Low energy accelerator-based neutron sources for neutron capture therapy

Neutron beams for neutron capture therapy (NCT) have been studied by a number of investigators, using high flux research reactors as the source of neutrons. There are very few research reactors in the world, probably less than ten, that can meet the conditions of both flux intensity and neutron energy spectrum. Hence the availability of NCT for patients with malignant brain tumors would be quite limited, even if all of these reactors would be able to accommodate the necessary beam design and hospital room atmosphere into the operational program. For a number of years it has been suggested that serious design work should be undertaken to develop accelerator-based sources for the neutrons, using compact accelerator designs. The relatively new RFQ (radiofrequency-quadrupole) linear accelerators have the promise of achieving this objective. This study investigates the optimization of the moderator and reflector system that would be used between the accelerator and the patient. The study is similar to that of Blue et al at Ohio State University, and Dolan at INEL, but goes further in attempting to optimize the useful neutron flux for boron NCT with respect to the unwanted dose from neutrons and gamma rays. This study also investigates both the Li-7 (p,n) and H-3 (p,n) proton accelerator target reactions. Though these two targets are studied separately, and each would involve an entirely different target design, a combination target of tritiated lithium hydride deserves to receive attention in future designs. The yields from the targets were calculated from the differential cross sections and stopping powers in the literature, assuming thick targets for the incident protons. The yields were divided into five angular groups, which were used as input to the MCNP Monte Carlo computer code. Neutron fluxes, neutron doses, and gamma doses were calculated at the patient position (skull) and at a tumor at various depths