From scission to metastability : Isomeric yield ratios in fission studies

Despite significant advances in nuclear physics, key questions remain open about how a nucleus splits into fragments at scission. The properties of the resulting fission fragments (FF) can be indirectly studied by detecting the products after their de-excitation. During the de-excitation process, FFs emit neutrons and γ-rays until they reach metastable states. Several nuclei have more than one long-lived metastable state, differing in spin and excitation energy, which are called isomers. The ratio of their production rates, known as the isomeric yield ratio (IYR), depends on the properties of the fission fragment and can be used to gain insight into its angular momentum. The IYRs of 22 products of the fission of 232Th induced by 28 MeV α particles were measured at the IGISOL facility at the University of Jyväskylä. Two measurement techniques were used, both relying on the mass-dependent spatial separation of the isomeric states: phase-imaging ion-cyclotron-resonance and multi-reflection time-of-flight mass-spectrometry. By comparing the newly measured IYRs with literature data from photofission and neutron-induced fission, it was found that the angular momentum of the compound nucleus undergoing fission has a significant impact on the fragments angular momentum, while its excitation energy has little to no influence. Additionally, the measured IYRs for yttrium and niobium isotopes showed an anomalously low IYR for 98Y. This unusual behavior is likely due to the shape coexistence in 98Y that was reported in previous studies. Furthermore, a model was developed to test sawtooth-like functional dependencies of the FF angular momentum on mass using the measured IYRs. Preliminary results give support to the observed sawtooth behavior of FF spin as function of mass, recently reported by Wilson et al. (2021), and the finding reported in this thesis regarding the impact of CN spin on the FFs angular momentum. Finally, a study of the level density models implemented in the nuclear model code TALYS is presented, where calculated and experimental IYRs from of a large number of nuclear reactions (excluding fission) from literature are compared. A bias was found in the models that favors the population of the high-spin states. The reason is attributed to a too wide spin-width distribution in the level density.