Optimization of target materials for a 3 MeV proton-driven accelerator neutron source for boron neutron capture...

Choosing the right target material for a 3 MeV (10 mA, 30 kW beam power) proton-driven accelerator-based boron neutron capture therapy (AB-BNCT) system is far from straightforward a material that produces the most neutrons under proton bombardment does not necessarily deliver the best therapeutic beam. This paper addresses that gap through a systematic two-phase PHITS (version 3.28) study of four candidate targets: lithium (Li), beryllium (Be), beryllium oxide (BeO), and iron (Fe), simulated over target thicknesses of 0.01-1.4 cm. In Phase 1 (proton-driven), lithium produced the highest raw thermal neutron flux - 7.69 × 10¹⁰ n/cm²·s at 1.4 cm - but beryllium's fast neutron dose per unit thermal flux was roughly 50% lower, indicating a considerably cleaner radiation field. Phase 2 (neutron-driven transport) told a very different story: beryllium developed the highest internal thermal neutron flux, 5.17 × 10¹⁴ n/cm²·s, driven by its neutron multiplication and moderation properties. Clinical dosimetry using three point detectors (F75, F85, F95) at z = 150, 155, and 160 cm along the BSA beam axis yielded, for the neutron-driven beryllium configuration: thermal flux of 6.03 × 10⁹, 5.65 × 10⁹, and 5.30 × 10⁹ n/cm²·s (Phase 2) and 5.34 × 10⁹, 5.00 × 10⁹, and 4.69 × 10⁹ n/cm²·s (Phase 1); average dose delivery rates (ADDR) of 7.29, 6.82, and 6.40 RBE-cGy/min (Phase 2) and 6.45, 6.04, and 5.67 RBE-cGy/min (Phase 1); and treatment times of 4.12, 4.40, and 4.69 min (Phase 2) and 4.65, 4.97, and 5.29 min (Phase 1) for a 30 Gy-eq prescription - a 6.6-fold improvement over the best published compact AB-BNCT design at comparable proton energy. Taken together, these results make a compelling case for beryllium as the optimal target material for compact 3 MeV AB-BNCT systems.