Silica-based Passive Dosimeters for sub-Gy Patient-focused Dose Evaluations

Abstract

In use of doped silica glass TL dosimeters, previous studies by others have focused on measurement of the radiation doses delivered during radiotherapy applications, with levels of dose typically in the fractions of a Gy to some several tens of Gy level. The present work is differentiated from such efforts, the thesis evaluating the utility of several types of enhanced-sensitivity doped silica glass TL dosimeter, novel forms that have been developed for measurement of low radiation doses. The range of interest is from several μGy to several tens of mGy level (ie doses some 0.1% and less of that of the therapy doses previously investigated), the focus being on medical applications within which such exposures arise. The stochastic effects of radiation are without threshold, the probability nevertheless increasing with dose. As such, the low radiation doses that can give rise to small but extant probability of inducing cancer need to be measured, allowing for dose mitigation measures that can be evaluated. Therefore, one of the main purposes of this research is to examine the development of new dosimeters that can measure such low radiation doses, either in diagnostic uses of x-rays or in regard to scattered radiation received to the lens of the eye during stereotactic radiosurgery (SRS) applications. The diagnostic applications of choice are those which find widespread use, from single exposure dental doses, through to chest radiography and mammographic imaging. As no dosimeter exists that can come close to approximating the ideal characteristics of an omni-utility sensor, a number of glass TL dosimeter forms have been investigated, differing in dopant concentration, size and fabrication, examined for use in the different dose regimes. The characterisation of GeB-PFc, Ge-PFc, GeB-FF, Ge-doped disc, Ge-SMF and LiF (all defined in detail in the thesis) have been performed, including the study of linearity, fading, energy response, dose response, and sensitivity. The outcome is one demonstrating successful accommodation of all of the dose ranges of interest. Additional sensitivity of between 20% to 30% has been shown to be available through thin (several tens of microns) high Z coatings to flat fibres (FF), the coatings acting as photoelectron and Auger electron conversion media. Being small in size, non toxic and devoid of any electrical contact, it is the intention that these TL forms take on the important role of safe and accurate dose evaluations as an aid in dose optimization measures.