Neutron and photon detector responses outside the lateral shield of a 3 MeV ⁷Li(p,n) accelerator-based BNCT neutron source: a PHITS and Geant4 study

摘要: Monitors around an accelerator-based boron neutron capture therapy (AB-BNCT) source measure a mixed neutron-photon field that differs from their calibration fields. This work characterises the field outside the lateral shield of a 3.0 MeV, 10 mA ⁷Li(p,n) source and the detector readings it produces. An energy-angle correlated source was generated with PHITS 3.35 from 10⁸ protons in thick natural lithium with evaluated proton data. It gave 2.37 × 10⁻⁴ neutrons per proton (1.48 × 10¹³ n s⁻¹ at 10 mA), with a mean energy of 0.51 MeV and 40% of neutrons emitted between 60° and 120°. This source was transported through a 22 cm wall of borated polyethylene, lead and concrete. Ambient dose equivalent H*(10) was scored at three ring-shaped monitoring positions, and NaI(Tl) and moderated ³He-counter responses were computed with Geant4 and PHITS. The wall reduced the neutron H*(10) 672-fold and the photon H*(10) 490fold. Outside it, neutrons dominated the dose (5.0 times the photon dose), and streaming around the beam-port end raised the neutron dose 2.3-fold. A forward-only source underestimated the lateral neutron dose 1.9-2.3-fold. Analytical checks confirmed the simulations, and the two codes agreed within 0.6% on the NaI(Tl) response. Moderated ³He counters would record about 2 × 10⁵ counts s⁻¹. The calibration factor of small spheres changed up to 1.8-fold between the shielded and streaming-affected fields, whereas that of a 10-inch sphere was unchanged within its 7% uncertainty. Lateral-field assessments of ⁷Li(p,n) sources need a target-level source, explicit streaming treatment and large or field-calibrated monitors.