Produção e caracterização de filmes OSL/RL baseados na fluorita natural brasileira para dosimetria em campos de radiação com taxas de dose alta e ultra alta

Abstract

The technological advancement of ionizing radiation applications in different fields has led to the need to establish procedures and develop new technologies for ionizing radiation dosimetry. For instance, the MOGNO beamline at the Brazilian Synchrotron Light Laboratory (LNLS) uses a quasi-monochromatic X-ray beams (22, 39, and 67 keV) for nano- and microtomography with synchrotron light in high-precision scientific investigations, yet it faces dosimetric challenges due to high dose rates (HDR – High Dose Rate) and small irradiation fields. Concurrently, ultra-high dose rate radiotherapy (UHDR) promises to revolutionize oncological treatments through the FLASH effect, which preserves healthy tissues while eradicating tumors, but specialized detectors capable of operating under ultrashort pulse parameters are still required. To address these challenges, flexible calcium fluoride (CaF₂) films were developed and characterized, adaptable for both optically stimulated luminescence (OSL) and radioluminescence (RL) techniques, functioning in passive and active (real-time) dosimetry systems. The production methodology involved rigorous thermal treatment of the sensitive material based on natural Brazilian fluorite (650°C for 30 minutes) to eliminate spurious signals and enhance sensitivity, followed by precise granulometric control through sieving (38–53 μm, 75–90 μm, and <150 μm), resulting in the production of two specialized batches. Batch 1, with dimensions of 14×14 cm², was optimized for OSL measurements, while Batch 2, in a 6×6 cm² format with CaF₂ concentrations of 27% and 54%, was intended for RL measurements. After characterizing the dosimetric response and properties with both techniques, the applicability of the films in HDR and UHDR was validated. In the microtomography beam at the MOGNO line (39 keV), under a photon flux on the order of 10¹⁰ photons/s, the both two Batch 1 films measured dose rates of (75 ± 10) mGy/s (T film) and (69 ± 8) mGy/s (S film), which are compatible with the reference value evaluated using commercial BeO OSL dosimeters (69 ± 4 mGy/s). Moreover, the possibility of mapping one-dimensional dose distributions during complete 360° rotational microtomography protocols was demonstrated. In the context of UHDR delivery, using the ElectronFlash accelerator with 7–9 MeV beams and pulses of 0.5–4 μs, Batch 2 films exhibited linear response over a wide dose range (0.02–235 Gy), with energy dependence of the RL signal in the transition between 7 and 9 MeV, and spatial agreement of 86–98% when compared to Gafchromic films for one- and two-dimensional dose distribution mapping. Formulations with higher CaF₂ concentration (54%) and grain sizes above 75 μm demonstrated optimized luminescent performance. In conclusion, these films represent a versatile and reliable platform for precision dosimetry in both UHDR and HDR conditions, validating applications at relatively low energies, such as synchrotron microtomography, as well as in FLASH radiotherapy (above 1 MeV), with high resolution in small irradiation fields.