  The X and Gamma Imaging Spectrometer instrument on-board the THESEUS mission
(selected by ESA in the framework of the Cosmic Vision M5 launch opportunity,
currently in phase A) is based on a detection plane composed of several
thousands of single active elements. Each element comprises a 4.5x4.5x30 mm 3
CsI(Tl) scintillator bar, optically coupled at both ends to Silicon Drift
Detectors (SDDs). The SDDs acts both as photodetectors for the scintillation
light and as direct X-ray sensors. In this paper the design of the XGIS
detection plane is reviewed, outlining the strategic choices in terms of
modularity and redundancy of the system. Results on detector-electronics
prototypes are also described. Moreover, the design and development of the
low-noise front-end electronics is presented, emphasizing the innovative
architectural design based on custom-designed Application-Specific Integrated
Circuits (ASICs).
