Space charge formation in γ-irradiated low density polyethylene

Energetic ionizing radiation can alter the chemical structure of polymeric materials and also may give rise to the presence of trapped charge within the material, the trapping characteristics of which may be influenced by these radiation-induced structural alterations. In the present work, the formation of space charge in gamma-irradiated LDPE (low-density polyethylene) was investigated using the technique of a LIPP (laser induced pressure pulse). Specimens of LDPE, similar to 350 mu m thick, were irradiated in a Co-60 gamma-source in room air to various doses and the results indicate that space charge distributions are dependent on both dose and electric stress, including the length of time during which the stress is applied. At low doses (less than or equal to 10 kGy), there is a large amount of positive charge adjacent to the cathode, leading to stress enhancement at the interface. Higher applied stress serves to extend the positive charge towards the anode. At high doses (greater than or equal to 50 kGy), the charge distribution is more complicated than that for low doses. Here there is initially negative charge present adjacent to the cathode and a substantial positive charge adjacent to the anode. With the passage of time, however, there is a polarity change from negative to positive at the cathode and a decrease in the density of the positive charge at the anode, with negative charge in the middle of the sample.