Preequilibrium Asymmetries in the ${}^{239}\mathrm{Pu}(n,f)$ Prompt Fission Neutron Spectrum

The physical properties of neutrons emitted from neutron-induced fission are fundamental to our understanding of nuclear fission. However, while state-of-the-art fission models still incorporate isotropic fission neutron spectra, it is believed that the preequilibrium prefission component of these spectra is strongly anisotropic. The lack of experimental guidance on this feature has not motivated incorporation of anisotropic neutron spectra in fission models, though any significant anisotropy would impact descriptions of a fissioning system. In the present work, an excess of counts at high energies in the fission neutron spectrum of ${}^{239}\mathrm{Pu}$ is clearly observed and identified as an excess of the preequilibrium prefission distribution above the postfission neutron spectrum. This excess is separated from the underlying postfission neutron spectrum, and its angular distribution is determined as a function in incident neutron energy and outgoing neutron detection angle. Comparison with neutron scattering models provides the first experimental evidence that the preequilibrium angular distribution is uncorrelated with the fission axis. The results presented here also impact the interpretation of several influential prompt fission neutron spectrum measurements.

Figure 1

An illustration of the reaction paths accessible to a ${}^{239}\mathrm{Pu}+n$ system undergoing nuclear fission, with the incident neutron energies, $E_n^{\mathrm{inc}}$. The fission fragments shown represent typical fragments emitted from the corresponding fissioning nucleus, though their precise identity, as well as the number of neutrons emitted from each nucleus, are arbitrarily chosen. See the text for a discussion.

Figure 2

The energy dependence of fission neutrons in ${}^{239}\mathrm{Pu}(n,f)$ is shown. The various line styles and shades of blue describe the PFNS for incident neutron energy ranges in which preequilibrium prefission neutrons are observed. Average incident neutron energies, $⟨E_n^{\mathrm{inc}}⟩$, over the plotted range are also specified. The width of each distribution reflects the statistical uncertainties in the measured spectrum. All spectra were scaled to have the same integral from in the 3–5 MeV outgoing neutron energy range.

Figure 3

The angular dependence of the fission neutron spectrum of ${}^{239}\mathrm{Pu}(n,f)$ is shown. Lines of different styles and shades of blue correspond to different detection angles. The width of the largest and smallest angles represents the statistical uncertainty. The uncertainties for the intermediate angles are not shown but are similar. The ENDF/B-VIII.0 [3] PFNS at $E_n^{\mathrm{inc}}=15\mathrm{MeV}$ is shown as the red dash-triple-dotted line and shaded region, the latter of which represents the evaluation uncertainty. Note that the preequilibrium neutrons are clearly observed even at angles above 90°. All spectra were scaled to have the same integral in the 3–5 MeV outgoing neutron energy range.

Figure 4

The ratios of the average PFNS energy over an outgoing range of $E_n^{\mathrm{out}}=1–3\mathrm{MeV}$ (blue open circles) and $E_n^{\mathrm{out}}=1–12\mathrm{MeV}$ (black diamonds) at a detection angle of 30° to the same quantity calculated at 150° as a function of incident neutron energy are shown. The uncertainties on the data points represent the uncertainty on the mean energy at 30°, while the width of the shaded region around a ratio of unity (red online) represents the uncertainty of the mean energy at 150° for $E_n^{\mathrm{out}}=1–12\mathrm{MeV}$. See the text for discussion.

Figure 5

The angular distributions of counts in the preequilibrium prefission neutron excess region in ${}^{239}\mathrm{Pu}(n,f)$ are shown as black diamonds. The red and cyan shaded regions are K-M fits to the experimental data with $l_{\max }=2$ and 3, respectively, as indicated on panel (b), and the widths of the distributions include the model uncertainties and parameter correlations from the fit. The solid blue lines are FKK predictions for the angular distributions calculated following Ref. [11] averaged over the lower and upper limits of the incident energy range shown on each plot. The statistical scatter of the data is reduced with increasing incident neutron energy because the effect of preequilibrium neutron emission is larger at higher incident energies. Note that the angles shown on the $x$ axis are defined with respect to the momentum direction of the incident neutron.