Calibration of neutron detectors for time-of-flight experiments by making use of a borated graphite standard neutron spectrum pile

Abstract

In order to obtain a standard field of neutron spectrum in the keV region, a borated graphite pile was designed and used. Theoretical calculations revealed that the 90° ( μ=0) angular lethargy spectrum of neutrons at about 22.5 cm from the center, where an isotropic photoneutron source was placed, had an essentially flat shape and little dependence on several factors. By the linac time-of-flight method with this pile, energy dependent neutron detection efficiencies of a 6Li glass scintillation counter bank and a 10Ba-vaseline plug NaI(Tl) counter were determined. As a subsidiary measurement, the spatial neutron distribution was measured by making use of the 58Ni(n,p) 58Co and 197Au(n,γ) 198Au reactions. It was found that neutrons distributed nearly isotropically around the photoneutron source, which verified the adequacy of the one-dimensional calculation for predicting the neutron spectrum in this pile. Radial attenuations of the reaction rates agreed with the predicted. By making use of these calibrated neutron detectors the neutron spectrum in an aluminum assembly was measured by the time-of-flight method. The values obtained with these two detectors agreed with each other and also with the theoretically predicted value.