Gamma irradiation effects in two epoxy resins and a polyimide

Changes in the electrical properties of insulators exposed to high levels of ionizing radiation can have a significant effect on the lifetime of equipment incorporating such material. In order to obtain a knowledge of the processes involved, a series of experiments were undertaken on the effects of gamma irradiation, at dose levels up to a few megagrays (MGy), on the DC conductivity and the thermally stimulated discharge current (TSDC) spectra of two epoxy resin samples and a polyimide. Among other findings it is shown that the curves for DC conductivity can be used to measure the glass transition temperature T_G. Initial differential scanning calorimetry measurements had shown that the T_G of the as-produced resin was 189°C. Irradiation to 1 MGy reduced this value to 90°C, and this was little changed by annealing. The activation energy below T_G decreased as a sequence of irradiation, but increased after annealing of the sample. Comparison with the TSDC spectra shows that the T_G of the system is lowered by the irradiation process, and it has also been shown that annealing increases the T_G, although at this dose level it does not return to its original value. Measurements on polyimide indicate that there is a change in conduction mechanism after irradiation