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The ORION chipset, a full-custom multichip readout and processing electronics for the X- $\gamma $ -ray imaging spectrometer (XGIS) on-board the transient high-energy sky and early universe surveyor (THESEUS) space mission, is presented. The XGIS detection plane is arranged in a matrix of 10 $\times $ 10 detection modules, each one composed of 64 CsI(Tl) scintillation bars (4.5 mm $\times $ 4.5 mm $\times $ 30 mm) optically coupled at the top and bottom ends to two 8 $\times $ 8 monolithic silicon drift detector (SDD) matrices. The top SDD, exposed to the X-ray entrance window, performs the double function of low-energy X-ray detection as well as scintillator’s readout, together with the bottom SDD, providing detection and spectroscopic energy range from 2 keV up to 20 MeV. The need to achieve a high-energy resolution, as well as a high sensitive area on the detection plane, led to the development of a chipset organized to have a minimum-area analog readout chip placed in close proximity of the SDD (ORION-FE) and a mixed-signal back-end (ORION-BE) placed a few centimeters further on the back-end board for the additional signal processing and digitization. The multichip readout electronics integrates two dedicated analog processors for low-energy photons up to 30 keV (X-processor) and high-energy photons up to 5 MeV ($\gamma $ -processor), allowing a spectroscopy-grade resolution in the 4 decades energy band (2 keV–20 MeV) of the XGIS, with a simulated power consumption of 1.55 mW/pixel. The ORION prototype was bonded to two ~25 mm2 SDDs, and extensively characterized in terms of pulse shaping, pulse discrimination, and stretching functionality, as well as linearity, dynamic range, and spectroscopic resolution. An optimum equivalent noise charge (ENC) at −20 °C of 24.3 el. r.m.s. on the X-channel [212 eV full-width at half-maximum (FWHM) on Si], and 39.6 el. r.m.s. on the $\gamma $ -channel [3.7 keV FWHM on CsI(Tl)] has been recorded. [ABSTRACT FROM AUTHOR]

In this paper, an X- and gamma-ray detector based on a silicon drift detector (SDD) coupled to a CsI(Tl) scintillating crystal is presented. The SDD is operated both as a direct X-ray detector for photons interacting in silicon and as an indirect detector for photons interacting in the scintillator. As interactions in silicon and in CsI yield different amounts of charge per unit energy deposited, discrimination of the place of interaction is necessary to obtain the correct energy. Discrimination of the interaction type is carried out by means of pulse shape discrimination performed with two parallel processing chains with different shaping times. The performance of the detectors are described and discussed. Pulse shape discrimination is possible throughout the detector energy range. [ABSTRACT FROM AUTHOR]