Isomeric yield ratios in nuclear fission

Abstract: Isomeric yield ratios (IYR), referring to the relative yield of the high spin states of a nucleus to the total yield of all observed states, is an observable of nuclear fission that has the potential to improve our understanding of the fission dynamics. Apart from that, systematic observations of IYRs can also contribute to other areas, such as the validation of fission models, modelling of the r-process in stellar nucleosynthesis, studies of the antineutrino mixing angle, the safety of present-day nuclear reactors, and the design of advanced reactor systems.With these motivations in mind, an IYR measurement in proton-induced fission was performed at the IGISOL facility in Jyväskylä, Finland. In the measurement, the Penning trap JYFLTRAP was used to separate the excited state from the ground state and to project those onto a position-sensitive MCP detector. The obtained phase images were used to train a Bayesian Gaussian Mixture model to identify the states. After considering corrections for the detector efficiency and radioactive decay, 19 IYRs were obtained.In this thesis, the measurement of IYRs with the IGISOL technique is described, and a systematic study of IYRs in proton-induced fission is presented. Furthermore, the measured ratios are compared with calculations using three different models: the Madland-England (ME) model, the fission model GEF, and the combination of GEF + TALYS. The experimental results show that, in general, the IYR decreases with the spins of measured states. While this, to some degree, confirms the ME model, variations in the IYR between nuclides with the same spin assignments reveal that the model is too simple to predict individual ratios. Furthermore,discrepancies in the IYRs between the measurement and GEF are observed in most cases, indicating a need to optimize the performance of GEF against nuclear data from proton-induced fission. The combination of GEF + TALYS results in an overall under estimation of the observedIYRs, which could be explained by the different assumptions used in GEF and TALYS.To investigate how the angular momenta of the primary fission fragments relate to the IYRs, a surrogate model of GEF has been developed. By reproducing the measured IYR with the calculated ratios from the model, the average angular momentum Jav, is deduced. The Jav for fission products with a mass number greater than 131 show a mass dependency which fits the parameterisation proposed by J. Wilson et al,. For IYRs in the mass region 119 ≤ A < 132, in which no measurements are presented by Wilson, a decrease in the Jav with increasing mass number is observed for the first time.Besides the study of IYRs, a benchmark of a multi-physics simulation model of the ion guide for neutron-induced fission products has been performed using γ-ray spectroscopy data. The results of the benchmark show that the high-energy part of the neutron flux from the simulation with MCNPX is overestimated by about 40 %, while the ion transportation simulated with GEANT4 agrees well with the experimental data. Based on the benchmark, the ion guide can be optimized to achieve high enough intensities of the collected ions to reach reasonable measurement times. In the next step, the addition of electric fields is considered to direct the ions in and to reduce the ion drifting time. However, this task lies outside the scope of this PhD thesis.