Determining the 239Np(n,f) cross section using the surrogate ratio method

Abstract

Background: Neutron-induced fission cross-section data are needed in various fields of applied and basic nuclear science. However, cross sections of short-lived nuclei are difficult to measure directly due to experimental constraints. Purpose: The first experimental determination of the neutron-induced fission cross section of 239Np at nonthermal energies was performed. This minor actinide is the waiting point to 240Pu production in a nuclear reactor. Method: The surrogate ratio method was employed to indirectly deduce the 239Np(n,f) cross section. The surrogate reactions used were 236U (3He,p) and 238U(3He,p) with the reference cross section given by the well-known 237Np(n,f) cross section. The ratio of observed fission reactions resulting from the two formed compound nuclei, 238Np and 240Np, was multiplied by the directly measured 237Np(n,f) cross section to determine the 239Np(n,f) cross section. Results: The 239Np(n,f) cross section was determined with an uncertainty ranging between 4% and 30% over the energy range of 0.5-20 MeV. The resulting cross section agrees closest with the JENDL-4.0 evaluation. Conclusions: The measured cross section falls in between the existing evaluations, but it does not match any evaluation exactly (with JENDL-4.0 being the closest match); hence reactor codes relying on existing evaluations may under- or overestimate the amount of 240Pu produced during fuel burnup. The measurement helps constrain nuclear structure parameters used in the evaluations. © 2013 American Physical Society.