Although the prompt fission neutron spectrum (PFNS) is an essential component of neutron-driven systems that has been measured for decades, there are still multiple glaring unknowns regarding the PFNS of major actinides in the fission neutron incident energy range, specifically with regard to multichance fission and pre-equilibrium neutron emission processes. The only impactful experimental $^{239}\mathrm{Pu}$ PFNS measurements included in recent nuclear data evaluations were measured over a limited outgoing neutron energy range at thermal and 1.5-MeV average incident neutron energy, while other potentially impactful measurements have been shown to contain errors that resulted in either large uncertainty increases or in complete exclusion from nuclear data evaluation. We report here a measurement of the $^{239}\mathrm{Pu}$ PFNS over a wide range of incident neutron energy (1--20 MeV) and three orders of magnitude in outgoing neutron energy (0.01--10 MeV) resulting from the Chi-Nu experiment at the Los Alamos Neutron Science Center. These results are the combination of separate PFNS measurements in the same experimental area, one using a Li-glass and the other a liquid scintillator detector array. Covariances between all PFNS data points from each detector and within each incident energy range were generated between all other data in both detector arrays and within all other incident neutron energy bins, yielding a single covariance matrix for all 1300 PFNS data points reported here. These covariances are based on a thorough assessment of systematic bias and uncertainties associated with the measurement, PFNS extraction technique, combination of data from each detector type, and other aspects of the analysis. The existence of covariances between PFNS data points in different incident neutron energy ranges yielded covariances between average PFNS energy values at each incident energy to be reported here as well, which allowed for firm statements to be made regarding a shape of a purely experimental mean PFNS energy trend for the first time. Although minor PFNS shape differences exist between the results reported here and recent nuclear data evaluations, the ENDF/B-VIII.0 and JEFF-3.3 PFNS evaluations agree reasonably well with the present results from 1-to 10-MeV incident neutron energy, which spans the well-measured 1.5-MeV incident neutron energy PFNS from Lestone and Shores as well as the onset of second-chance fission. However, while the pre-equilibrium component of the PFNS above 12-MeV incident neutron energy roughly agrees in position and magnitude with ENDF/B-VIII.0 and JEFF-3.3, clear differences relating to the relative magnitude of third-chance fission PFNS features are present in the PFNS shape and in the mean PFNS energy trends.

中文翻译：

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测量由能量为 1-20 MeV 的中子诱发的从 10 keV 到 10 MeV 的 Pu239(n,f) 瞬发中子谱

尽管瞬发裂变中子能谱 (PFNS) 是中子驱动系统的重要组成部分，并且已经被测量了几十年，但关于裂变中子入射能量范围内主要锕系元素的 PFNS，特别是关于多机会的 PFNS，仍然存在多个明显的未知数。裂变和平衡前中子发射过程。最近的核数据评估中唯一有影响的实验 $^{239}\mathrm{Pu}$ PFNS 测量是在有限的外发中子能量范围内以热和 1.5-MeV 平均入射中子能量进行测量的，而其他可能有影响的测量已经显示包含导致不确定性大幅增加或完全排除在核数据评估之外的错误。我们在此报告了在大范围的入射中子能量 (1--20 MeV) 和三个数量级的出射中子能量 (0.01--10 MeV) 上 $^{239}\mathrm{Pu}$ PFNS 的测量结果来自洛斯阿拉莫斯中子科学中心的 Chi-Nu 实验。这些结果是在同一实验区域中单独 PFNS 测量的组合，一个使用锂玻璃，另一个使用液体闪烁体探测器阵列。来自每个探测器的所有 PFNS 数据点和每个入射能量范围内的协方差在两个探测器阵列中的所有其他数据和所有其他入射中子能量箱内生成，为此处报告的所有 1300 个 PFNS 数据点生成单个协方差矩阵。这些协方差基于对与测量相关的系统偏差和不确定性的彻底评估，PFNS 提取技术、来自每种探测器类型的数据组合以及分析的其他方面。不同入射中子能量范围内 PFNS 数据点之间存在协方差，导致此处报告的每个入射能量的平均 PFNS 能量值之间存在协方差，这使得可以对纯实验平均 PFNS 能量的形状做出明确的声明第一次出现趋势。尽管此处报告的结果与最近的核数据评估之间存在细微的 PFNS 形状差异，但 ENDF/B-VIII.0 和 JEFF-3.3 PFNS 评估与当前 1 到 10 MeV 入射中子能量的结果相当吻合，这跨越了来自 Lestone 和 Shores 的经过充分测量的 1.5-MeV 入射中子能量 PFNS 以及二次机会裂变的开始。然而，