Glass transition temperature of Risø B3 radiochromic film dosimeter and its importance on the post-irradiation heating procedure

Full-color development and stability of measured absorbance of the commercially used Risø B3 radiochromic film dosimeter can be achieved by applying a post-irradiation heat treatment procedure, typically at 60 °C for 10 min. However, the choice of post-irradiation heating duration and temperature has only been based on a trial and error method, and thus, the thermal stability of the material has not been fully studied before. We show that B3 film undergoes a rapid thermal decomposition of the PVB polymer base at 400 °C, with an initiation of the process at 70 °C. It means that higher irradiation temperatures, which can occur e.g. at high-dose electron beam irradiation, may cause polymer degradation. We also show that the post-irradiation heating temperature agrees with the polymer material's glass transition temperature as measured by differential scanning calorimetry (DSC). The post-irradiation heating procedure successfully removes the film's thermal history. This can be seen as an endothermic peak in the first heating cycle, which becomes smaller for higher irradiation doses. Glass transition temperatures of irradiated and post-irradiation heated B3 film are lower than those of the non-irradiated film, and decrease with increasing the dose. That implies a probable chain scission or degradation of the base polymer upon irradiation using high dose rates with no irradiation temperature control.