Abstract
Prompt fission neutron spectrum (PFNS) evaluations use provide nuclear data for the PFNS across a wide range of incident and outgoing neutron energies. However, experimental data underlying the evaluation are sparse, inconsistent, and incomplete with respect to the desired incident and outgoing energy coverage. As such, evaluations sometimes predict features of the PFNS, such those relating to multi-chance fission and pre-equilibrium pre-fission neutron emission, without any experimental validation. The Chi-Nu experiment at Los Alamos National Laboratory has recently obtained high-precision results for the 239Pu and 235U PFNS which, for the first time in both cases, have shed light on multi-chance fission and pre-equilibrium contributions to the observed fission neutron spectrum. In addition to providing the first experimental data on some of these fission properties, the angular coverage of the Chi-Nu experiment allows for the extraction of angular distributions of pre-equilibrium pre-fission neutrons. PFNS results of multi-chance fission and pre-equilibrium pre-fission neutron emission are discussed in this proceedings in terms of the observed neutron spectrum and the average PFNS energies.
Highlights
Knowledge of the energy spectrum of neutrons emitted promptly following neutron-induced fission, i.e., the prompt fission neutron spectrum (PFNS), is essential simulating for any system relating to neutron-driven chain reactions
Measurements of the PFNS are commonly tagged on a fission signal, but strictly speaking, neutrons measured in coincidence with this fission signal are not guaranteed to be “prompt fission neutrons”, i.e., neutrons emitted promptly following fission
Existing data are lacking in that few experiments measure the actinide PFNS at multiple incident neutron energies, and the few previous whitesource PFNS measurements of 239Pu resulted in discrepancies
Summary
Knowledge of the energy spectrum of neutrons emitted promptly following neutron-induced fission, i.e., the prompt fission neutron spectrum (PFNS), is essential simulating for any system relating to neutron-driven chain reactions. A common simplified picture of neutron-induced fission on, for example, 239Pu is that the excited 240Pu daughter nucleus fissions into, typically, two fragments that emit neutrons and γ rays. This type of fission is referred to as first-chance fission and is the most likely type of fission. [1, 15,16,17,18,19] and references therein for further theoretical discussions of these fission processes
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