An investigation of the chemical stability of a monomer/polymer gel dosimeter

The aim of this work is to investigate the temporal stability of a polyacry lamide gelatin hydrogel used for 3D monomer/polymer gel dosimetry technique s involving different methods of analysis. Long-term instabilities for a si milar gel have recently been reported, but differ markedly from those descr ibed in this work. Two kinds of long-term instabilities are described. One affects the slope o f the dose-R-2 plot and is related to post-irradiation polymerization of th e comonomer/polymer aggregates. It is observed that post-irradiation polyme rization only lasts 12 hours after irradiation. The other instability affec ts the intercept of the dose-R-2 plot, lasts for up to 30 days and is relat ed to the gelation process of gelatin. Further studies were performed on gelatin gels of varying compositions to o btain a better understanding of the molecular mechanism that causes the ins tability due to gelation. The studies included observations of the spin-spi n and spin-lattice relaxation rates in combination with diffusion measureme nts and optical measurements. It is shown that the heating history during the manufacture of the gel affe cts the absolute R-2 value of the gel but not its variation. The findings presented in this study may help in producing more stable and reproducible monomer/polymer gel dosimeters.