Influence of radiation environments on space charge formation in γ-irradiated LDPE

In the present paper the formation of space charge in irradiated low-density polyethylene (LDPE) was investigated using the pulsed electroacoustic (PEA) technique. Specimens of LDPE, similar to 100 mu mm thick, were irradiated to various doses at room temperature in a Co-60 gamma source in one of three different environments: ambient room air, oxygen-free nitrogen and vacuum. The results obtained indicate that the space charge distribution is dependent on the irradiation environment. There is a large concentration of positive charge evident in the sample irradiated in air compared to very little for material irradiated in either vacuum or nitrogen under similar conditions. This indicates that the presence of oxygen during irradiation, and thereby oxidation itself, has a major influence on the mechanism by which space charge evolves. Thus the monitoring of space charge may in itself be an indicator of the degree of oxidation and of aging in a material. The evolution of space charge and its decay are discussed.