Calculation and influence study of moderator thermal neutron scattering cross section of beam shaping assembly in AB-BNCT

Boron neutron capture therapy (BNCT) is an emerging binary targeted radiotherapy technique. The beam shaping assembly (BSA) is a critical component of BNCT, and the accurate evaluation of neutron beam parameters at the BSA exit is essential for treatment efficacy. Many studies have identified MgF 2 /AlF 3 as ideal moderator materials for the BSA, but the thermal scattering effects of these materials can impact the accuracy of neutron beam evaluation. This study first employed the first-principles computational software VASP and PHONOPY to calculate material parameters of MgF 2 and AlF 3 . Then, using both the phonon expansion method and the mean square displacement matrix method, the modified NJOY was used to calculate the thermal scattering cross section. Neutron transport simulations for the BSA under both step-by-step and intelligently optimized design schemes were performed using the Monte Carlo code MCNP. The results showed that after using the thermal scattering cross section of MgF 2 and AlF 3 , the thermal neutron flux decreased by 8.65% and 5.11% under the two schemes, respectively. In addition, the intelligent design yielded higher thermal and epithermal neutron fluxes at the BSA exit, proving that this BSA structure is better. This study is essential for enhancing the effectiveness of treatment planning.