Postfission properties of uranium isotopes: A hybrid method with Langevin dynamics and the Hauser-Feshbach statistical model

Background: Precise understanding of nuclear fission is crucial for experimental and theoretical nuclear physics, astrophysics, and industrial applications; however, the complete physical mechanism is unresolved due to the complexities.

Purpose: In this study, we present a new method to describe the dynamical-fission process and following prompt-neutron emission, where we combine the dynamical fission calculation based on the Langevin method and the Hauser-Feshbach statistical model.

Methods: Two methods are connected smoothly within the universal charge distribution and the energy conservation, allowing us to calculate a sequence of fission dynamics and postfission phase, including prompt neutron emission.

Results: Using a certain set of model parameters, we successfully reproduce the experimental primary-fission yields, total kinetic energy, independent-fission yields, and prompt neutron emissions for the neutron-induced fission of ²³⁶U, a compound nucleus of n +²³⁵U . We elucidate the physical mechanism of the characteristic features observed in previous experiments, such as shell properties. Additionally, we apply our calculation to two very neutron-rich uranium isotopes, i.e., ²⁵⁰U and ²⁵⁵U, which are not experimentally confirmed but are important for r-process nucleosynthesis. Theoretical results indicate that ²⁵⁰U exhibits an asymmetric multiple-peak fission yield distribution, while the neutron-rich ²⁵⁵U has a single peak due to symmetric fission. Our method predicts postneutron emission fragments, where ²⁵⁰U shows a stronger neutron emissivity than ²⁵⁵U.

Conclusions: Our framework is highly reproducible in the experiments and shows that the number of emitted neutrons after fission differs significantly in neutron-rich uranium fission depending on distributions of fission variables.