Development Studies Regarding the Construction of Epithermal-Enriched Neutron Field for Medical Purposes at the University of Tokyo YAYOI Fast Reactor

For boron neutron capture therapy, irradiation by epithermal neutrons has an advantage in that the neutrons show deeper penetrations in tissue than do thermal neutrons. An epithermal-enriched neutron field for medical irradiation purposes was constructed at the fast-neutron source reactor YAYOI. Depth-flux curves for neutrons and gamma-ray doses in a head phantom were measured with combinations of layers of iron, graphite, polyethylene, lead, and bismuth surrounding the core. Combinations and the thickness of the layers were optimized according to the following limitations:

1. sufficient neutron flux for therapeutical purpose with good penetration in tissue, 3 × 10¹² thermal-neutron fluence at a 2.4-cm depth in the phantom
2. tolerable contamination level of fast neutrons and gamma rays, 2000 rem.

Collimation of the neutron beam and shielding of the irradiation room were attained with ⁶Li-enriched flexible sheets and a laminated shield plate. The characteristics of the optimized field are as follows: =_thh = 3.2 × 10⁸ n/cm²·s at a 2.4-cm depth in the phantom, contaminating fast-neutron and gamma-ray local doses are 990 and 357 rem during the required irradiation time, 2.6 h.