New constraint on the Np237(n,γ)Np238 integral cross section using the Godiva-IV critical assembly

Background: Accurate knowledge of the ²³⁷Np(n,γ)²³⁸Np cross section at fast neutron energies is important for applied nuclear science. The presently available experimental data have large disagreements in the fast neutron region.Purpose: Perform a model-independent measurement of the ²³⁷Np(n,γ)²³⁸Np integral cross section using a well characterized fast neutron source and compare the result with previous measurements and current nuclear data evaluations. Provide an integral measurement that can be used as a benchmark for current evaluations.Methods: Multiple samples of ²³⁷Np were irradiated in the Godiva-IV critical assembly. Following the irradiation, the samples placed in a γ-ray counting setup and the γ rays emitted from the decay of ²³⁸Np were measured over a time period of approximately 7 d. Multiple γ-ray decay branches of ²³⁸Np were observed.Results: The observed activity of ²³⁸Np was used to calculate the amount of ²³⁸Np produced during the irradiation via the ²³⁷Np(n,γ)²³⁸Np reaction and an integral cross section of 342(11) $mb$ was measured for the Godiva-IV neutron spectrum. The ²³⁸Np half-life has been measured with a result of 50.31(5) h.Conclusion: The ²³⁷Np(n,γ)²³⁸Np integral cross section measured in this work is in agreement with overlapping 1σ error bands to ENDF/B-VIII.0. However, the measured value is 3σ away from the calculated integral cross section using JENDL-5. This measurement offers a reliable benchmark for future ²³⁷Np(n,γ)²³⁸Np cross section evaluations.