Determining the [superscript 239]Np(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 [superscript 239]Np at nonthermal energies was performed. This minor actinide is the waiting point to [superscript 240]Pu production in a nuclear reactor. Method: The surrogate ratio method was employed to indirectly deduce the [superscript 239]Np(n, f) cross section. The surrogate reactions used were [superscript 236]U([superscript 3]He, p) and [superscript 238]U([superscript 3]He, p) with the reference cross section given by the well-known [superscript 237]Np(n, f ) cross section. The ratio of observed fission reactions resulting from the two formed compound nuclei, [superscript 238]Np and [superscript 240]Np,was multiplied by the directly measured [superscript 237]Np(n, f ) cross section to determine the [superscript 239]Np(n, f ) cross section. Results: The [superscript 239]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 [superscript 240]Pu produced during fuel burnup. The measurement helps constrain nuclear structure parameters used in the evaluations.