Caliste-SO: the x-ray spectrometer unit of the STIX instrument onboard the Solar Orbiter space mission

Caliste-SO is a hybrid detector integrating in a volume of 12 × 14 × 18 mm 3 a 1 mm-thick CdTe pixel detector, a frontend IDeF-X HD ASIC and passive parts to perform high resolution spectroscopy in the 4-200 keV energy range with high count rate capability (10 4 -10 5 photons/s/cm 2 ). The detector hybridization concept was designed by CEA and 3DPlus to realize CdTe cameras for space astronomy missions with various pixel patterns. For the STIX instrument onboard the Solar Orbiter mission, the imaging system is made by 32 collimators that sample the visibilities of the spatial Fourier transform and doesn’t require fine pitch pixels. The Al-Schottky CdTe detectors produced by Acrorad are then patterned and tested by the Paul Scherrer Institute to produce 12 pixels surrounded by a guard ring within 1 cm 2 . Electrical and spectroscopic performance tests of the Caliste-SO samples are performed in France at key manufacturing steps, before sending the samples to the principal investigator to mount them in the Detector Electronics Module of STIX in front of each collimator. Four samples were produced in 2013 to be part of the STIX engineering model. Best pixels show an energy resolution of 0.7 keV FWHM at 6 keV (1 keV resolution requirement for STIX) and a low-level detection threshold below 3 keV (4 keV requirement for STIX). The paper describes the design and the production of Caliste-SO and focuses on main performance tests performed so far to characterize the spectrometer unit.