ORION, a Multi-Chip Readout Electronics for Satellite Wide Energy Range X-/γ-Ray Imaging Spectroscopy: design and characterization of the analog section

The ORION chipset, a full-custom multi-chip readout and processing electronics for the X-γ Imaging Spectrometer (XGIS) on-board the THESEUS space mission, is presented. The XGIS detection plane is arranged in a matrix of 10×10 detection modules, each one composed by 64 CsI(Tl) scintillation bars (4.5 mm×4.5 mm×30 mm) optically coupled at the top and bottom ends to two 8×8 monolithic Silicon Drift Detector 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 a 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 multi-chip 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 (γ-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 ~25mm2 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 FWHM on Si), and 39.6 el. r.m.s. on the γ-channel (3.7 keV FWHM on CsI(Tl)) has been recorded.