Effects of crosslinker fraction in polymer gel dosimeters using FT Raman spectroscopy

Radiation-induced chemical changes in polyacrylamide gels (PAGs) used in 3D radiation dosimetry have been studied using Fourier transform Raman spectroscopy. Consumption of monomer and crosslinker were characterized for polymer gel dosimeters with initial fraction of crosslinker (%C) varying from to 100%. Results indicate that the rate of monomer/crosslinker consumption is highly nonlinear over %C. This indicates that maximum polymer gel dosimeter sensitivity (i.e. slope of the monomer consumption curves) is dependent not only on initial %C but also on dose range. For low-dose regions (<5 Gy) 30%C PAGs are the most sensitive dosimeters, their maximum sensitivity being a factor of 1.8 times greater than 50%C PAGs. In mid-dose range (S-15 Gy), however, 50%C PAGs are the most sensitive dosimeters, being potentially a factor of 1.3 more sensitive than any other PAG studied. In high-dose regions (> 15 Gy) 70%C PAGs are at least 1.5 times more sensitive than any other PAG studied. A qualitative model is presented which helps to explain the variation in overall monomer consumption rate as a function of initial %C. The variation in the shapes (i.e. mathematical course) of the consumption curves for different %C PAGs is, in part, qualitatively explained by consideration of the average number of monomer to crosslinker molecules available for reaction in each PAG as it is irradiated to higher doses. Finally, we demonstrate the importance of an existing polymer network on the subsequent monomer consumption rate in a 50%C PAG.