Thermoluminescence and Phosphorescence in Polyethylene Under Ultra-Violet Irradiation

The thermoluminescence of polyethylene induced at low temperature by ultra-violet radiation has been studied and compared with that induced by γ-radiation. Although considerable similarity exists between these two, the total thermoluminescence output under u. v. irradiation is found to be dose rate dependent, whereas under γ-irradiation it is independent of dose rate. Study of the low temperature u. v. induced phosphorescence of polyethylene, together with data from the preceding paper, enables a model to be suggested to explain this basic difference. This involves the double excitation and consequent ionization of carbonyl molecules attached to the molecular chains when exposed to u. v. radiation, while γ-radiation is able to cause direct ionization. Measurement of the phosphorescence intensity variation with temperature for a typical low-density polyethylene showed very clearly the two main polyethylene structural transitions noticed in the previous thermoluminescence studies.