Thermally stimulated currents in polyethylene irradiated by γ rays at low temperature

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Abstract
The influence of morphology and thermal history on the thermally stimulated current (TSC) of polyethylene samples irradiated with γ rays at low temperature has been studied. In low density polyethylene (LDPE) and high density polyethylene (HDPE), the TSC maximum occurs at about 50 and 110 °C, respectively, with further peaks observable at lower and higher temperatures. The amount of charge carriers detrapped increases by an order of magnitude when the crystallinity of LDPE changes from 42% to 55%. Annealing polyethylene samples at elevated temperature with subsequent slow cooling results in samples which produce TSC several times larger when compared with samples cooled from elevated temperatures with a high rate. The results are discussed in terms of one mechanism involving trapped ion pairs which undergo geminate recombination on detrapping causing only small TSC at low temperature. A second mechanism involves charges separated sufficiently to undergo volume recombination after detrapping causing the TSC maxima mentioned above. The traps of the latter mechanism are thought to be located mainly at the amorphous crystalline interface.