  A future compact and modular X and gamma-ray spectrometer (XGS) has been
designed and a series of prototypes have been developed and tested. The
experiment envisages the use of CsI scintillator bars read out at both ends by
single-cell 25 mm2 Silicon Drift Detectors. Digital algorithms are used to
discriminate between events absorbed in the Silicon layer (lower energy X rays)
and events absorbed in the scintillator crystal (higher energy X rays and
gamma-rays). The prototype characterization is shown and the modular design for
future experiments with possible astrophysical applications (e.g. for the
THESEUS mission proposed for the ESA M5 call) are discussed.
