Reproducibility, dose rate dependence and fading of CaSO4:Dy and LiF:Mg,Ti TL responses to medical electron beams

This work aims to evaluate the reproducibility, the dose rate dependence and the fading of CaSO4:Dy (developed and produced by the Laboratory of Dosimetric Materials of the Radiation Metrology Centre) and LiF:Mg,Ti (TLD-100, Harshaw) TL responses to electron beams of nominal energies of 4, 6, 9, 12 and 16 MeV generated by a Clinac 2100-C linear accelerator (Varian). The dosimeters, previously divided in groups according to their individual sensitivities to 60Co gamma-radiation in air and electronic equilibrium conditions, were irradiated at the depth of dose maximum of each beam in a polymetyl methacrylate (PMMA), a RMI-457 (solid water) or a water phantom in the remaining conditions suggested by the TRS-398 Code of Practice. The dose of 0.50 Gy was delivered at the dose rates of 6.67 Gy.s-1 (400 MU.min-1) and, to study the dose rate dependence, 5.33 Gy.s-1 (320 MU.min-1). A TL reader model QS 3500 (Harshaw) was used to read the TL signal from 36 to 44 h after each irradiation to evaluate the reproducibility and the dose rate dependence and hourly between 24 and 55 h to assess the fading of the TL response. Regardless the phantom, the reproducibility of the LiF:Mg,Ti TL response is 2.4% for all studied electron beams while CaSO4:Dy reproducibility is 2.4% for the 16 MeV electron beam and 2.5% for the other studied energies. Despite the variations in the TL signal with the phantom and the nominal energy, the dose rate has no significant influence in the read TL signal. A first order exponential decay fitted to the experimental data allowed to estimate the decays of CaSO4:Dy and LiF:Mg,Ti in 5.00 ± 0.16% and 9.9 ± 0.3% per year, respectively, according to the expectations based on literature data for the dosimetry of the 60Co gamma-radiation applying CaSO4:Dy and LiF:Mg,Ti dosimeters. (author)