Dependence of the TLD-300 glow curve on the photon field direction of incidence. Studies in air and in phantom for mammography and CT x-rays

Abstract

The glow curve of the CaF2:Tm thermoluminescent dosimeter (TLD-300) has the remarkable property of showing sensitivity to the energy of the radiation field. Careful analyses of the TLD-300 glow curve enable to determine the effective energy of the incident low- and intermediate-energy X ray photons, with a typical accuracy of 1 keV. The effect is consequence of different efficiencies for the high- and low-temperature peaks as a function of the secondary spectrum ionization density. Since the TLD-300 dosimeters have parallelepiped shape (3.2 × 3.2 × 0.89 mm3) and the spectra incident on its different sides do suffer a distinctive spectral hardening as they traverse the dosimeter, we have investigated the dependence of the emitted light and the glow curve shape on the beam direction of incidence with respect to the dosimeter geometrical shape. We have exposed dosimeters to photon spectra with effective energies between 15 and 60 keV (mammography and computed tomography, CT, beams) and studied the integrated TL response and the glow curve, for irradiations in air (essentially uni-directional) and inside phantoms (with an important multi-directional component from scattered radiation). Two limit cases were investigated: beam incidence on the main face (3.2 × 3.2 mm2) and incidence on a side face (3.2 × 0.89 mm2). A rigorous glow curve deconvolution process was applied to obtain 7 peaks, following our laboratory protocol. The observations with CT beams were interpreted by Monte Carlo simulations. Results showed that for irradiation in air, the ratio of integrated thermoluminescent response after side- and main-face exposures was about 0.3 at mammography energies (15 - 17 keV effective energy) and about 0.8 at CT energies (45 - 58 keV effective energy). These ratios became 0.6 - 0.7 and 0.9 - 1.0, respectively, inside plastic phantoms appropriate for each clinical modality. The glow curve analysis showed that, both for mammography and CT energies inside a phantom, there is no statistically significant difference between the effective energies obtained after main- and side-face exposures. We have concluded that exposure of TLD-300 inside phantoms can determine the radiation field effective energy for beams which are similar to those in this study, independent of the dosimeter orientation with respect to the primary beam.