Characterization of the focal plane of the microchannel X-ray telescope at the metrology beamline of SOLEIL synchrotron for the space astronomy mission SVOM

The Microchannel X-Ray Telescope is one of the four instruments of the SVOM Sino-French space mission to observe the afterglow of gamma-ray bursts in the 0.2–10 keV energy range and localize them within 2 arcmin. Inside the 9 kg camera, the detection chain consists of a detector assembly equipped with a fully depleted 256 × 256 pixels pnCCD and two CAMEX front-end ASICs, and a “US-free” front-end electronics unit to control the focal plane and extract photon events. The flight spare model was installed and characterized in November 2021 at SOLEIL synchrotron on the soft X-ray branch of Metrology beamline operating from 40 eV to 1800 eV. The goals of this campaign were to measure the quantum efficiency and characterize the spectral response, i.e., the energy calibration law, the charge transfer efficiency and the energy resolution at different positions on the detector. This implied the development of a full-custom cryostat to interface with the beam line and a special tuning of the beam line to get the appropriate photon flux for single photon counting in the pixels of 75 μm size without pile-up. This paper presents the configuration, the test plan, the test results, and the modeling of the detector response. This beam line was successfully used for spectral characterizations of the MXT detector from 300 to 1800 eV with excellent spectral purity and with a photon flux 7 orders of magnitude lower than the beamline nominal output.