Spectral performance of ECLAIRs flight detectors on SVOM mission
Résumé
The hard X-ray imager of the Sino-French SVOM mission, called ECLAIRs, is dedicated to the detection and near real time localization of cosmic transients in the 4−150 keV energy range. ECLAIRs is a wide-field camera using a detection plane consisting of 6400 Schottky CdTe detectors, which makes images of the hard X-ray sky thanks to a coded mask. The detection plane is highly modular, being made of 200 hybrid modules of 4 x 8 pixels (called “XRDPIX”). Based on both an extremely low noise design and the use of innovative technologies, the ECLAIRs instrument will achieve the unprecedented low-energy threshold of 4 keV over the entire detection plane.
The CdTe semiconductor detectors of 4 × 4 mm 2 size and 1 mm thickness are biased with a high voltage going to −450V, and regulated at −20∘ C to minimize the leakage current and maximize the polarization time due to Schottky barrier lowering. The low-energy threshold required for the space mission is achieved thanks to an extensive characterization of the elements constituting the XRDPIX modules.
In this paper, we introduce the astrophysical context that led to the design of the mission, with an emphasis on the study of GRBs with ECLAIRs. We provide a detailed description of the XRDPIX hybrid modules, and we explain the experimental setup used for testing and qualifying 45 flight models. We present the results of the measurements performed to validate the module performance with a radioactive source of Americium 241. We study the spectral resolution, the low energy threshold, the gain of the detectors, and their counting performance. We also present two side studies concerning the impact of low temperatures and the stability of the performance over time. To conclude, we propose optimal settings for two key configuration parameters: the high voltage bias and the peaking time, that permit obtaining the challenging 4 keV energy threshold required for the SVOM mission.