标题： prefission 中子能谱和 PFNS 的角度各向异性$^{240}$Pu$(n,F)$ 显示英文标题

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

摘要： 1972年，在$^{239}$Pu+n的中子发射光谱(NES)中观察到了次级中子的角度各向异性，并且在2019年$^{239}$Pu $(n,F)$的瞬发裂变中子谱(PFNS)中也发现了这一点，因此现在可以预测$^{240}$Pu(n,F)的PFNS。 在NES中，角度各向异性是由集体能级的直接激发以及（n,nX）反应中第一中子发射的非平衡/半直接机制（连续态中的状态被激发）引起的；而在PFNS中，它是由（n,xnf）反应的预裂变中子的专属谱引起的。 在$^{239}$Pu $(n,xnf)$和$^{240}$Pu(n,xnf)反应中，观察到的PFNS显示出对前向或后向半球内发射的第一个预裂变中子的不同响应，这取决于相对于入射中子动量的角度。 由于（n,nf）中子的能量及其平均值依赖于发射角θ与入射中子动量之间的夹角，因此所观测到的PFNS、平均瞬发裂变中子多重性、裂变截面、平均总动能TKE等也会很大程度上依赖于角度θ。 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. 显示英文摘要

摘要： 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.