICs handshaking. The system architecture is based on that of the IBIS/PICsIT instrument onboard the INTEGRAL satellite, at present fully operative. The instrument has been designed as a calorimeter module suitable to work in conjunction with the MEGA Compton telescope prototype, developed at the Max-Planck Institut fur Extraterrestrische Physik (MPE), Garching, Germany. The calorimeter module will detect photons scattered by the MEGA silicon tracker. The improvements in energy resolution provided by SDD/CsI(Tl) detectors should improve the overall energy and angular resolution of the Compton telescope. The detectors properties and the system performance, from analogue signal conditioning to the digital data handling will be reported and discussed.