Research on neutron beam design for BNCT at the Musashi reactor

Remodeling of the neutron irradiation facility for the Musashi reactor was carried out for BNCT in 1975. After that, ninety-four patients of brain tumor and four patients of melanoma were treated using the reactor until the end of March 1989. Although treatment has been successfully performed since 1976, it has become necessary to improve the depth-dose distribution and reduce the irradiation time. The present report first addresses the basic experiments made, focusing on the enhancement of thermal neutrons by means of single-crystal silicon and measurement of total epithermal neutron flux using gold foils. Some aspects of the design calculations are described including the principles and procedure for different calculations such as source calculation and structure optimization calculation. In particular, the structure optimization calculation is describe in detail focusing on source intensity normalization with experiments, trial for enhancement of thermal and epithermal neutron flux, trial for suppression of fast neutrons, another trial for suppression of fast neutrons, and gamma-ray build-up with phantom. Investigations have shown that the thermal and epithermal neutron flux can be extracted by using single-crystal silicon or aluminum, but the fast neutron flux is also enhanced. (N.K.)