Angular anisotropy in prefission neutron spectra and PFNS of $^{240}$Pu$(n,F)$

Abstract

Angular anisotropy of secondary neutrons was evidenced in neutron emission spectra (NES) of $^{239}$Pu+n in 1972, and prompt fission neutron spectra (PFNS) of $^{239}$Pu$(n,F)$ in 2019, it might be predicted for $^{240}$Pu(n,F) PFNS now. In case of NES angular anisotropy is due to direct excitation of collective levels and pre-equilibrium/semi-direct (states in the continuum are excited) mechanism of neutron emission of first neutron in (n,nX) reaction, while in case of PFNS it is due to exclusive spectra of pre-fission neutrons of (n, xnf) reactions. In $^{239}$Pu$(n,xnf)$ and $^{240}$Pu(n,xnf) reactions observed PFNS envision different response to the emission of first pre-fission neutron in forward or backward semi-spheres with respect to the momentum of incident neutrons. Since energies of (n,nf) neutrons and their average values depend on angle of emission theta with respect to the incident neutron momentum, the observed PFNS, average prompt fission neutron multiplicity, fission cross section, average total kinetic energy TKE, etc. also would be quite dependent on angle theta. Exclusive spectra of (n, xnf) neutrons at theta of 90 degrees are consistent with $^{240}$Pu(n, F)($^{239}$Pu$(n,F)$, $^{239}$Pu$(n,2n)$) observed cross sections and neutron emission spectra of $^{239}$Pu+n interaction at En up to 20 MeV. The correlations of the angular anisotropy of PFNS with the relative contribution of the $(n,nf)$ fission chance to the observed fission cross section and angular anisotropy of pre-fission neutron emission are ascertained.