Impact of oxygen on the accuracy and precision of normoxic polymer gel dosimeters

Diffusion of oxygen in normoxic polymer gel dosimeters was studied. Gel samples in glass tubes were exposed to oxygen for different periods and then irradiated. Oxygen inhibited polymerization at the surface and polymerization occurred at larger depths. However the interface between these two regions revealed a local increase of polymerization. R2 values in these distinct regions are analyzed in terms of the concentration of oxygen and of antioxidant. Perhaps counter-intuitively, a small concentration of oxygen increases the sensitivity of the gel by consuming antioxidant molecules. The presence of antioxidant molecules is shown to cause a decrease of sensitivity, presumably through interactions with water free radicals and/or polymer radicals. Mixing and filling procedures as well as oxygen penetration from plastic caps or walls of gel containers are potential sources of oxygen-related discrepancies in normoxic polymer gel dosimeters. This effect may cause dosimetric inaccuracies of up to 60%.