A theoretical investigation of embedded scintillator optical fibre sensors for use in real-time X-ray and γ-ray dosimetry

• Main Author: Ding, Liang
• Published: Cardiff University 2017
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.742800

This thesis reports on an inorganic scintillator embedded optical fibre dosimeter (ISOFD) sensor designed and optimised using a Monte-Carlo (MC) simulation method. The major factors that hinder the application of ISOFD are: the low coupling efficiency between the scintillation-domain and the optical fibre, the incident energy dependent response, and the lack of theoretical evaluation of the sensor performance. Regarding these problems, a MC simulation method was developed in this thesis to investigate the key factors that determine the radiation sensitivity, leading to an optimized sensor design. The radiation energy absorption by the scintillator material Gd2O2S and optical light transmission inside the ISOFD were simulated with respect to diagnostic X-rays and therapeutic γ-rays separately. The MC simulation method of ISOFD models for X-ray detection integrated the simulation of photon interactions and the ray-tracing simulation of light-rays. The simulation confirmed that the energy-dependence of ISOFDs is caused by the photon interaction cross sections of Gd2O2S for different incident energies. The packing density of the scintillation-domain has been identified the most prominent parameter that enhances the energy absorption efficiency. A thorough analysis of light reflection and transmission at boundaries showed that the coupling efficiency can be improved by decreasing the refractive index (RI) difference between the scintillation-domain and surrounding materials. The simulation method of the γ-ray detection further included the MC simulation of electron and positron interactions. The theoretical calculation of the photon interactions in Gd2O2S showed that the electron energy attenuation by scintillators contributes to the overall energy absorption of γ-rays. The simulated parameters of electron interactions are in good agreement with the ESTAR database. The simulation of the electron transport in a single Gd2O2S particle model verified that the electron energy is attenuated via inelastic collisions; meanwhile the energy deposition inside the particle is determined by the average size of the scintillator-particle.