The unified interaction model applied to the gamma ray induced supralinearity and sensitization of peak 5 in LiF:Mg,Ti (TLD-100)

Abstract

We describe the development of a comprehensive theory of thermoluminescence (TL) supralinearity and sensitization, the unified interaction model (UNIM), based on both radiation absorption stage and recombination stage mechanisms. The UNIM incorporates both the track interaction model (TIM) for heavy charged particles (HCPs) and the defect interaction model (DIM) for isotropically ionizing gamma rays and electrons, in a unified and self-consistent conceptual and mathematical formalism. The model is applied to explain the unique features of gamma-induced supralinearity and sensitization of peak 5 in LiF:Mg,Ti, especially the strictly linear, then supralinear behaviour and the dependence of the supralinearity on ionization density (gamma ray energy and particle type). Both features arise from a localized trapping entity (the track for HCPs, spatially correlated trapping centres and luminescent centres (TC/LC pairs) for gamma rays and electrons, which dominate the dose response at low dose and are not subject to intra-track competitive processes, thus leading to linear dose response behaviour. The decreasing efficiency of the competitive processes relative to the luminescence recombination processes, as a function of dose, leads to the supralinear behaviour. The decrease of the supralinearity with decreasing gamma ray energy (increasing ionization density) arises from the increasing probability of the TC/LC pair to simultaneously capture an electron - hole pair, leading to geminate recombination not subject to competitive processes. The UNIM is shown to be capable of yielding excellent fits to the experimental data with many of the variable parameters of the model strongly constrained by ancillary optical absorption and sensitization measurements.