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Title: Preequilibrium Asymmetries in the Pu 239 ( n , f ) Prompt Fission Neutron Spectrum

Abstract

The physical properties of neutrons emitted from neutron-induced fission are fundamental to our understanding of nuclear fission. However, while state-of-the-art fission models still incorporate isotropic fission neutron spectra, it is believed that the preequilibrium prefission component of these spectra is strongly anisotropic. The lack of experimental guidance on this feature has not motivated incorporation of anisotropic neutron spectra in fission models, though any significant anisotropy would impact descriptions of a fissioning system. In the present work, an excess of counts at high energies in the fission neutron spectrum of 239Pu is clearly observed and identified as an excess of the preequilibrium prefission distribution above the postfission neutron spectrum. This excess is separated from the underlying postfission neutron spectrum, and its angular distribution is determined as a function in incident neutron energy and outgoing neutron detection angle. Comparison with neutron scattering models provides the first experimental evidence that the preequilibrium angular distribution is uncorrelated with the fission axis. The results presented here also impact the interpretation of several influential prompt fission neutron spectrum measurements.

Authors:
 [1];  [1];  [1];  [1];  [1];  [1];  [1];  [1];  [1];  [1];  [1];  [1];  [1];  [2];  [2];  [2];  [2]
  1. Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
  2. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Publication Date:
Research Org.:
Los Alamos National Lab. (LANL), Los Alamos, NM (United States); Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Sponsoring Org.:
USDOE National Nuclear Security Administration (NNSA)
OSTI Identifier:
1501816
Alternate Identifier(s):
OSTI ID: 1515359; OSTI ID: 1546325
Report Number(s):
LA-UR-18-31568; LLNL-JRNL-765375
Journal ID: ISSN 0031-9007; PRLTAO
Grant/Contract Number:  
89233218CNA000001; AC52-07NA27344; AC52-06NA25396
Resource Type:
Accepted Manuscript
Journal Name:
Physical Review Letters
Additional Journal Information:
Journal Volume: 122; Journal Issue: 7; Journal ID: ISSN 0031-9007
Publisher:
American Physical Society (APS)
Country of Publication:
United States
Language:
English
Subject:
73 NUCLEAR PHYSICS AND RADIATION PHYSICS; Physics - Nuclear physics and radiation physics

Citation Formats

Kelly, K. J., Kawano, T., O’Donnell, J. M., Gomez, J. A., Devlin, M., Neudecker, D., Talou, P., Lovell, A. E., White, M. C., Haight, R. C., Taddeucci, T. N., Mosby, S. M., Lee, H. Y., Wu, C. Y., Henderson, R., Henderson, J., and Buckner, M. Q. Preequilibrium Asymmetries in the Pu239(n,f) Prompt Fission Neutron Spectrum. United States: N. p., 2019. Web. doi:10.1103/PhysRevLett.122.072503.
Kelly, K. J., Kawano, T., O’Donnell, J. M., Gomez, J. A., Devlin, M., Neudecker, D., Talou, P., Lovell, A. E., White, M. C., Haight, R. C., Taddeucci, T. N., Mosby, S. M., Lee, H. Y., Wu, C. Y., Henderson, R., Henderson, J., & Buckner, M. Q. Preequilibrium Asymmetries in the Pu239(n,f) Prompt Fission Neutron Spectrum. United States. doi:10.1103/PhysRevLett.122.072503.
Kelly, K. J., Kawano, T., O’Donnell, J. M., Gomez, J. A., Devlin, M., Neudecker, D., Talou, P., Lovell, A. E., White, M. C., Haight, R. C., Taddeucci, T. N., Mosby, S. M., Lee, H. Y., Wu, C. Y., Henderson, R., Henderson, J., and Buckner, M. Q. Fri . "Preequilibrium Asymmetries in the Pu239(n,f) Prompt Fission Neutron Spectrum". United States. doi:10.1103/PhysRevLett.122.072503. https://www.osti.gov/servlets/purl/1501816.
@article{osti_1501816,
title = {Preequilibrium Asymmetries in the Pu239(n,f) Prompt Fission Neutron Spectrum},
author = {Kelly, K. J. and Kawano, T. and O’Donnell, J. M. and Gomez, J. A. and Devlin, M. and Neudecker, D. and Talou, P. and Lovell, A. E. and White, M. C. and Haight, R. C. and Taddeucci, T. N. and Mosby, S. M. and Lee, H. Y. and Wu, C. Y. and Henderson, R. and Henderson, J. and Buckner, M. Q.},
abstractNote = {The physical properties of neutrons emitted from neutron-induced fission are fundamental to our understanding of nuclear fission. However, while state-of-the-art fission models still incorporate isotropic fission neutron spectra, it is believed that the preequilibrium prefission component of these spectra is strongly anisotropic. The lack of experimental guidance on this feature has not motivated incorporation of anisotropic neutron spectra in fission models, though any significant anisotropy would impact descriptions of a fissioning system. In the present work, an excess of counts at high energies in the fission neutron spectrum of 239Pu is clearly observed and identified as an excess of the preequilibrium prefission distribution above the postfission neutron spectrum. This excess is separated from the underlying postfission neutron spectrum, and its angular distribution is determined as a function in incident neutron energy and outgoing neutron detection angle. Comparison with neutron scattering models provides the first experimental evidence that the preequilibrium angular distribution is uncorrelated with the fission axis. The results presented here also impact the interpretation of several influential prompt fission neutron spectrum measurements.},
doi = {10.1103/PhysRevLett.122.072503},
journal = {Physical Review Letters},
number = 7,
volume = 122,
place = {United States},
year = {2019},
month = {2}
}

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