Radiological characterization of metal oxide semiconductor field effect transistor dosimeters

Abstract

Metal oxide semiconductor field effect transistor (MOSFET) dosimeters are increasingly utilized in radiation therapies and radiation diagnosis. While MOSFET dosimeters have already proved to be excellent in high-energy photon radiation therapies, research on the detailed dosimeter characteristics has not yet been done for the diagnostic range of photons (i.e., low energies and low doses). This paper reports our effort to develop a very detailed 3D Monte Carlo simulation model of the High-Sensitivity MOSFET dosimeter using MCNP 4C. Efforts were put in developing and validating a methodology to accurately determine energy deposition in the sensitive volume, which is extremely thin, of the MOSFET dosimeter. Our simulation results show that the MOSFET dosimeter has maximum energy dependence (5.2 times) at about 40 keV of photon energy. The energy dependence curve was also found to follow the ratio of the mass energy absorption coefficient of silicon to that of air (= ( en/ )silicon / ( en/ )air), but show variation due to the lack of charged particle equilibrium at the sensitive volume of the dosimeter. We also used the Monte Carlo model to study the depth dependence and the angular dependence of the dosimeter. Finally, this study investigated the distribution of regional electron contributions to the sensitive volume of the MOSFET dosimeter, which is important to the manufacturer for optimization of the dosimeter for low energy photon applications.