Prediction of the SVOM MXT camera end of life spectral performance based on proton irradiation results

TL;DR

This study predicts the spectral performance of the SVOM MXT camera throughout its mission by analyzing proton irradiation effects, demonstrating it will meet requirements and discovering a novel CTI inversion phenomenon.

Contribution

It provides the first detailed assessment of proton irradiation effects on MXT's spectral performance and reports a previously unobserved CTI inversion effect.

Findings

MXT will meet performance requirements over the mission duration

Proton irradiation causes an inversion of CTI trend with energy

A new CTI inversion phenomenon was observed and characterized

Abstract

SVOM, the Space-based Variable astronomical Object Monitor, launched on June 22nd 2024, is a Chinese-French mission focused on exploring the brightest phenomena in the cosmos - Gamma-Ray Bursts. Among the four instruments on board is the Micro-channel X-ray Telescope (MXT). The MXT camera features a 256x256 pixel pnCCD detector to perform X-ray imaging and spectroscopy in the 0.2-10 keV energy range. Cruising in a low-Earth orbit (600 km) that crosses the South Atlantic Anomaly, the MXT focal plane is exposed to radiation, primarily protons, that will lead to performance degradation over time. The challenge for MXT, and possibly for future missions with similar mass and mechanical constraints, is to maintain spectral performance all along the mission duration. To assess the expected radiation-induced performance degradation, a spare flight model of MXT focal plane underwent an irradiation campaign with 50 MeV protons at the Arronax cyclotron facility in June 2022. Then, the proton irradiated spare model was characterized in detail at the X-ray Metrology beamline of the SOLEIL Synchrotron facility in June 2023, as well as with a laboratory X-ray fluorescence source. We find through the evaluation of key indicators of performance such as the charge transfer inefficiency (CTI) and the low energy threshold, that MXT will remain compliant to its requirements over the SVOM mission lifetime. We also report an unexpected effect of proton irradiation that is the inversion of the trend of CTI with energy, recovered with two different sources illuminating the detector, and never reported in literature so far.