高エネルギー加速器研究機構における黒鉛パイル熱中性子校正場の開発

Abstract

A thermal neutron reference field using a graphite pile of 190cm×190cm×250cm was developed at the High Energy Accelerator Research Organization. Inside the pile, the thermal neutron flux was measured by the gold foil activation technique using the Cadmium difference method, and outside the pile by the 3He proportional counter using the Cadmium difference method. Besides the gold foil, the optical fiber coupled to the ZnS(Ag) scintillator with the Li converter was used to measure the neutron flux inside the pile to verify its availability. The absolute values of thermal neutron fluxes measured inside the pile could be obtained with an accuracy of 7.5-7.8%. The measured thermal neutron fluxes inside the pile were compared with those calculated by use of the MCNP-4B Monte Carlo code. The calculated thermal neutron fluxes agreed very well with the measured ones. Outside the pile, the measured thermal neutron flux was 7.38×10-6 (n/[s·cm2·source neutron]), and the contribution of neutrons other than thermal neutrons was less than 1%. The thermal neutron fluxes measured by the optical fiber with the scintillator showed very good agreement with those measured with the gold foil activation method. The optical fiber detector was found to be quite useful to measure the thermal neutron flux distribution in the pile because of its much higher detection efficiency than that of gold foil activation.