Measurement and analysis of supralinearity in LiF TLD-100 irradiated by 1.5 keV x-rays.

Measurements of the response of lithium fluoride TLD-100 to 1.5 keV ultrasoft x-rays, 240 keV x-rays and 137Cs gamma-rays at doses below 70 Gy are presented and the implications for the track interaction model are discussed. This model proposes that thermoluminescence is due to the combination electrons and holes formed along secondary charged particle tracks. As the dose is increased, the average distance between particle tracks is reduced and there is a greater probability of combination between neighbouring tracks; this mechanism is used to explain the supralinear response of lithium fluoride to certain radiations. Measurements using ultrasoft x-rays are interesting in this context because the energy from each photon is deposited in a small volume, therefore the probability of combination between neighbouring events should only be significant at high doses. The results, however, show that lithium fluoride exhibits a significant supralinear response to 1.5 keV x-rays after only a few tens of grays and suggests that the track interaction model alone cannot explain the phenomenon of supralinearity.