Modelling of post-irradiation events in polymer gel dosimeters

The nuclear magnetic resonance (NMR) spin-spin relaxation time (T-2) is rel ated to the radiation-dependent concentration of polymer formed in polymer gel dosimeters manufactured from monomers in an aqueous gelatin matrix. Cha nges in T-2 with time post-irradiation have been reported in the literature but their nature is not fully understood. We investigated those changes wi th time after irradiation using FT-Raman spectroscopy and the precise deter mination of T-2 at high magnetic field in a polymer gel dosimeter, A model of fast exchange of magnetization taking into account ongoing gelation and strengthening of the gelatin matrix as well as the polymerization of the mo nomers with time is presented. Published data on the changes of T-2 in gela tin gels as a function of post-manufacture time are used and fitted closely by the model presented. The same set of parameters characterizing the vari ations of T-2 in gelatin gels and the increasing concentration of polymer d etermined from Fr-Raman spectroscopy are used successfully in the modelling of irradiated polymer gel dosimeters. Minimal variations in T-2 in an irra diated PAG dosimeter are observed after 13 h.