Abstract
When polymers are used for electrical insulation in space or at a nuclear power plant, radiation-induced space charges or surface charges may give rise to an unexpected surface discharge or breakdown. The charge generation and accumulation processes can be understood by examining the distribution of space charge inside the polymers irradiated by low-dose radioactive rays. Therefore, in this report, residual space charge distribution in gamma-irradiated polyethylene is examined. To measure the space charge distribution, the pulsed electroacoustic (PEA) method was employed. The PEA method is one of the advanced nondestructive techniques to measure the space charge distribution with high sensitivity and reproducibility. Samples of a commercially available low density polyethylene were irradiated by a60Co source to a total dose of 1.2 to 12 kGy which is high enough to excite charges and low enough to avoid chemical damages such as oxidation. After gamma-irradiation, dc voltages of up to 15 kV were applied to the sample, and the time dependence and the applied voltage dependence of space charge distribution were measured using the PEA method. From these measurements, it became clear that a large amount of positive charges exist near the cathode in gamma-irradiated polyethylene, and these positive charges enhance the electric field at the interface between the cathode and the polyethylene. On the other hand, almost no space charge was observed in nonirradiated polyethylene. It is considered that the mobile carriers are electrons, and that such moved electrons and the remaining positive ions form the observed space charge distributions.
