The Unified Interaction Model Applied to the Gamma Induced Supralinearity and Sensitisation of Peaks 4 and 5 in LiF:Mg,Ti (TLD-100)

Abstract

A comprehensive theory of thermoluminescence (TL) supralinearity and sensitisation, the Unified Interaction Model (UNIM) is described, based on both radiation absorption stage and recombination stage mechanisms. The UNIM can be applied to both heavy charged particles and isotropically ionising 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 sensitisation of peaks 4 and 5 in LiF:Mg,Ti, especially the strictly linear, then supralinear behaviour and the dependence of the supralinearity on ionisation density (gamma ray energy and particle type). A model is proposed for peak 4, based on its mainly 'hole-trapping' characteristics, which explains its diametrically opposed behaviour in sensitised material, i.e. a decreased sensitivity and supralinear dose response. All features arise from a localised 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 decrease of the supralinearity with decreasing gamma ray energy (increasing ionisation density) arises from the increasing probability of the TC/LC pair to capture simultaneously 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 measurements.