The relationship between radiation-induced chemical processes and transverse relaxation times in polymer gel dosimeters

The effects of ionizing radiation in different compositions of polymer gel dosimeters are investigated using FT-Raman spectroscopy and NMR T-2 relaxat ion times. The dosimeters are manufactured from different concentrations of comonomers (acrylamide and N,N'-methylene-bis-acrylamide) dispersed in dif ferent concentrations of an aqueous gelatin matrix. Results are analysed us ing a model of fast exchange of magnetization between three proton pools. T he fraction of protons in each pool is determined using the known chemical composition of the dosimeter and FT-Raman spectroscopy. Based on these resu lts, the physical and chemical processes in interplay in the dosimeters are examined in view of their effect on the changes in T-2 The precipitation o f growing macroradicals and the scavenging of free radicals by gelatin are used to explain the rate of polymerization. The model describes the changes in T-2 as a function of the absorbed dose up to 50 Gy for the different co mpositions. This is expected to aid the theoretical design of new, more eff icient dosimeters, since it was demonstrated that the optimum dosimeter (i. e, with the lowest dose resolution) must have a range of relaxation times w hich match the range of T-2 values which can be determined with the lowest uncertainty using an MRI scanner.