Evaluations of Energy Spectra of Neutrons Emitted Promptly in Neutron-induced Fission of 235 U and 239 Pu

Journal Article · 01 February 2018 · Nuclear Data Sheets

DOI: https://doi.org/10.1016/j.nds.2018.02.006 · OSTI ID:1427374

Neudecker, Denise  ^[1]; Talou, Patrick ^[1]; Kawano, Toshihiko  ^[1]; Kahler, Albert C. ^[1]; White, Morgan C.  ^[1]; Taddeucci, Terry N. ^[1]; Haight, Robert C. ^[1]; Kiedrowski, Brian ^[2]; O'Donnell, John M. ^[1]; Gomez, Jaime A.  ^[1]; Kelly, Keegan J. ^[1]; Devlin, Matthew  ^[1]; Rising, Michael E.  ^[1]

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1. Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)
2. Univ. of Michigan, Ann Arbor, MI (United States)

The energy spectra of neutrons emitted promptly in the neutron-induced fission reactions of ²³⁵U and ²³⁹Pu were re-evaluated for ENDF/B-VIII.0. The evaluations presented here 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 ²³⁵U and ²³⁹Pu 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 ²³⁹Pu PFNS was retained for E_inc ≤ 5 MeV awaiting more conclusive experimental evidence.

• Research Organization: Los Alamos National Laboratory (LANL)
• Sponsoring Organization: USDOE National Nuclear Security Administration (NNSA)
• Grant/Contract Number: AC52-06NA25396
• OSTI ID: 1427374
• Report Number(s): LA-UR-17-25589
• Journal Information: Nuclear Data Sheets, Journal Name: Nuclear Data Sheets Journal Issue: C Vol. 148; ISSN 0090-3752
• Publisher: ElsevierCopyright Statement
• Country of Publication: United States
• Language: English