The luminescent centres have since been identified as impurities by Boustcad and Charlesby (1967)\cite{Boustead_1967}, and the traps (Boustead PhD Thesis 1969) assigned to the unsaturated groups on the main chain of the polymer and to ‘cavities’ (suitable molecular configurations) presumed to be present in the amorphous parts. Also the boundary regions between the amorphous and crystalline phases of the material were considered to be likely sites for ‘cavity’ trapping.
Keyser (PhD Thesis 1970) has investigated trapped electrons using ESR on samples containing varying amounts of unsaturation. He concludes that unsaturation is not involved in electron trapping. Thus the nature of the electron trap still presents a problem.
The mechanism proposed for the TL is that after ionization some electrons are trapped at various sites in the polymer, leaving behind positive ions. On subsequent warming the electrons are expelled from the traps and recombine with the positive charges. The energy released in this process appears as a luminescence which is characteristic of impurity molecules present in the polyethylene. The work described below was undertaken in order to get direct evidence of the charge trapping and charge movement implied by this model. By simultaneous examination of the TL and TSC after irradiation, correlations in the behaviour of the two phenomena would provide support for a mechanism involving the untrapping and movement of electrons.