Avalanche photodiodes for the CATSAT gamma-ray burst mission

five-sigma significance level. In approximately half of these cases, it should be possible to discriminate between the hypothesis that there is an absorbing hydrogen column of density 1 x 10 sup 2 sup 2 cm sup - sup 2 and the hypothesis that there is no column. This thesis firstly describes efforts to characterise large-area, high-gain, Avalanche Photodiodes (APDs), manufactured by Radiation Monitoring Devices (RMD) Inc. of Massachusetts. These are relatively new devices in the field of X-ray spectroscopy and the research presented here attempts to increase our understanding of their behaviour as X-ray detectors and their underlying internal physical processes. Models are suggested for Quantum Detection Efficiency and for Photopeak Fraction in these devices. Measurements of these properties as a function of energy constrain the models, revealing new information about the internal structure of APDs and providing powerful predictive tools for detector response. The intrinsic silicon dead layer of a typical device is found to be 2 mu m thick, whilst the sensing layer is 21 mu m thick. Secondly, this thesis provides detailed accounts of how the new tools mentioned above have been utilised to characterise an ensemble of APDs and how that characterisation has been used to simulate the behaviour of an APD-based astrophysical instrument: CATSAT's Soft X-ray Spectrometer (SXR). This work includes simulated SXR observations of the diffuse soft X-ray background, the crab nebula and CATSATs target objects: Gamma-ray Bursts. The results of these simulations are presented, leading to an analysis of CATSAT's ability to meet its scientific objectives. It is estimated that the SXR will observe approximately 7 bursts per year above th