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

Abstract

We differentiate the event-by-event n-γ multiplicity data from 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 γ-ray emission around 0.7 and 1.2 MeV, but the only directional alignment was observed for E γ ≤ 0.7 MeV and tended to be parallel and antiparallel to neutrons emitted in the same event. The emission of γ 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 γ-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-γ angular correlations for fission fragments near closed shells as a weakening of the alignment process for spherical nuclei. Lastly, we have observed that statistical γ rays are emitted isotropically, indicating tha the average angular momentum removed by this radiation is small. These results, and the analysis tools presented in this work, representing a stepping stone for future analysis of n-γ emission correlations and their connection to angular momentum properties. I. INTRODUCTIONNeutrons and γ rays emitted from fission fragments reveal important features of the nuclear fission process and the state of the fragments immediately following fission. Among several open questions in fission, the n-γ angular correlations are particularly interesting because of their intimate relation to the fission fragment angular momenta. The angular momentum of a fragments plays a pivotal role in the emission of γ rays and the n-γ angular distribution. The characterization of the fragment angular momenta is one of the most important open questions in fission physics.