First Characterization of the Focal Plane of the Micro-Channel X-Ray Telescope on board the SVOM Mission and Preparation of Radiation Tests to Predict its Performances in Low-Earth Orbit

Abstract

This paper describes the simulated in-orbit environment and the first on-ground experimental characterisation of the focal plane of the Micro-channel X-ray Telescope (MXT) on board the Chinese-French Space-based multi-band astronomical Variable Object Monitor (SVOM), for the study of Gamma-Ray Bursts (GRBs). MXT will mount a back-illuminated 450 µm-thick fully-depleted frame-store pnCCD at the focal plane of lobster-eye optics to study the afterglow of GRBs in the 0.2 to 10 keV energy range. The performances of the MXT detector are predicted to degrade due to the radiation environment of the low Earth orbit of the SVOM satellite, which includes regular passages through the South Atlantic Anomaly (SAA). Of special concern are the leakage current and the Charge Transfer Efficiency (CTE), affecting the low-energy threshold and the spectral resolution of the camera, which will be operated at -65°C. In-orbit degradation will be experimentally studied on ground at the beginning of 2019 with proton irradiation tests at the Paul Scherrer Institute (PSI) in Switzerland. In preparation for this test campaign, simulations were performed to evaluate the particle flux on the detector and hence the displacement damage at the end of the mission, with emphasis on the contribution of protons trapped in the radiation belts. The simulations used the SPENVIS web service and the Geant4 toolkit, and were based on a simplified geometrical model of the camera, increasingly detailed around the focal plane of MXT. In parallel, a first laboratory characterisation was performed on the Engineering Model (EM) of the MXT focal plane.