Experimentele en theoretische studies van β-vertraagde fissie

The process of β-delayed fission (βDF) consists of a mother nucleus β-decaying to different states of the daughter, and fission of the latter with a probability that strongly depends on the energy of the states populated by the decay. The energy window of βDF is limited by the Q-value of the mother decay, and because this is always smaller than, or at most a few MeV higher than the primary fission barrier of the daughter nucleus, the fission step of βDF is classified as low-energy fission. As such, the microscopic properties of the nuclei involved are predominant in βDF, which then offers a great tool to investigate how these nuclear characteristics influence the outcome of the whole process. Moreover, βDF is an interesting process from the nuclear astrophysics point of view due to the role it might play in the termination of r-process nucleosynthesis. Ideally one should estimate the βDF probability (PβDF) of the very neutron-rich nuclei involved in the r-process. However, these cannot be produced in the laboratory, and theoretical models need to be developed to make predictions where the experimental results cannot be obtained. So far, most of the βDF cases studied experimentally can be found in the neutron-deficient side of the nuclear chart, while only a handful of cases have been observed in the neutron-rich. As a consequence, accurate models are needed to make extrapolations to nuclei that are extremely far away from the experimental values they are benchmarked on. To provide more experimental results on the neutron-rich side, an experiment was performed at the ISOLDE facility (CERN) with the goal of studying βDF of 230,232Fr and 230,232,234Ac. No fission fragments were observed for any of the nuclei investigated, so only upper limits for their PβDF could be determined. At the same time, a theoretical framework to calculate βDF probabilities was developed. This approach uses energy-density functional based models to obtain the different inputs needed for the final probabilities calculations, which are done with the code TALYS. In particular, the proton-neutron quasiparticle random-phase approximation (pnQRPA) with the D1M parametrisation of the Gogny effective interaction was used to obtain the β-feeding function of the mother nucleus. On the other hand, the BSkG3 parametrisation of the Skyrme effective interaction was used to calculate the potential energy surface of the daughter nucleus from which the fission path was extracted using the code PyNEB. The calculations were performed for all the experimentally known cases of βDF. The nuclei for which finite PβDF values are reported were used to calculate a root-mean-square (RMS) deviation between the calculated probabilities and the experimental ones. After some optimisation of the theoretical framework to better reproduce the experimental PβDF, an RMS deviation of about two orders of magnitude was found. Even though more improvements to the theoretical framework could be applied in the future, it can already be used as prediction tool to identify new βDF cases that can be experimentally measured in different facilities. To do this the RMS deviation of two orders of magnitude found from the comparison with the experimental results is used as uncertainty range on the newly calculated probabilities.