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 ${}^{239}$Np at nonthermal energies was performed. This minor actinide is the waiting point to ${}^{240}$Pu production in a nuclear reactor.

Method: The surrogate ratio method was employed to indirectly deduce the ${}^{239}\text{Np}(n,f)$ cross section. The surrogate reactions used were ${}^{236}\text{U}{(}^3\text{He},p)$ and ${}^{238}\text{U}{(}^3\text{He},p)$ with the reference cross section given by the well-known ${}^{237}\text{Np}(n,f)$ cross section. The ratio of observed fission reactions resulting from the two formed compound nuclei, ${}^{238}$Np and ${}^{240}$Np, was multiplied by the directly measured ${}^{237}\text{Np}(n,f)$ cross section to determine the ${}^{239}\text{Np}(n,f)$ cross section.

Results: The ${}^{239}\text{Np}(n,f)$ 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 ${}^{240}$Pu produced during fuel burnup. The measurement helps constrain nuclear structure parameters used in the evaluations.

• Received 23 December 2012

DOI:https://doi.org/10.1103/PhysRevC.87.034613