Electrical conduction in irradiated low-density polyethylene

A programme of experiments has been undertaken to examine transient charging/discharging currents and steady state currents in low-density polyethylene (LDPE) under the application of direct fields. This has been undertaken for pristine material and for material which has received doses of radiation between 10⁴ and 10⁶ Gy from either a ⁶⁰Co γ-source or a research reactor. The material was irradiated in ambient air or dry nitrogen. Measurements were made for applied fields in the range 6.7 × 10⁵-5.3 × 10⁷ V m ^-1 and temperatures between ambient and 90°C. With pristine material at low fields, transient charging/discharging currents decreased monotonically with time. However, the mechanism changed at higher fields with a peak occurring in the charging transient indicating a space-charge limited process. Substantial charge injection was also in evidence as demonstrated by anomalous discharging currents. This transient response was echoed by the current/voltage characteristics of steady state behaviour. Gamma and neutron irradiation brought about a change in this situation and the charge transport mechanism altered gradually from space-charge-limited conduction to an ohmic process with increasing dose. The role played by charge traps appears to be significant.