X/Gamma-ray Imaging Spectrometer Instrument onboard the THESEUS ESA M7 candidate mission: experimental tests and Monte Carlo simulations

Understanding the formation of the first generations of stars and galaxies, the evolution of the early Universe, and the nature of high-energy transient phenomena remains a major challenge in modern astrophysics. High-redshift Gamma-Ray Bursts (GRBs), thanks to their extreme luminosities, are powerful probes of the distant Universe and important targets for multi-messenger astronomy. The Transient High-Energy Sky and Early Universe Surveyor (THESEUS) has been proposed to address these scientific goals. Within its payload, the X/Gamma-ray Imaging Spectrometer (XGIS) is designed to detect transient sources from the X-ray to the gamma-ray domain, combining Silicon Drift Detectors, CsI(Tl) scintillator bars, coded-mask imaging, and dedicated readout electronics. This thesis presents an experimental and simulation study of an XGIS detector prototype. The work focuses on the characterization of a four-pixel prototype equipped with ORION electronics, including the calibration of the X and gamma branches, the determination of the effective test capacitance, the evaluation of electronic noise, and the study of scintillation light output and response uniformity along the bars. The experimental activity is complemented by Monte Carlo simulations developed with the Geant4 toolkit. A detector geometry was implemented to reproduce the laboratory configuration and investigate the detector response. The simulations were then extended to include detector-noise effects and depth-of-interaction information, with particular attention to their possible use for future background studies and event classification. By combining laboratory measurements and simulations, this work contributes to the characterisation and development of the XGIS detection concept and provides useful input for future optimization of the instrument within the THESEUS mission and other future high-energy transient Missions.