SPACE-CHARGE FORMATION IN GAMMA-IRRADIATED LOW-DENSITY POLYETHYLENE

Energetic ionizing radiation can alter the chemical structure of polym eric materials and also may give rise to the presence of trapped charg e within the material, the trapping characteristics of which may be in fluenced by these radiation-induced structural alterations. In the pre sent 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 de pendent 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 appli ed stress serves to extend the positive charge towards the anode. At h igh doses (greater than or equal to 50 kGy), the charge distribution i s more complicated than that for low doses. Here there is initially ne gative charge present adjacent to the cathode and a substantial positi ve charge adjacent to the anode. With the passage of time, however, th ere 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 ne gative charge in the middle of the sample.