Conceptional design of an adjustable moderator for BNCT based on a neutron source of 2.8 MeV proton bombarding with Li target

Beam shaping assembly (BSA) is a vital component in Boron Neutron Capture Therapy (BNCT) for obtaining epithermal neutron beams. Several feasible designs of BSA for accelerator-based BNCT (AB-BNCT) neutron source are carried out based on neutrons by bombarding a natural lithium target with 10 mA, 2.8 MeV proton beams. The calculation results demonstrate that a thickness of 45 cm is appropriate for general moderators referring to the therapeutic parameter of Advanced Depth (AD). A series of optimizations are performed and two results are confirmed: One is that employing the configuration of MgF2 and FLUENTAL combined by 1:1 could improve the therapeutic rate (TR) of tumors at a depth of middle region, and the other one is that the TR of superficial tumors can be increased by adding a 5 cm thick boron-11 secondary moderator at the end of general moderators. As a result, an innovative conception of an adjustable moderator is recommended to BNCT. Compared to the MgF2 moderator with a fixed thickness of 45 cm, the TR value can be improved by a maximum of 47.7 % by using the adjustable moderator. Furthermore, the configuration of adjustable moderator has been designed with regulation method for treating tumors of different depths.