Development of a silicon detector for dose imaging and measurement in external beam radiotherapy

The aim of this research project is to characterise two silicon epitaxial diode types with structures differing with the presence of n+ guard ring in one type. These detectors will be evaluated and ultimately assessed on their potential as a reliable commercial detector system for newer radiation therapy modalities, with focus on its use in Intensity Modulated Radiation Therapy Quality Assurance. The detectors comprise of a p+ layer as its substrate and a p-type epitaxial layer, with n+ junction implanted. One of the detector types had an n+ guard ring surrounding the diode, which was designed to minimise leakage current and limit the sensitive volume of the detector. This investigation showed that the guard ring was effective in minimising leakage current when grounded, but left unconnected greatly increased the overall detector noise and lead to a large decrease in collection efficiency. However, it was also seen that having the guard ring grounded acted as a pathway to ground for charge carriers created during incident radiation at low voltages, thus disqualifying it for use in passive mode. Comparing results with the diode without the guard ring, the latter showed a slightly higher leakage current, but overall higher collection efficiency and a low depletion voltage, thus it was more suitable to be used as a during a clinical setting. Clinical irradiations showed that the detector is dose rate independent across the range between 50 and 600 MU/min, it shows a linear response to dose over the range of 0 to 100 MU and a depth dose profile congruent with results produced by ion chamber measurements. It was therefore concluded that the detector without the guard ring was the most suitable option for measurements of IMRT radiation therapy.