Development of LiF-based ceramics for shielding neutron fluxes in boron neutron capture therapy and their performance...

In boron neutron capture therapy (BNCT) facilities, neutron beam-shielding materials are crucial for preventing radiation exposure and adverse events caused by beams leaking around the neutron source and gaps around the irradiation port. Owing to their stability in severe thermal, radiative, and mechanical environments, lithium-fluoride-based ceramics have been developed to shield neutron fluxes in BNCT facilities. Bulk ceramics as large as 15 cm × 15 cm × 5 cm have been successfully developed in ternary LiF-MgF₂-CaF₂ systems. Boron- or gadolinium-added quaternary LiF-MgF₂-CaF₂ systems have also been developed. These ceramics demonstrate high bulk densities of over 95 %. The particle and heavy ion transport code system (PHITS) simulation indicated that some of these ceramic tiles provided better shielding performance than commercially available 50 wt%LiF-polyethylene composite materials. We propose a variable-shaped jig that utilises the developed LiF-based ceramic beads with polyethylene beads to shield the thermal and fast neutron beams, which might leak between the irradiation port and the patient. The shielding performance of some jig models was evaluated by PHITS simulations using the initial free beam profile of the linear-accelerator-type iBNCT001 at Tsukuba. When using both the 100 wt% LiF ceramic and the 70 wt% LiF-based ternary ceramic beads containing small amounts of boron, the expected shielding performance is approximately 1/20 for thermal neutrons and approximately 1/10 for fast neutrons against the initial free beam, while the γ-ray dose rate increases by about two times. Ceramic tiles and jigs are expected to be applicable to various places in BNCT facilities.