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TLproperties of many materials and their interaction with various types of radiation fields.+ The thermoluminescent efficiency, ak, is the ratio of the mean energy emitted as TL light, E+, to the mean energy, E, imparted to the TL material by the radiation field, a k = E+/B. d is sometimes referred to as the 'integral' dose in the irradiated volume and is defined as the expectation value of the imparted energy. $ The relative TL response, q k , , of a TL material of mass m, and volume V, is the ratio of the TL efficiences for the two radiation fields, k, I at 'low dose' Do, i.e. where .E+ is increasing linearly with dose:ak(Do)/ar(Do). 9: A tissue-equivalent material is one with atomic composition which closely approximates to the atomic composition of tissue. TL materials such as LiF, Li2B407, Be0 are nonetheless referred to as approximately 'tissue-equivalent' materials in gamma radiation fields because their effective atomic number and mass energy absorption coefficients for photoelectric absorption approximate to those of tissue (e.g. Jayachandran 1971).Y S Horo w itz irradiated material. Johnson also describes the use of an integrating period in the readout cycle which approximately cancels the growth of peaks 4-5 due to storage by including a certain fraction of peaks 2-3 which exhibit stronger fading. Ehrlich (1974) using fast preheating to 160°C did not observe 'sensitivity-transfer' in TLD-700; however, fading increased to 15% after 4 days. Indeed the fading of 'unannealed' LiF-TLDS is strongly dependent on the preheat cycle (Webb and Phykitt 1971, De Planque et a1 1980). Mason et a1 (1976) studied the effect of Ti in TLD-700 ( T1 = 300 "C, tl = 30 min) on the relative trap distributions associated with peaks 2 and 5 and concluded that the results of Booth etal (1972) were inconsistent with their data.'7. '7.' 7.

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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.

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