IMPROVED RECIPES FOR POLYMER GEL DOSIMETERS CONTAINING N-ISOPROPYLACRYLAMIDE
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Experimental studies were undertaken to improve the radiation dose response and ease of manufacture of polymer gel dosimeters that use N-isopropyl acrylamide (NIPAM) as the monomer. An alternative carageenan gelling agent was tested in place of gelatin. Although the carageenan did reduce the gelling time for the dosimeter solution, the dose response of the dosimeters was unsatisfactory. An alternative antioxidant system, ascorbic acid and Cu2+, was investigated with the aim of reducing the toxicity of dosimeter materials and providing opportunities for commercial production of prepackaged dosimeter kits. Unfortunately, the new antioxidant was ineffective for the NIPAM-based dosimeters that were studied. Three cosolvents, glycerol, N-propanol and isopropanol, were used to increase the solubility of N,N’-methylene-bisacrylamide (Bis) crosslinker in polymer gel dosimeter recipes that use NIPAM. These cosolvents enabled the manufacture of polymer gel dosimeters with higher levels of dissolved crosslinker than was previously possible. Preliminary results using x-ray computed tomography to read the resulting gels are very promising, due to enhancements in dose sensitivity. Dosimeters with high N,N’-methylene-bisacrylamide content that used isopropanol or glycerol as cosolvents had good optical clarity prior to irradiation, but did not produce reliable optical CT results for non-uniformly-irradiated gels. Further experiments and recipe optimization are required to determine whether gels with cosolvents and high levels of N,N’-methylene bisacrylamide can be used effectively for verifying spatially non-uniform dose distributions using x-ray computed tomography. A mathematical model that includes inhibition of NIPAM-Bis polymerization was developed and the inhibition effects of MEHQ and oxygen in polymer gel dosimeters were simulated. Kinetic parameters were obtained from the literature and were estimated using experimental data obtained by our research group. Good agreement was obtained between model predictions and experimental data with and without oxygen contamination. Simulation results indicate that MEHQ has little influence on the duration of the inhibition period and the rate of polymerization when no oxygen contamination is present, so that removal of MEHQ from dosimeter recipes is not required. Effective oxygen removal is very important to achieve reliable dosimeter results.