Evaluation of silicon photodiodes for detection of ionising radiation.

The main objective of this research project was to investigate the suitability of the silicon photodiode as a light sensor for scintillation detection of ionising radiation. The type of instrument originally envisaged by Fisher would be particularly applicable to measurement of gamma-ray dose rates. During the course of the research, it was found that some modern silicon photodiodes are very effective as direct semiconductor detectors for both charged particles, and also for photons in the energy range 8 - 140 keV. A summary of this section of the work was published (GOODA and GILBOY, 1987) and has generated considerable interest in the subject. An associated medical physics project in Denmark was developed on the basis of these observations. The development of the photodiode as a scintillation light detector also proved highly successful. Using a CsI(Tl) scintillator and commercially available photodiodes, pulse height energy spectra rivalling those obtainable from conventional photomultiplier-NaI(Tl) assemblies were achieved. By comparing scintillation pulses with direct gamma absorption events in the photodiode, the light output of CsI(Tl) was determined to be significantly higher than that of NaI(Tl), which is usually accepted as the most efficient scintillator at room temperature. The detector assembly developed was successfully employed in the acquisition of data for a gamma ray transmission computer tomography system. A gamma dose rate instrument based on the CsI(Tl)-photodiode combination is also clearly feasible, but more work needs to be done to ascertain the range and sensitivity of this device. In addition to the developmental side of the project, some investigations were made into scintillation pulse shapes induced by gamma rays and alpha particles in CsI(Tl), with particular attention paid to afterglow. The Bollinger-Thomas method employed was modified by the inclusion of a spectrophotometer to investigate the wavelength dependence of pulse shapes