Reproducibility, Dose Rate Dependence and Fading of CaSO_4:Dy and LiF:Mg,Ti TL Responses to Medical Electron Beams

摘要

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) andLiF:Mg,Ti (TLD-100, Harshaw) TL responses to electron beams of nominal energies of 4, 6, 9, 12 and 16 MeVgenerated by a Clinac 2100-C linear accelerator (Varian). The dosimeters, previously divided in groups accordingto their individual sensitivities to ~(60)Co gamma-radiation in air and electronic equilibrium conditions, wereirradiated at the depth of dose maximum of each beam in a polymetyl methacrylate (PMMA), a RMI-457 (solidwater) or a water phantom in the remaining conditions suggested by the TRS-398 Code of Practice. The dose of0.50 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 to44 h after each irradiation to evaluate the reproducibility and the dose rate dependence and hourly between 24 and55 h to assess the fading of the TL response. Regardless the phantom, the reproducibility of the LiF:Mg,Ti TLresponse is 2.4% for all studied electron beams while CaSO4:Dy reproducibility is 2.4% for the 16 MeV electronbeam and 2.5% for the other studied energies. Despite the variations in the TL signal with the phantom and thenominal energy, the dose rate has no significant influence in the read TL signal. A first order exponential decayfitted to the experimental data allowed to estimate the decays of CaSO_4:Dy and LiF:Mg,Ti in 5.00 ± 0.16% and9.9 ± 0.3% per year, respectively, according to the expectations based on literature data for the dosimetry of the~(60)Co gamma-radiation applying CaSO4:Dy and LiF:Mg,Ti dosimeters.