Abstract The energy spectra of neutrons emitted promptly in the neutron-induced fission reactions of 235 U and 239 Pu were re-evaluated for ENDF/B-VIII.0. These evaluations are based on a careful modeling of all relevant physics processes, an extensive analysis of experimental data and a detailed quantification of pertinent uncertainties. Energy spectra of neutrons emitted in up to fourth chance fission are considered and both compound and pre-equilibrium processes are included. Also, important nuclear model parameters, such as the average total kinetic energy of the fission fragments and the multiple chance fission probabilities, and their uncertainties are estimated based on experimental knowledge, model information and evaluated data. In addition to experimental information already available for ENDF/B-VII.1, these new evaluations make use of recently published experimental data either of high precision or spanning a broad incident energy range, information on legacy measurements explaining discrepancies and recently measured data of the average total kinetic energy as a function of incident neutron energy. The resulting evaluated data and covariances agree well with the experimental database used for the evaluation. However, the evaluated spectra are softer than the 235 U and 239 Pu ENDF/B-VII.1, JENDL-4.0 and JEFF-3.2 evaluations for incident neutron energies E inc ≤ 1.5 MeV and E inc ≤ 5 MeV , respectively. For E inc > 5 MeV , the evaluated spectra show structures due to the improved modeling which are not present in ENDF/B-VII.1 and JEFF-3.2 but can be observed in JENDL-4.0 evaluations. Part of these new evaluations were adopted for ENDF/B-VIII.0, while the ENDF/B-VII.1 239 Pu PFNS was retained for E inc ≤ 5 MeV awaiting more conclusive experimental evidence.

中文翻译：

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235 U 和 239 Pu 中子诱发裂变中快速发射的中子能谱评估

摘要 对 ENDF/B-VIII.0 的 235 U 和 239 Pu 的中子诱导裂变反应中迅速发射的中子的能谱进行了重新评估。这些评估基于对所有相关物理过程的仔细建模、对实验数据的广泛分析以及对相关不确定性的详细量化。考虑了高达第四次机会裂变中发射的中子的能谱，并且包括复合过程和预平衡过程。此外，重要的核模型参数，如裂变碎片的平均总动能和多次机会裂变概率，及其不确定性是根据实验知识、模型信息和评估数据估计的。除了 ENDF/B-VII.1 已有的实验信息外，这些新的评估利用了最近公布的高精度或跨越广泛的入射能量范围的实验数据、解释差异的传统测量信息以及最近测量的平均总动能与入射中子能量的函数关系。得到的评估数据和协方差与用于评估的实验数据库非常吻合。然而，对于入射中子能量 E inc ≤ 1.5 MeV 和 E inc ≤ 5 MeV 的评估光谱分别比 235 U 和 239 Pu ENDF/B-VII.1、JENDL-4.0 和 JEFF-3.2 评估软。对于 E inc > 5 MeV，评估的光谱显示了由于改进的建模而产生的结构，这些结构在 ENDF/B-VII.1 和 JEFF-3.2 中不存在，但可以在 JENDL-4.0 评估中观察到。ENDF/B-VIII 通过了这些新评估的一部分。