Extensive testing of Schottky CdTe detectors for the ECLAIRs X-/gamma-ray camera on board the SVOM mission

We report on an on-going test campaign of more than 5000 Schottky CdTe detectors (4x4 x1 mm3), over a sample of twelve thousands, provided by Acrorad Co., Ltd (Japan). Six thousand four hundred of these detectors will be used to build the detection plane of the ECLAIRs camera on the Chinese-French gamma-ray burst mission SVOM. These tests aim at eliminating, from the flight model, detectors showing high leakage current and unstable behaviors. They are mandatory to fulfill the prime requirement of ECLAIRs to detect gamma-ray burst photons down to 4 keV. For better functional performance and stability, the detectors will be operated at -20 oC under a reverse bias of 600 V. Under these conditions, we found that 78% of the detectors already tested could be considered for the flight model. They show a low initial leakage current (with a mean value around 20 pA), and remain stable below 100 pA within 2 h. Using a standard analog electronics chain, we measured a mean energy resolution of 1.8 keV at 59.6 keV using an 241Am source. Because the Schottky detectors are well known to be unstable due to the bias-induced polarization effect, the high voltage power supplies on ECLAIRs will have to be switched off at regular time intervals. We investigated the polarization effect first at room temperature and low bias voltage for faster analysis. We found that the spectroscopic degradation in quantum efficiency, gain and energy resolution, starts as soon as the bias is turned on: first slowly and then dramatically after a time tp which depends on the temperature and the voltage value. Preliminary tests under in-flight conditions (-20 oC, -600 V) showed that the detectors should remain stable over a timescale larger than a day. As a by product of our test campaign, we measured the mean activation energy of 170 Schottky CdTe detectors. We found evidence for two distinct populations of detectors: the main one centered at 0.64 eV, interpreted as due to cadmium vacancies in the crystal, and the second population centered at 0.54 eV, correlated with a lower apparent resistivity.