The Impact of Irradiation Particle Type and Energy on Radiation-induced Trap Properties in CCDs

This thesis describes a study on the impact of irradiation particle type and particle energy on silicon based detectors. Radiation damage is a major limiting factor to achieving high accuracy scientific observations in space and can reduce a mission’s operational lifetime. Highly energetic particles can penetrate through satellites, reflect off the mirrors in the focal plane (e.g., Chandra) or interact with the body of the satellite to produce secondary particles that are energetic enough to fluoresce or damage the devices. The energetic particle can displace a silicon atom from its regular lattice position and the resulting vacancy (the lattice position absent of a silicon atom) can migrate through the device until it finds an energetically stable pairing. This degrades the performance of the detector and therefore quality of the science delivered. There is currently no ideal solution to fully mitigate radiation damage, instead any remaining damage must be corrected post-irradiation. This correction requires a complete understanding of the damage mechanisms and a real time measurement of the damage sites in the imaging area. Upcoming missions such as Euclid require a high level of post-imaging corrections to meet the ambitious scientific goals and will be utilising the “trap pumping” technique in-orbit to perform live characterisations on the active defects. The trap pumping technique is used extensively in this study to identify radiation induced defects in the silicon lattice by characterising defects introduced by energetic interactions at various energies and doses, including irradiation by protons, photons, neutrons and electrons. Different particle types and energies can cause ionisation damage, single atom displacement damage or clusters of atomic displacements. In this study several new and important results have been identified relating to n-channel defects. These defects are specific to particle type and energy. In particular the presence of a “fast tail” of emission time constants of the divacancy are found for proton and neutron irradiations and absent in gamma irradiations. Furthermore, a spread of emission times is found for all particle irradiations which indicates traps may occupy a distribution of energies/cross sections around an energy level rather than a single fixed emission time constant for each trap species. This work also shows that potential pockets can be falsely identified as traps through the trap pumping technique when insufficient datapoints are used. These results improve our fundamental understanding of trap dynamics in n-channel silicon and can help shape the strategy for damage mitigation and testing strategy for trap pumping in CCD based satellite missions