Filter/moderator materials for neutron beam optimization for medical irradiation field

An investigation on structural materials to produce thermal and epithermal neutron beam was made using nuclear reactor, TRIGA -II. Structural composition and materials for optimization of those beams for each field of thermal column and thermalizing column were determined according to Monte Carlo Code, MCNP. With regard to neutron filter, graphite was appropriate as a moderator and the ratio; the dose of fast neutron/thermal neutron flux could be reduced by increasing the density of graphite. A combination of Al and AlF3 was found most suitable for epithermal neutron beam filter, which allows an increase in neutron flux and a decrease in the ratio, fast neutron dose/epithermal neutron fluence. Graphite and lead fluoride (PbF2) were most appropriate as the beam reflector for epithermal neutron beam and epithermal one, respectively. When bismuth (Bi) and PbF2 were used as γ-ray shielding material for thermal beam and epithermal one, respectively, neutron beams of good quality were obtained. In addition, graphite and PbF2 were suitable as beam collimator materials for thermal beam and epithermal one, respectively. These results demonstrate that the optimum neutron beams for neutron capture therapy are obtainable through positive utilization of PbF2 as the structural materials for epithermal neutron beam. Thus, boron neutron capture therapy (BNCT) would be spread in Japan when the nuclear reactor, TRIGA is modified so as to use in medical field. (M.N.)