Advances in $^{239}$Pu($n$,$f$) Prompt Fission Neutron Spectrum Measurements from the Chi-Nu Experiment

The prompt fission neutron spectrum (PFNS) is of vital importance for understanding the behavior of systems driven by neutron-induced fission. The $^{239}$Pu($n$,$f$) PFNS was recently measured at LANL with the Chi-Nu experimental approach in which results from a Li-glass and from a liquid scintillator array were combined to form a single measurement from 10 keV to 10 MeV outgoing neutron energy for each of 20 incident neutron energy ranges from 1-20 MeV. Data analysis incorporated a highly-detailed covariance assessment, including correlations between all 1300 reported PFNS data points. These correlations allowed for the determination of the covariance matrix of the mean spectrum energies as a function incident neutron energy, which had never before been reported for an experimental PFNS measurement. Clear observations of second-chance fission and pre-equilibrium neutron emission was observed, though third-chance fission is much less prominent than predicted in ENDF/B-VIII.0 and other evaluations.