Comparison of Results from Recent NNSA and CEA Measurements of the 239Pu(n, f) Prompt Fission Neutron Spectrum

Abstract

The National Nuclear Security Administration (NNSA)/DP French Alternative Energies and Atomic Energy Commission (CEA)/DAM agreement on cooperation on fundamental science is a U.S.-French collaborative effort to combine intellectual and experimental resources and further the relevant nuclear science. Recently, both the NNSA and CEA experimental teams performed high-statistics measurements of the 239 Pu(n,f ) prompt fission neutron spectrum (PFNS) at the Los Alamos Neutron Science Center, both of which were recently published in the journal Physical Review C. These separate measurements used the same experimental area and a common neutron detector array, but differ in many aspects including background assessments, data acquisition systems and philosophies, fission detectors, and PFNS extraction techniques. Hence, some aspects of the experimental methods and associated uncertainties are highly correlated while others are independent. The results from both measurements broke new ground for PFNS measurements given their higher accuracy and more detailed study of corrections necessary for the measured quantity compared to existing literature measurements, and both will significantly impact PFNS nuclear data evaluations for the foreseeable future. The focus of this work is to document a comparison of the results from these distinct measurements in terms of the acquired data, the PFNS results, and the measured average PFNS energies. While systematic differences between the PFNS results are present on the 1-3% level, the acquired data relative to each respective measurement at low incident neutron energies are in remarkable agreement, as are the conclusions regarding the magnitude and position of features in the PFNS relating to second-chance fission, third-chance fission, and pre-equilibrium neutron emission.

Nuclear Science for the Manhattan Project and Comparison to Today’s ENDF Data

“…The NUEX result was quite important in a subsequent 2016 international assessment led by Capote with a collaboration organized through the IAEA. 104 This was a consequence of the international regard for Los Alamos' reputation in nuclear science, and it was only five years later that these data could be independently corroborated by high-precision lab experiments by the CEA, Los Alamos, and Livermore (The LANL-LLNL "Chi-Nu" data [97][98][99] But any pride in our Laboratories' technical accomplishments must, in the end, play second fiddle to the importance of sustaining a scientific questioning culture as articulated by our first Director, J. Robert Oppenheimer: "There must be no barriers to freedom of inquiry. There is no place for dogma in science.…”

“…ENDF/B-V to VII.1) 110 (Fig. 18 For comparison, also shown is a recent Los Alamos measurement from the LANSCE/Chi-Nu experiment 98,99 (red symbols), and Lestone and Shore's 53,111 NUEX experiment from test data (orange points), which is further discussed in the Conclusions. In summary, tremendous progress on the PFNS had been made by the end of the Manhattan project.…”

“…This is no longer the case, and nowadays our best theoretical treatments tend to be calibrated to high-precision measured data. In the last decade, flagship experiments [97][98][99] at Los Alamos, by LANL, LLNL and CEA scientists have focused on precise PFNS measurements.…”

“…17 shows LA-84 (May, 1944) [104][105][106] ). For comparison, some recent 2020 high-accuracy 235 U "Chi-Nu" data from Los Alamos' LANSCE facility [97][98][99] are shown as red points. In April 1944 William's group also saw small differences between the 235 U and 239 Pu PFNS (Ref., 27 p.196) between 1 and 2.5 MeV outgoing neutron energy.…”

Shedding light on the 239Pu fission source term with new high-precision experiments and advanced fission modeling

“…This new information addresses the questions mentioned above on fission and thus better constrains nuclear data. The measurements decisively inform the evaluations either by covering for the first time for broad incident and outoing neutron energy ranges the 239 Pu PFNS with high precision [22][23][24], or by utilizing novel methods or detectors to obtain high-precision (n,f) cross sections and ] p [25][26][27]. We also use the post-scission fission event generator CGMF [28].…”