Measurement and Analysis of Capture Reaction Rate of ²³⁷Np in Various Thermal Neutron Fields by Critical Assembly and Heavy Water Thermal Neutron Facility of Kyoto University

Capture reaction rate ratios of ²³⁷Np relative to ¹⁹⁷Au were measured in 11 thermal neutron fields provided by the Kyoto University Critical Assembly and the Kyoto University Reactor Heavy Water Neutron Irradiation Facility. In the measurement, both samples of ²³⁷Np and ¹⁹⁷Au were irradiated at the same time, and their gamma activities were measured. The typical experimental error was 3.5%. The analysis was performed by three steps: full-core calculation, self-shielding correction of the sample, and perturbation correction of the sample. Three full-core calculations by a continuous-energy Monte Carlo code (MVP), a transport code (TWOTRAN), and a diffusion code (CITATION) were made with the JENDL-3.2 library. The self-shielding factors were derived by an analytical formula, and the perturbation factors were calculated by another MVP calculation. The reaction rates were derived by multiplying the neutron spectrum, the two correction factors, and the capture cross sections of ²³⁷Np and ¹⁹⁷Au.

As a result, the three full-core calculations provided almost the same neutron spectra at the sample position and gave almost the same calculated-to-experimental values (C/Es) for the capture reaction rate ratios of ²³⁷Np relative to ¹⁹⁷Au. Based on the capture cross section of ²³⁷Np taken from the JENDL-3.2 library, the C/Es were between 0.97 and 1.04, and the average C/E among the 11 cores was 1.01. On the other hand, the C/Es using the ENDF/B-VI and the JEF-2.2 were 1.02 to 1.06 for harder spectrum cores, whereas the C/Es for the softer spectrum cores were 1.08 to 1.16. It is concluded that the JENDL-3.2 library has good accuracy for the capture cross section of ²³⁷Np but the ENDF/B-VI and the JEF-2.2 libraries overestimate that of ²³⁷Np >10% in the thermal neutron energy region.