Directional-dependence of the event-by-event neutron-$\gamma$ multiplicity correlations in $^{252}$Cf(sf)

TL;DR

This study investigates how neutron and gamma-ray emissions in $^{252}$Cf spontaneous fission depend on emission energies and directions, revealing correlations with fragment angular momentum and excitation energy, and providing insights into fission dynamics.

Contribution

It introduces a detailed analysis of directional and energy-dependent neutron-gamma correlations in $^{252}$Cf(sf), highlighting their relation to fragment angular momentum and excitation.

Findings

Neutron emission enhances gamma emission at 0.7 and 1.2 MeV.

Directional gamma emission aligned with neutrons at energies ≤0.7 MeV.

Statistical gamma rays are emitted isotropically, indicating small angular momentum transfer.

Abstract

We differentiate the event-by-event n-$\gamma$ multiplicity data from \ce{^{252}Cf}(sf) with respect to the energies of the emitted particles as well as their relative angles of emission. We determine that neutron emission enhances $\gamma$-ray emission around $0.7$ and $1.2$ MeV, but the only directional alignment was observed for $E_\gamma \leq 0.7$ MeV and tended to be parallel and antiparallel to neutrons emitted in the same event. The emission of $\gamma$ rays at other energies was determined to be nearly isotropic. The presence of the emission and alignment enhancements is explained by positive correlations between neutron emission and quadrupole $\gamma$-ray emission along rotational bands in the de-exciting fragments. This observation corroborates the hypothesis of positive correlations between the angular momentum of a fragment and its intrinsic excitation energy. The results of this work are especially relevant in view of the recent theoretical and experimental interest in the generation of angular momentum in fission. Specifically, we have determined an alignment of the fragments angular momenta in a direction perpendicular to the direction of motion. We interpret the lack of $n$-$\gamma$ angular correlations for fission fragments near closed shells as a weakening of the alignment process for spherical nuclei. Lastly, we have observed that statistical $\gamma$ rays are emitted isotropically, indicating that the average angular momentum removed by this radiation is small. These results, and the analysis tools presented in this work, represent a stepping stone for future analysis of $n$-$\gamma$ emission correlations and their connection to angular momentum properties.