Development and application of room temperature operated semiconductor radiation detectors

The application of radiation detector materials, capable of operating at room temperature, has been studied. The advantage of such materials is that they do not need to be cooled to reduce thermally generated leakage currents. Therefore, systems utilising these detectors can be smaller and more portable than ones containing HpGe and Si(Li) detectors which need to be cooled using liquid nitrogen to reduce leakage current. Three materials, silicon, cadmium telluride and mercuric iodide, have been investigated. The silicon photodiode showed very good energy resolution down to photon energies of 13.4keV, but its low detection efficiency restricts its use to energies below 200keV for direct detection. Cadmium telluride produced energy spectra with worse energy resolution, due mainly to charge collection, leakage current and fabrication. But, cadmium telluride was able to produce spectra up to 661.6keV with reasonable efficiency and resolution. Mercuric iodide produced spectra with energy resolution on a par with the silicon photodiode but with a higher detection efficiency. Mercuric iodide crystals have been produced at Surrey using the polymer assisted transport growth method. A large crystal with dimensions exceeding 1cm3 has also been produced. Due to problems with application of electrical contacts no useful spectra were obtained with these devices. The importance of the polymer in the starting material was investigated. The addition of polyethylene was found to favour the growth of platelets. The size of the platelets was also found to increase with increasing proportion of polymer in the stalling mix. A computer simulation enabling the performance of semiconductor radiation detector materials to be predicted is also presented. The model follows processes from photon interactions in the detector through to charge collection. Energy spectra may be produced from the output of this model and energy resolution and detection efficiency may be analysed as a function of detector thickness, applied bias and photon energy. This model could be used to optimise performance of radiation detectors.