Electromagnetic Transition Rate Measurements in 28Mg and Identification of Exotic Molecular Symmetries in Actinide Nuclei

Synopsis

The nuclear interaction is responsible for holding protons and neutrons together to form nuclei. While much is known about the nuclear interaction, a mathematically consistent formulation which correctly describes observable properties of nuclei and predicts the results of experiments has remained elusive. In this Thesis, the goal of developing a better understanding of the nuclear interaction is addressed through two main projects.

The primary project of this Thesis was an experiment performed at TRIUMF, Canada's particle accelerator centre. In this experiment, electromagnetic transition rates were measured to probe the structure of 28Mg and study the emergence of the non-central tensor components of the nuclear interaction near the N=20 island of inversion. Using two Doppler-shift lifetime methods, measurements of six upper limits, one lower limit, and four precision lifetimes were made for excited states in 28Mg, including precise measurements of both the 4+ to 2+ and 2+ to 0+ transitions in the yrast band. Corresponding B(E2) values for these transitions suggest that the yrast band of 28Mg has decreasing collectivity with increasing spin, a trend not predicted by most current theoretical models. They also highlight the structural evolution across the Mg isotopic chain allowing for deeper insights into the influence of tensor components of the nuclear interaction.

The second project included in this Thesis was a search through experimental data for evidence of exotic molecular symmetries. Recent advances in mean-field and point-group theoretical methods have allowed for the prediction of both the existence of exotic symmetries in nuclei and the resulting experimental signatures. Eight nuclei were identified in the actinide region of the nuclear landscape, exhibiting various levels of evidence for C2v symmetry - the symmetry of the H2O molecule. While theoretical analysis is ongoing, these eight nuclei found in a single region of the nuclear landscape and containing high-fold degeneracies motivates further studies to explore exotic symmetries of nuclear shapes.