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Title: Measurement of the 239Pu$(n, f$) prompt fission neutron spectrum from 10 keV to 10 MeV induced by neutrons of energy 1–20 MeV

Abstract

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 239Pu 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. Here, we report a measurement of the 239Pu 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 andmore » 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.« less

Authors:
ORCiD logo [1];  [1];  [1];  [1];  [1];  [1];  [1];  [1];  [1];  [2];  [2]; ORCiD logo [1];  [1]; ORCiD logo [1];  [1]; ORCiD logo [1]; ORCiD logo [1];  [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.:
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Sponsoring Org.:
USDOE National Nuclear Security Administration (NNSA)
OSTI Identifier:
1668481
Report Number(s):
LLNL-JRNL-800997
Journal ID: ISSN 2469-9985; 1004328
Grant/Contract Number:  
AC52-07NA27344; 89233218CNA000001
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Physical Review C
Additional Journal Information:
Journal Volume: 102; Journal Issue: 3; Journal ID: ISSN 2469-9985
Publisher:
American Physical Society (APS)
Country of Publication:
United States
Language:
English
Subject:
nuclear physics; radiation physics; fission; neutron physics; scintillators; time-of-flight neutron spectroscopy

Citation Formats

Kelly, K. J., Devlin, M., O'Donnell, J. M., Gomez, J. A., Neudecker, D., Haight, R. C., Taddeucci, T. N., Mosby, S. M., Lee, H. Y., Wu, C. Y., Henderson, R., Talou, P., Kawano, T., Lovell, A. E., White, M. C., Ullmann, J. L., Fotiades, N., Henderson, J., and Buckner, M. Q. Measurement of the 239Pu$(n, f$) prompt fission neutron spectrum from 10 keV to 10 MeV induced by neutrons of energy 1–20 MeV. United States: N. p., 2020. Web. doi:10.1103/physrevc.102.034615.
Kelly, K. J., Devlin, M., O'Donnell, J. M., Gomez, J. A., Neudecker, D., Haight, R. C., Taddeucci, T. N., Mosby, S. M., Lee, H. Y., Wu, C. Y., Henderson, R., Talou, P., Kawano, T., Lovell, A. E., White, M. C., Ullmann, J. L., Fotiades, N., Henderson, J., & Buckner, M. Q. Measurement of the 239Pu$(n, f$) prompt fission neutron spectrum from 10 keV to 10 MeV induced by neutrons of energy 1–20 MeV. United States. doi:10.1103/physrevc.102.034615.
Kelly, K. J., Devlin, M., O'Donnell, J. M., Gomez, J. A., Neudecker, D., Haight, R. C., Taddeucci, T. N., Mosby, S. M., Lee, H. Y., Wu, C. Y., Henderson, R., Talou, P., Kawano, T., Lovell, A. E., White, M. C., Ullmann, J. L., Fotiades, N., Henderson, J., and Buckner, M. Q. Mon . "Measurement of the 239Pu$(n, f$) prompt fission neutron spectrum from 10 keV to 10 MeV induced by neutrons of energy 1–20 MeV". United States. doi:10.1103/physrevc.102.034615.
@article{osti_1668481,
title = {Measurement of the 239Pu$(n, f$) prompt fission neutron spectrum from 10 keV to 10 MeV induced by neutrons of energy 1–20 MeV},
author = {Kelly, K. J. and Devlin, M. and O'Donnell, J. M. and Gomez, J. A. and Neudecker, D. and Haight, R. C. and Taddeucci, T. N. and Mosby, S. M. and Lee, H. Y. and Wu, C. Y. and Henderson, R. and Talou, P. and Kawano, T. and Lovell, A. E. and White, M. C. and Ullmann, J. L. and Fotiades, N. and Henderson, J. and Buckner, M. Q.},
abstractNote = {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 239Pu 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. Here, we report a measurement of the 239Pu 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.},
doi = {10.1103/physrevc.102.034615},
journal = {Physical Review C},
issn = {2469-9985},
number = 3,
volume = 102,
place = {United States},
year = {2020},
month = {9}
}

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