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UBC Theses and Dissertations

Fourier transform Raman spectroscopy of polyacrylamide gels for use in radiation dosimetry Jirasek, Andrew

Abstract

Polyacrylamide gel dosimeters are three dimensional, tissue equivalent, dose integrating, high spatial resolution dosimeters that show promise for use as dose verification tools in radiotherapy. To date, however, implementation of this class of dosimeters has been limited, partly due to the lack of complete understanding of the gel dose response mechanisms. The aim of this thesis is to further the understanding of chemical changes occurring in irradiated polyacrylamide gels. Fourier transform Raman spectroscopy is used to probe these chemical changes under a range of experimental conditions. Monomer consumption and polymer formation curves are constructed by cross-correlating relative Raman peak intensities for spectra acquired on individual gel samples which have each been irradiated to known doses. The results of the thesis are divided into three main categories. The first component involves preliminary work pertaining to establishing adequate experimental parameters. Sample housing, data acquisition parameters, data analysis, and spectral reproducibility are all examined. Reproducibility in spectral peak area is established to be better than 1% for spectra acquired on an individual gel sample, and ~1.5% or ~3.5% for spectra acquired on irradiated gels manufactured intra or inter batch, respectively. Secondly, monomer consumption and polymer formation is studied for gels irradiated with 6 MV x-rays. The monomer consumption data are extended to include gels of varying initial composition. Results indicate that monomer consumption is, in general, highly non-linear as a function of absorbed radiation dose. A qualitative model, based on the structure of formed polymer, is used to explain the differences between the consumption curves for the different gels. It is also shown that, within any given gel, the polymer structure varies as a function of absorbed dose. Finally, the dependence of the gel response on ionizing density is studied. Polymer gels are irradiated in two different regions of a 74 MeV spread out Bragg peak proton beam (i.e. two regions of differing incident particle linear energy transfer (LET). Monomer consumption curves are compared with 6 MV x-ray irradiated gel curves, thus arriving at a gel "relative effectiveness" (RE) as a function of LET. The theory of track structure is used to model the system and predict the gel RE in these same two regions. Track structure calculations confirm the LET dependence of the gels' response, indicating that the dependence is primarily due to the saturation of gel sensitive elements close (~ 10⁻⁶ cm) to the proton track. Track calculations are extended to different experimental situations and to gels of varying initial composition.

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