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Preequilibrium Asymmetries in the ^{239}Pu(n,f) Prompt Fission Neutron Spectrum.
K J Kelly1, T Kawano1, J M O'Donnell1
1Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA.
Physical Review Letters
|March 9, 2019
Summary
Preequilibrium fission neutrons from plutonium-239 are anisotropic, contrary to current models. This study provides the first experimental evidence that their angular distribution is uncorrelated with the fission axis.
Area of Science:
- Nuclear Physics
- Nuclear Fission
Background:
- Neutron spectra from nuclear fission are crucial for understanding fission processes.
- Current fission models often assume isotropic neutron spectra, despite evidence suggesting preequilibrium components are anisotropic.
Purpose of the Study:
- To experimentally investigate the anisotropy of the preequilibrium prefission neutron component.
- To provide data for improving nuclear fission models.
Main Methods:
- Analysis of high-energy neutron spectra from ^{239}Pu induced fission.
- Separation of preequilibrium and postfission neutron contributions.
- Determination of the angular distribution of the preequilibrium component.
Main Results:
- An excess of high-energy neutrons, attributed to the preequilibrium prefission distribution, was observed.
- The angular distribution of this preequilibrium component was determined as a function of incident neutron energy and detection angle.
- Experimental evidence shows the preequilibrium angular distribution is uncorrelated with the fission axis.
Conclusions:
- The preequilibrium component of fission neutron spectra exhibits anisotropy.
- This finding challenges current isotropic fission models and necessitates their revision.
- Results impact the interpretation of prompt fission neutron spectrum measurements.
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