标题： 提示裂变中子在$^{239}$Pu($n,f$) 反应中的作用 显示英文标题

标题： Prompt Fission Neutrons in the $^{239}$Pu($n,f$) Reaction

摘要： 脉冲裂变中子谱从$^{239}$Pu($n,f$) 在洛斯阿拉莫斯中子科学中心的武器中子研究设施（WNR）对入射中子能量从$0.7$到$700\,$MeV 进行了测量。 设计了一种新型高效裂变室，与高度分段的 Chi-Nu 数组耦合，以检测裂变事件中发出的中子。 使用双时间飞行技术，从散裂靶得出入射中子能量，并从裂变室得出出射中子能量。 脉冲裂变中子能谱（PFNS）相对于$^{252}$Cf 自发裂变进行了测量，最低至$200\,$keV，最高约为$12\,$MeV，对于所有入射中子能量，典型的不确定度远低于$2\%$，直到约$10\,$MeV 的出射中子能量。 PFNS 的总体趋势在 JEFF3.3 和 ENDF-B\VIII .0 评价中得到了很好的再现。 然而，在光谱的低能部分观察到了差异，其中评价结果高估了发射中子的数量。 实验上获得了中子产额和平均动能随入射中子能量变化的数据，报告的不确定度低于$0.4\%$。 中子多重性在$6\,$ MeV 以上与一些较早的数据不一致，表明需要使用高效率的裂变探测器，以避免数据偏差。 测量的平均动能与最新数据一致。 评估结果大致再现了趋势，但在其不确定性范围内未能再现实验值。 显示英文摘要

摘要： Prompt fission neutron spectra from $^{239}$Pu($n,f$) were measured for incident neutron energies from $0.7$ to $700\,$MeV at the Weapons Neutron Research facility (WNR) of the Los Alamos Neutron Science Center. A newly designed high-efficiency fission chamber was coupled to the highly segmented Chi-Nu array to detect neutrons emitted in fission events. The double time-of-flight technique was used to deduce the incident-neutron energies from the spallation target and the outgoing-neutron energies from the fission chamber. Prompt fission neutron spectra (PFNS) were measured with respect to $^{252}$Cf spontaneous fission down to $200\,$keV and up to about $12\,$MeV for all the incident neutron energies with typical uncertainties well below $2\%$ up to about $10\,$MeV outgoing-neutron energy. The general trend of PFNS is well reproduced by JEFF3.3 and ENDF-B\VIII.0 evaluations. Discrepancies were however observed for the low-energy part of the spectra, where evaluations overestimate the number of emitted neutrons. Neutron multiplicities and average kinetic energies as a function of incident-neutron energy are obtained experimentally with reported uncertainties below $0.4\%$. Neutron multiplicities disagree with some older datasets above $6\,$ MeV, indicating the need of using a high-efficiency fission detector, which does not bias the data. The measured mean kinetic energies agree with the most recent data. Evaluations fairly reproduce the trend, but fail to reproduce the experimental values within their uncertainties.