Simulations and experimental tests of the X/Gamma-ray Imaging Spectrometer for THESEUS and other mission opportunities

Gamma-Ray Bursts (GRBs) are unique tools for investigating the Early Universe and for advancing Multi-Messenger Astrophysics. The Transient High-Energy Sky and Early Universe Surveyor (THESEUS) has been proposed in the ESA M7 framework for exploiting the potential of such events as cosmological probes. The THESEUS payload consists of two GRB monitors and a telescope, respectively, the Soft X-ray Imager (SXI), the X/Gamma-ray Imaging Spectrometer (XGIS) and the InfraRed Telescope (IRT). The extremely wide Field of View and the unprecedented energy pass-band of the monitors (2 keV – 10 MeV) are combined with an onboard estimation of the redshift, provided by the IRT. The detection plane of XGIS is composed by an array of Silicon Drift Detectors coupled with CsI(Tl) scintillator bars. In this Thesis, I have verified XGIS scientific capabilities by doing functional, performance and environmental tests on a demonstration module. Concerning the former activity, an XGIS prototype, coupled to specific readout electronics (ORION), has been characterized, through both radioactive sources and synthetic electric signals. Energy spectra were obtained and analysed, with particular attention to the correct elaboration of data, the full scale energy range of the instrument and its energy resolution. Due to its flexible design, XGIS can be adapted to different instrument sizes, shapes and mission costs. Through an exploratory simulation, I have evaluated the GRB rate estimation performance of an experiment based on an alternative configuration of XGIS.