Study of the radiation effects on the Swift-XRT CCD camera in low Earth orbit

In this thesis, the damage caused by space radiation on the Swift X-ray telescope CCD is investigated. The analysis reveals the presence of damaged pixels affected by charge traps that results in the degradation of the detector energy resolution. The software developed for the trap mapping analysis is presented. The implementation of the trap corrections recovers a significant fraction of the lost resolution. Data from XRT calibration radioactive sources are analysed to characterise the energy and temperature dependence of the charge losses. The trap measurements are exploited in the attempt to derive the value of the ionisation energy of silicon using a novel statistical method. The large charge losses affecting the damaged pixels are at odds with expectations from CCD irradiation by protons, that should generate single electron defects. Neutrons, instead, generated on board the XRT in the detector aluminium proton shield, may displace multiple lattice atoms, as they interact directly with the nuclei. The two scenarios were investigated exposing the same kind of CCD on board the XRT, irradiated before the Swift launch with 10 MeV protons, to a dose of 14 MeV neutrons comparable to that of the XRT during a few years in orbit, as derived from simulations developed using ESA’s space radiation modelling system. A laboratory program was undertaken at the Leicester Camera test facility to investigate the damage caused by protons and neutrons. In both cases, pixels affected by large energy losses are identified and characteristic trap energy levels are derived. In the context of satellite missions using CCDs, the observed spatial nonuniformity of the damage suggests that the classical approach of an average correction for the charge transfer inefficiency applied over the entire detector is not accurate and may produce misleading results. Optimisation of the CCD shielding design is discussed based on this investigation.