The effect of oxygen on the thermoluminescence of irradiated polyethylene

The main thermoluminescence glow curve, the a, B, y glow curve, of polyethylene irradiated by y-rays at 77 °K has been found to be greatly affected by molecular oxygen absorbed in the material before irradiation. The oxygen removes the B and y peaks almost entirely at low irradiation doses and adds another peak, the e peak, at a lower temperature than these to form an a, e glow curve. Kinetic studies showed that the e peak has a higher probability constant at low temperatures than any of the a, B, y peaks but that it shares the same common activation energy. Since the e luminescence cannot be observed at 77 °K it seems that some change, perhaps a structural change, must occur during warming from 77 °K before it can begin to appear. The oxygen effect can be removed by pumping or by a pre-irradiation followed by preheating. The latter method is shown to remove the oxygen by conversion into peroxide radicals, produced when the oxygen combines with alkyl radicals formed during the preirradiation. The e peak was found to occur only in the presence of oxygen molecules, but not with oxygen in the form of peroxides. Peroxide radical formation was observed by means of the electron spin resonance (e.s.r.) method. E.s.r. measurements also showed that thermoluminescence in polymers was not a direct product of free radical reactions. Various possible thermoluminescence mechanisms are discussed and it is shown that the polyethylene thermoluminescence phenomena can be explained on the basis of a model involving the trapping of electrons followed by their recombination with different types of luminescence centres.