Radiation effects in CCD X-ray detectors
The outcome of several studies into future X-ray astronomy satellite missions has favoured the approach of using grazing incidence optics in conjunction with Charged Coupled Devices (CCD's) as focal plane detectors. This approach allows astrophysical investigation of unprecedented sensitivity by maximising source detectability and by resolving the source spectra. This thesis covers work performed in the development and evaluation of EEV CCD's for use as focal plane X-ray detectors. Due to the nature of the space environment, the interactions of radiation with CCD's also form a major part of this thesis. The subject of X-ray astronomy is introduced and the considerations affecting the operation of CCD's in space are discussed. The construction and operation of the CCD is thoroughly reviewed. The equipment and techniques used to evaluate the performance CCD's to X-rays are presented. A model is developed to simulate the interaction of X-rays with CCD's and this is followed by detailed measurements of X-ray performance. Further improvements to the design of CCDs to optimise their use as X-ray detectors are also discussed. These improvements include increased high and low energy quantum efficiency, larger area, low noise, reduced dark current and increased tolerance to radiation damage. Sources of background signal are investigated which lead to background rejection schemes for different CCD designs. Modelling is also performed to simulate an application of CCD's in space. Measurements of the radiation damage suffered by the CCD's are given and the damage mechanisms which will ultimately lead to device failure in space are discussed. The radiation damage covers irradiation using Co60 and heavy ions, but focuses on the effects of protons of doses below 5 krads. The impact of such a fluence on the scientific performance of an X-ray satellite is evaluated.