Characterization indexes of moderator assembly for accelerator-based BNCT using 7Li(p,n) neutrons at proton energy of 2.5 MeV

This study demonstrates the usage of the indexes to characterize and optimize the moderator assembly for the 7Li(p,n) neutrons, especially in terms of its diameters. The indexes checked are treatable protocol depth (TPD) which is the greatest depth of the region satisfying the dose requirements in BNCT protocol, proton current necessary to complete BNCT by 1 hour irradiation, and the heat flux deposited in the Li target which should be removed. Assumed materials are D2O for moderator, and mixture of polyethylene and LiF with 50 wt% for collimator. Doses have been computed with MCNP codes. Consequently, TPD does not exhibit an apparent dependence on moderator diameter. Necessary proton current and heat flux decrease as the moderator diameter increases and this tendency saturates at around 60 cm of the moderator in this study. As to the collimator, increase in inner diameter is suitable for increasing TPD and decreasing necessary proton current and heat flux, while these indexes do not show apparent difference for collimator inner diameters over 14 cm for the parameters treated here. The practical viewpoint is to increase TPD, within the technically possible condition of accelerated proton current and heat removal from the Li target. (author)