Design of a mixed material moderator in a beam-shaping assembly for proton accelerator-based boron neutron capture therapy

Boron Neutron Capture Therapy is being promoted with the development of accelerator neutron sources, and many new accelerator-based BNCT facilities are being built. In Particle Accelerator Facility project of Sun Yat-sen University, we plan to build a terminal for BNCT research based on an 8 MeV, CW 3 mA proton accelerator. In this paper, we present a beam-shaping assembly for this proton accelerator with such low 24 kW beam power, using composite moderator materials composed of five elements: Mg, Al, F, O, and Li. The calculation result of FLUKA with ENDF/B and JENDL libraries shows that the epithermal neutron beam flux is $1.57\times {10}^{9}n/{\text{cm}}^2/s$ with the CW 3 mA proton beam. The fast neutron component and the gamma ray component under free-air condition are $1.49\times {10}^{-13}\text{Gy}\bullet {\text{cm}}^2$ and $8.12\times {10}^{-14}\text{Gy}\bullet {\text{cm}}^2$ respectively, in line with IAEA-TECDOC-1223 design recommendations. The thermal analysis shows that the maximum temperature of beryllium target is 706.5 K, and the structure materials of BSA do not melt.