A feasibility study on the hyper-thermal neutron irradiation field for neutron capture therapy

Neutron capture therapy was restarted at the heavy water thermal neutron irradiation facility on February 1990. Control of the energy spectrum of incident neutrons is examined as a method to improve thermal neutron flux distribution in the depth of a body. Hyper-thermal neutron is defined as neutron which can be approximated with Maxwell distribution of higher temperature than 300K. A region of 0.1 to 3 eV is regarded to be the hyper-thermal region. Thermal neutron flux distribution in vivo with the hyper-thermal neutron irradiation is calculated by simulation with MCNP-V3 code. Scattering material to obtain hyper-thermal neutron is examined. The results of the study show that the reaction rate is improved by the use of hyper-thermal neutron to enable treatment of the tumor 2 cm deeper than the previous thermal neutron field, ZrH1.7 at 1200 K is appropriate as a scattering material and the use of Bi layer can eliminate mixing gamma ray without deterioration of the irradiation field. (T.H.)