(A) study on absolute neutron emission rate measurement of A252 Cf source by the manganous sulfate bath method

Abstract

An initial effort has been made to measure absolutely the neutron emission rate of a $^252Cf$ source in order to establish the national standards for neutron source measurements and disseminate the neutron standards to other research institutes and end users of neutron radiations through the national traceability. At the first stage, a circulated manganous sulfate ($MnSO_4$) bath system was developed for the standardization of neutron source measurements. This bath system was made up of a circulation system filled with a $MnSO_4$ solution and a $^56Mn$ γ-ray detection system. To improve the counting statistics for the neutron emission rate measurements, the bath was converted to flow counting operation. The $^56Mn$ γ-ray detection sensitivity was improved by the circulation system which enables the $^56Mn$ activity to be measured during neutron activation of $MnSO_4$ solution in the bath. Dependence on an assumed hydrogen to manganese therml neutron capture cross section ratio was eliminated by varying the $MnSO_4$ solution concentration. The performance test of the bath system was carried out using the neutron source $^252Cf$, mounted at the center of the spherical bath. In the experiment, the characteristics of the bath system were determined in terms of the bath system efficiency, saturated $^56Mn$ activity, and manganese neutron capture fraction. In particular, the correction factors necessary for neutron capture in the source, captures of fast and thermal neutrons by competing reactions in the $MnSO_4$ solution, and neutron loss by leakage of neutrons from the bath were described in details. By the manganous sulfate bath method, the observed neutron emission rate, Q, was determined to be $(2.544 ± 0.338)×10^7 n/s$ for the $^252Cf$ source at the reference date, January 1, 1987 in the absence of impurities in the $MnSO_4$ solution. Under this assumption, the thermal neutron capture cross section ratio of hydrogen to manganese, $σ_H/\σ_Mn$, has been determine...