Proton induced damage after laboratory cold irradiation in CCD47-20 CCDs for CHEOPS

The CCD47-20 detector of CHEOPS (CHaracterising ExOPlanet Satellite, ESA/Switzerland), operated at -45 degrees C, is developing an increasing number of hot pixels in orbit, due to particle-induced radiation damage in its 700 km, sun-synchronous dawn/dusk orbit. While the satellite offers the possibility to raise the detector's temperature to well above +20 degrees C, it is not clear whether annealing in this temperature range, and for how long, could have a beneficial effect on the hot pixel distribution. To inform the mission on the potential benefits of in-orbit annealing, we irradiated CCD47-20s from the CHEOPS flight production batch at -45 degrees C with protons. After irradiation, the hot pixels were monitored for 7 days at -45 degrees C. Subsequently, the temperature was raised step-wise (up to +80 degrees C) for 12 hours at each temperature, each step followed by characterisation at -45 degrees C. A significant reduction in the hot pixel count was observed at each step. Based on the results, a recommendation for a possible in-orbit annealing procedure has been derived.