Discovery eliminates gap in neutron reaction modeling

A neutron interacting with a nucleus at low energies shows a strong resonating behavior due to the formation of a compound nucleus. As neutron energy increases, individual resonances start overlapping and the compound nucleus becomes more unstable. In a new paper published in Physical Review C, scientists from Los Alamos National Laboratory’s Theoretical division demonstrate for the first time how fluctuations in the nuclear interaction are generated by the resonances and then die away as the energy increases.
 
Read the paper
 
Why this matters: With a better understanding of fluctuations in nuclear phenomena, scientists can develop simulations that more accurately predict what happens in nuclear applications. Current codes approximate the transition region from low to high energy using fictitious resonances. 

• In the context of this paper, reactions on uranium-238, following a neutron striking a U-238 nucleus, produce an energetic nucleus that can emit its energy through a number of methods, which need to be modeled in a consistent manner.
• Although U-238 does not undergo nuclear fission, the method is applicable to fission reactions, which are important drivers for energy production and reactors that produce plutonium.
• For national security missions, this work could reduce nuclear data uncertainties at energy ranges relevant to nuclear weapons. 

What they did: The authors proposed a path around this longstanding nuclear theory deficiency by describing the smooth transition using the Random Matrix Theory, where the interactions in the compound nucleus are constructed through the Gaussian Orthogonal Ensemble. 
 
Funding: The Laboratory Directed Research and Development program at Los Alamos National Laboratory. 

LA-UR-26-21283