Effects of proton energy on optimal moderator system and neutron-induced radioactivity of compact accelerator-driven 9Be(p,n) neutron sources for BNCT

Abstract

Accelerator-based BNCT systems using 9Be(p, n) neutron sources have been constructed and under construction. This reaction become preferable over about 10 MeV since the neutron production becomes more efficient, but the radioactive products increase with the proton energy. In case of construction near a hospital we should consider not only the neutron production rate but also the radioactivity. Therefore, it is necessary to obtain information on the required beam power of an accelerator and the activity as a function of the proton energy to decide the optimal proton energy. We performed simulation calculations and first decided optimal thickness of MgF2 moderator and Fe filter to fulfill the neutronic conditions of BNCT. Then, we deduced accelerator power and found that the required accelerator power is approximately 30 kW around 10 MeV proton energy and decreases gradually with the proton energy. On the other hand, at energies less than 11 MeV, the photon dose rate around the target became lower for level for working around the target after seven days cooling time. Furthermore, the dose rate at the opening of a collimator reduced drastically after one hour cooling time at energy less than 10 MeV and got to a dose level at which 3 hours work/day can be done within the limits of regulation. We have therefore concluded that the optimal proton energy for BNCT is around 10 MeV from the results obtained here.