  A compact and modular X and gamma-ray imaging spectrometer (XGIS) has been
designed as one of the instruments foreseen on-board the THESEUS mission
proposed in response to the ESA M5 call. The experiment envisages the use of
CsI scintillator bars read out at both ends by single-cell 25 mm 2 Silicon
Drift Detectors. Events absorbed in the Silicon layer (lower energy X rays) and
events absorbed in the scintillator crystal (higher energy X rays and
Gamma-rays) are discriminated using the on-board electronics. A coded mask
provides imaging capabilities at low energies, thus allowing a compact and
sensitive instrument in a wide energy band (~2 keV up to ~20 MeV). The
instrument design, expected performance and the characterization performed on a
series of laboratory prototypes are discussed.
