Reevaluation of neutron energy spectrum in Heavy-Water neutron irradiation facility of Kyoto University research Reactor using multifoil activation method

Abstract

The Heavy-Water Thermal Neutron Facility at Kyoto University Research Reactor (KUR) has been operational for boron neutron capture therapy (BNCT) since May 1974. Following a facility construction update in 1996, the facility name was changed to the Heavy-Water Neutron Irradiation Facility (HWNIF), and the neutron energy spectra were evaluated using the multifoil activation method. In May 2010, the KUR transitioned its operations from high-enrichment fuel to low-enrichment fuel. However, the neutron energy spectrum in the KUR-HWNIF has not been reevaluated with high accuracy since then. This paper reports the reevaluation of the neutron energy spectrum for the standard epithermal-neutron irradiation mode using the multifoil activation method. Based on the reevaluated results, the epithermal- and fast-neutron fluxes increased by approximately 34 % and 19 %, respectively. The neutron absorption dose rate at evaluation point was approximately 17 % lower than the previous one; however, it remained acceptable from the perspective of BNCT biological irradiation.