Disentangling the influence of excitation energy and compound nucleus angular momentum on fission fragment angular momentum

This is the authors' abstract. We don't add key points for this paper.

The origin of the large angular momenta observed for fission fragments is still a question under discussion. To address this, we study isomeric yield ratios (IYRs), i.e., the relative population of two or more long-lived metastable states with different spins, of fission products. We report on IYRs of 17 isotopes produced in the 28-MeV α -induced fission of Th 232 at the IGISOL facility of the University of Jyväskylä. The fissioning nuclei in this reaction are U * 233 , 234 , 235 . We compare our data to IYRs from thermal neutron-induced fission of U 233 and U 235 , and we observe statistically significant larger IYRs in the Th 232 ( α , f ) reaction, where the average compound nucleus (CN) spin is 7.7 ℏ , than in U 233 , 235 ( n th , f ), with average spins of 2.6 and 3.6 ℏ , respectively. To assess the influence of the excitation energy, we study literature data of IYRs from photon-induced fission reactions, and find that, within current uncertainties, the IYRs indicate no dependency of the CN excitation energy. We conclude that the different IYRs seem to be due to the different CN spins alone. This would imply that the fission fragment angular momentum only partly comes from the fission process itself and is, in addition, influenced by the angular momentum present in the CN.