The effect of cooling rate on the precision of measurement of glow peak 5 in the thermoluminescence of LiF:Mg,Ti (TLD-100).

The effect of natural rapid cooling and oven slow cooling on the precision of thermoluminescence measurements of LiF:Mg,Ti is investigated. Three separate series of measurements resulted in average precisions of 5.1 and 5.0%, respectively. However, the highest precision of 1.7% (1 SD) was achieved for an oven-cooled material.

THE THERMOLUMINESCENCE (TL) DOSE RESPONSE OF COMPOSITE PEAK 5 IN LIF:MG,TI (TLD-100): DEPENDENCE ON THE ORDER OF KINETICS.

A seeming contradiction in the prediction of the spatially correlated trapping center/luminescent center model applied to LiF:Mg,Ti has been the linear/supralinear behavior of the dose response of glow peak 5a. In the TC/LC model, the localised electron-hole recombination, giving rise to glow peak 5a, is expected to result in an extended region of linear dose response. Deconvolution of the glow curves based on first order kinetic peak shapes results, however, in a dose response of peak 5a, which closely resembles the linear/supralinear dose response of peak 5. It is demonstrated herein that when general-order kinetics peak shapes are used for peak 5a, the analysis can result in a linear dose response of glow peak 5a up to dose levels as high as 30 Gy, well beyond the 1-Gy onset of supralinearity of peak 5. The extended linearity suggests a resolution of the contradiction.

SEARCH FOR EXPERIMENTAL EVIDENCE OF DOSE-RATE AND WALL SCATTERING EFFECTS IN THE THERMOLUMINESCENCE RESPONSE OF LIF:MG,TI (TLD-100).

An experimental investigation into the possibility of dose-rate effects and wall scatter in the thermoluminescent response of LiF:Mg,Ti (TLD-100) was carried out. The investigation was motivated by theoretical simulations predicting the possible presence of dose-rate effects coupled with the lack of detailed experimental studies. The dose rate was varied by changing the source to sample distance, by the use of attenuators, sources of 137Cs of various activities, filtration and the construction of identical geometrical irradiators of Teflon and stainless steel. Four levels of dose in the linear dose response region were studied at 10-2 Gy, 1.5 × 10-2 Gy, 0.1 Gy and 0.5 Gy to avoid complications in interpretation due to supralinearity above 1 Gy. At the dose of 1.5 × 10-2 Gy, the dose rate was varied by five orders of magnitude from 4.9 × 10-3 Gy s-1 to 4.9 × 10-8 Gy s-1. At the other levels of dose, a one to two orders of magnitude in dose rate was achieved. Within the measurement uncertainty of 5-10%, no dose-rate effects were observed in any of the experimental measurements and no changes in the shape of the glow curve were observed. The maximum wall scatter effect (Teflon to stainless steel) was measured at ~8% within the experimental uncertainty and well below expectations. The results are encouraging with respect to the accurate and reproducible use of LiF:Mg,Ti under various experimental conditions of irradiation.

INVESTIGATION OF THE TL CHARACTERISTICS OF COMPOSITE PEAK 5 IN THE GLOW CURVE OF LIF:MG,TI (TLD-100) USING NATURALLY AND FURNACE-COOLED SAMPLES FOLLOWING THE 400°C PRE-IRRADIATION ANNEAL.

The cooling rate to room temperature following the 400°C pre-irradiation anneal is known to affect the thermoluminescent properties of LiF:Mg,Ti (TLD-100) as a result of migration and clustering of defects during the cooling down process. In this investigation the dose response over an extended dose range from 0.01 to 7000 Gy in both naturally cooled and the much slower furnace-cooled samples has been measured. Glow curve deconvolution based on first-order kinetics is employed to extract the dose response of the various glow peaks. Of especial interest is the behaviour of glow peaks 4, 5a and 5 as a function of dose. The idea is to modify the supralinear dose response of peak 5 from 1 to 30 Gy to a linear behaviour. This dose range is important for clinical therapy and a linear dose response is of substantial advantage leading to both improved accuracy and precision.

THE UNIFIED INTERACTION MODEL: SIMULATIONS OF TL DOSE RESPONSE AND EXPERIMENTAL VERIFICATION OF INTENDED DOSE RESPONSE LINEARITY BY POSTIRRADIATION PHOTON EXCITATION.

The Unified Interaction Model (UNIM) simulates the linear/supralinear dose response of the glow peaks of LiF:Mg,Ti (TLD-100) and other thermoluminescent materials and the dependence of the supralinearity on photon/electron energy. The UNIM is based on the radiation action of spatially correlated trapping centres (TCs) and luminescent centres (LCs), which results in localised (geminate) electron/hole recombination by quantum mechanical tunnelling. The linear dose response is mainly attributed to geminate recombination. UNIM simulations of the dose response of glow peak 5 in LiF:Mg,Ti following 500 and 8 keV electron/photon irradiation are discussed. In addition, simulations of postirradiation photo-excitation that redistribute the electrons and holes in the various TCs and LCs are demonstrated to extend the region of linear dose response and reduce the supralinearity. Experimental verification of dose-response linearity for levels of dose ≤30 Gy following both 3.65 and 5 eV photon excitation is presented.