To fission or not to fission: 230Ac

Licentiate thesis, 2024

Around half of the elements heavier than iron found in the Universe are believed to have been formed during the rapid neutron capture process (r-process). During the r-process, very neutron-rich nuclei are created through a series of neutron captures and β−-decay reactions. Eventually, the formed isotopes become so heavy that they can undergo nuclear fission. Thus, fission limits the mass of isotopes formed in the r-process. Fission probabilities and fission barrier heights of neutron-rich elements are essential inputs for r-process model calculations. However, studying the fission of these isotopes is extremely challenging due to the lack of available stable beam and target combinations that can result in the formation of interesting isotopes.
A novel technique employing a light-ion transfer reaction in inverse kinematics has been developed for obtaining fission data. As proven by a pilot experiment using a stable 238U beam, the extraction of fission barrier heights using a (d,pf ) reaction in inverse kinematics in a solenoidal spectrometer is possible, indicating promising future prospects.
This licentiate thesis focuses on improving the experimental approach used in the pilot experiment. In particular, the detection efficiency can be increased dramatically by using segmented silicon detectors covering a larger solid angle compared to the gas-filled detectors for fission fragments used in the pilot experiment. The new approach is more compact and allows also for the detection of γ-rays emitted from the fission fragments.
This development work was motivated by an upcoming experiment on the search for fission of 230Ac using a radioactive 229Ac beam. Importantly, this approach is versatile and can be applied to other isotopes of interest.