Properties of correlated fission fragments from neutron induced fission of Np-237 at incident neutron energies between 200 keV and 100 MeV

Abstract

Background: Measurements of the neutron-induced fission of major actinides are vital to applications such as nuclear energy, nuclear forensics, nuclear nonproliferation and stockpile stewardship. Contemporary fission models are informed by additional correlated fission data, advancing toward a fully predictive fission model. Of particular interest are the average total kinetic energy ( T KE ) of the fission fragments, fission fragment mass yield distributions and correlations between mass and T KE as they evolve with excitation energy of the fissioning system. Neptunium-237 is produced in significant quantities in the nuclear fuel cycle. Its presence in spent reactor fuel and ∼ 2 × 10 6 year half life make a complete and precise understanding of its production and destruction desirable to inform reactor design. Little T KE data exists for 237 Np(n, f ), and until recently, there was no T KE nor mass yield data for 237 Np(n, f ) at incident neutron energies above E n = 5.55 MeV. The most recent measurement suffered from low statistics and relatively poor energy resolution compared to previous studies, so a higher quality data set is desired.Purpose: This study aimed to measure post-neutron evaporation mass yields and average total kinetic energy release of correlated fission products from neutron-induced fission of 237 Np over a wide range of incident neutron energies (particularly incident neutron energies above E n = 5.55 MeV), as well as to analyze how these quantities evolve with incident neutron energy. Method:The aforementioned measurements were obtained using a twin Frisch-gridded ionization chamber (TFGIC) and a thin-backed 237 Np target at the Los Alamos Neutron Science Center -Weapons Neutron Research (LANSCE -WNR) facility, which provides a collimated beam of neutrons with energies ranging from 100s of keV to 100s of MeV. The data were analyzed using the double-energy (or 2E) method, with mass-dependant corrections for prompt-fission neutrons and pulse height defect.Results: A total of 1.15 × 10 6 coincident fission events were recorded and analyzed using the 2E method. Preand post-neutron evaporation T KE values are reported for 54 incident neutron energies in the energy range 0.20 ≤ E n ≤ 100.0 MeV and compared to existing data and models. Pre-and post-neutron evaporation mass yields were extracted with a full width at half maximum (FWHM) resolution of approximately 4u and compared to existing data and evaluations. Conclusions:The present T KE and mass yield data agree with previous results and also with statistical models of 237 Np(n, f ) at incident neutron energies between E n = 0.2 − 20.0 MeV. A flattening of the T KE data is observed (relative to the GEF model's prediction) above E n = 20.0 MeV. However, the interpretation of this discrepancy is unclear as the analysis method's neglect of incomplete momentum transfer at high energies as well as pre-equilibrium pre-fission phenomena likely have a significant impact on the measurement at such high incident neutron energies.