The work aims to analyze the radiation field characteristics and optimize the beam shaping assembly (BSA) and peripheral shielding structure for a medical radiological device driven by a 2.7 MeV/30 mA proton beam based on the 7Li(p,n)7Be reaction, so as to reduce stray radiation dose and enhance radiological safety performance. Monte Carlo simulations were conducted using the MCNPX code. A stepwise optimization was performed on the moderator, reflector, thermal neutron filter, and collimator, while the neutron and gamma shielding effectiveness of the gamma shield and an outer borated polyethylene layer (10 wt% B4C, 10 cm thick) was evaluated. Radiation field parameters, including neutron flux, energy spectra, and dose equivalent rates at the BSA exit and peripheral points of interest, were calculated. Three moderator configurations of single-layer MgF2, single-layer AlF3, and a double-layer structure (MgF2 in front, AlF3 at rear) were compared in terms of their effects on radiation field quality and stray radiation. The results showed that, for the optimized double-layer moderator, the epithermal neutron flux Φepi at the BSA exit reached 1.84×109 cm-2·s-1, the thermal-to-epithermal ratio Φth/Φepi was reduced to 0.025, the fast-neutron dose ratio Df/Φepi was 1.80×10-13 Gy·cm2, the gamma dose ratio Dγ/Φepi was 1.52×10-13 Gy·cm2, and the beam directionality J/Φ was 0.71, all superior to the IAEA recommended limits. Compared with the single-layer AlF3 scheme (37 cm), the total moderator thickness was reduced by 5 cm. The peripheral borated polyethylene shield reduced the neutron dose equivalent rate to 2.3 μSv/h and the gamma dose equivalent rate to 0.8 μSv/hat a lateral distance of 30 cm from the facility, meeting the national radiological protection standard GBZ 130-2020 (≤2.5 μSv/h). The proposed double-layer composite moderator not only improves the therapeutic beam quality but also effectively controls stray radiation from the facility, providing a theoretical basis for radiation field optimization and shielding design of medical accelerator-based neutron sources.
Key words
boron neutron capture therapy (BNCT) /
medical radiological device /
radiation field characteristic /
beam shaping assembly (BSA) /
shielding optimization /
Monte Carlo simulation (MCNPX)
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