Performance and qualification of CdTe pixel detectors for the Spectrometer/Telescope for Imaging X-rays

The Spectrometer/Telescope for Imaging X-rays (STIX) is a remote sensing instrument on-board the ESA Solar Orbiter spacecraft. STIX is designated to the study of energetic phenomena in solar flares. A Fourier-imaging technique using tungsten grid collimators in front of CdTe pixel detectors is employed, covering the 4 to 150 keV energy range with a full-width-half maximum resolution around 1 keV at low energies. Acrorad CdTe detectors of 1mm thickness with a planar aluminum Schottky contact are used as basis for a subsequent patterning process into eight large pixels, four small pixels, and a guard ring. The patterning is done by means of microfabrication technologies. The area of the patterned sensor is 10x10mm(2). Test equipment has been developed for selecting the detectors with best performance prior to integration with the read-out system, and for qualification purposes. The set-up allows pixel-based dark current measurements at low temperatures. Pixel dark currents below 60 pA are needed to avoid excess noise in the read-out ASIC. The best pixels show dark currents below 10 pA at 300 V bias and -20 degrees C. Spectroscopic measurements with Ba-133 sources confirm the good performance. This paper briefly explains the mission context of the CdTe detectors and then gives details of the production and testing procedures. Typical results are shown, with emphasis on performance degradation studies from displacement damage by proton irradiation. This is expected to be the dominant degradation mechanism for this application.