Extensive testing of Schottky CdTe detectors for the ECLAIRs X-Gamma-ray Camera on board the SVOM mission

摘要

We report on an on-going test campaign of more than 5000 Schottky CdTedetectors (4x4x1 mm^3), over a sample of twelve thousands, provided by AcroradCo., Ltd (Japan). 6400 of these detectors will be used to build the detectionplane of the ECLAIRs camera on the Chinese-French gamma-ray burst mission SVOM.These tests are mandatory to fulfill the prime requirement of ECLAIRs to detectgamma-ray burst photons down to 4 keV. The detectors will be operated at -20Cunder a reverse bias of 600 V. We found that 78% of the detectors alreadytested could be considered for the flight model. We measured a mean energyresolution of 1.8 keV at 59.6 keV. We investigated the polarization effectfirst at room temperature and low bias voltage for faster analysis. We foundthat the spectroscopic degradation in quantum efficiency, gain and energyresolution, starts as soon as the bias is turned on: first slowly and thendramatically after a time t_p which depends on the temperature and the voltagevalue. Preliminary tests under in-flight conditions (-20C, -600 V) showed thatthe detectors should remain stable over a timescale larger than a day. We alsomeasured the mean activation energy of 170 Schottky CdTe detectors. We foundevidence for two distinct populations of detectors: the main one centered at0.64 eV, interpreted as due to cadmium vacancies in the crystal, and the secondpopulation centered at 0.54 eV, correlated with a lower apparent resistivity(abridged).