Buildup of electric charge in foil dielectrics during gamma irradiation

Theoretical and experimental studies of electric volume charge building up in polymer foils under ionizing radiation reveal two mechanisms at work here. One is the difference between the absorption capacities of the foil material and of the other material in contact with it, the second mechanism is knockout and slowdown of Compton electrons. Seven industrial grades of polymer materials in foils of 0.2-2.0 mm thickness were exposed to gamma radiation in doses of 1.3-6 from a 60Co source at 100 C: SF-2-50 and SF-2N-50 (glass Textolite), FDM-2 (glass Textolite), GF-2-50, PEVD (high density polyethylene), PKT-5, FLAN-10. Data were obtained on the buildup of volume charge density as function of the irradiation dose and as function of the intensity of the external electric field (one electrode grounded, and electrode connected to electrometer amplifier) at two fixed irradiation doses, also on the volume charge relaxation as function of time. The data were evaluated in terms of thermal stimulation current, the charge calculated by graphical integration of thermostimulated current curve. The results indicate that both processes do depend in a complex manner on the kind and the thickness of the dielectric foil material as well as on the conditions of irradiation and storage respectively.