The effects of gamma-irradiation on thermally stimulated discharge current spectra in low-density polyethylene

Abstract

An investigation has been made of permanent effects of γ-radiation on additive-free low-density polyethylene (LDPE) using thermally stimulated discharge current techniques (TSDC) over the temperature range -100–100°C. In the low temperature regime (-100–0°C) a peak occurring around -30°C was introduced by irradiation in air but not in oxygen-free nitrogen. It was found that this peak is due to dipole orientation of oxidation products such as carbonyl groups. The movement of the peak towards higher temperatures with increasing radiation absorbed dose, is explained in terms of radiation-induced crosslinking which increases with increasing dose. In the high temperature region, (0–100°C) a peak observed at approx. 45°C in pristine samples, is due to the release of charge carriers injected from the electrodes. This peak was marked in irradiated samples by charge carriers created by the radiation. In the latter case, a peak occurred in the temperature range 60–100°C depending on dose and polarisation voltage. This behaviour can be explained by assuming that irradiation in air introduces shallow traps within the material which can be filled from deeper traps by a field assisted process during repeated polarisations associated with successive measurements. The process does not occur for irradiation in nitrogen and it is suggested that the shallow trapping levels are the result of oxidation, particularly with the production of carbonyl groups.