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Title: Reexamining the role of the ( n , γ f ) process in the low-energy fission of U 235 and Pu 239

In this paper, the $$(n,{\gamma}f)$$ process is reviewed in light of modern nuclear reaction calculations in both slow and fast neutron-induced fission reactions on $$^{235}\mathrm{U}$$ and $$^{239}\mathrm{Pu}$$. Observed fluctuations of the average prompt fission neutron multiplicity and average total $${\gamma}$$-ray energy below 100-eV incident neutron energy are interpreted in this framework. The surprisingly large contribution of the $M1$ transitions to the prefission $${\gamma}$$-ray spectrum of $$^{239}\mathrm{Pu}$$ is explained by the dominant fission probabilities of $${0}^{+}$$ and $${2}^{+}$$ transition states, which can only be accessed from compound nucleus states formed by the interaction of $s$-wave neutrons with the target nucleus in its ground state, and decaying through $M1$ transitions. The impact of an additional low-lying $M1$ scissors mode in the photon strength function is analyzed. We review experimental evidence for fission fragment mass and kinetic-energy fluctuations in the resonance region and their importance in the interpretation of experimental data on prompt neutron data in this region. In conclusion, calculations are extended to the fast energy range where $$(n,{\gamma}f)$$ corrections can account for up to 3% of the total fission cross section and about 20% of the capture cross section.
Authors:
 [1] ;  [1] ;  [2]
  1. Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
  2. Alternative Energies and Atomic Energy Commission (CEA), Saint-Paul-lez-Durance (France). Physics Studies Lab.
Publication Date:
Report Number(s):
LA-UR-17-31364
Journal ID: ISSN 2469-9985
Grant/Contract Number:
AC52-06NA25396
Type:
Accepted Manuscript
Journal Name:
Physical Review C
Additional Journal Information:
Journal Volume: 97; Journal Issue: 6; Journal ID: ISSN 2469-9985
Publisher:
American Physical Society (APS)
Research Org:
Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
Sponsoring Org:
USDOE National Nuclear Security Administration (NNSA)
Country of Publication:
United States
Language:
English
Subject:
73 NUCLEAR PHYSICS AND RADIATION PHYSICS; electromagnetic transitions; fission; nuclear reactions
OSTI Identifier:
1441321
Alternate Identifier(s):
OSTI ID: 1439903