Structure of neutron-rich nuclei in the sd and fp shells

This research is concerned with improving our understanding of the nuclear force that holds protons and neutrons together in nuclei and governs their interactions. By studying the gamma rays emitted by particular unstable isotopes, information can be obtained on how these sub-atomic particles behave both individually and collectively. The 'nuclear shell model' views the protons and neutrons in nuclei as occupying discrete single-particle states with different energies and angular momenta. These states appear to be grouped into shells, separated by large energy gaps at so-called 'magic numbers'. Originally, the shell model was based on stable, or near-stable nuclei. As we move away from stability, new effects become apparent that adjust the structure in unexpected ways. One such change is the evolution of magic numbers with increasing neutron excess. Interactions between the constituent particles in neutron-rich nuclei are raising and lowering the energies of states, resulting in changes in their spacing and ordering. These effects can be measured by exciting nuclei, such that their nucleons occupy higher-energy states than usual. Gamma rays are then emitted as the nuclei return to the ground state; this radiation can tell us about the energy spacing and angular momentum of the excited states. In certain circumstances, new nuclei, more neutron-rich than any previously observed, can be created in excited states. This proposal aims to use both methods to determine further information about these exotic nuclear structures. There are three specific objectives of this work. The first is to study magnesium-30 nuclei. With 12 protons and 18 neutrons, this isotope is neutron-rich compared to the stable magnesium-26. Using a particle accelerator, magnesium-30 can be created at high excitation energies by colliding (for example) uranium-238 and magnesium-26 nuclei. If done inside a large array of detectors the resulting gamma rays, emitted as the nuclei relax to the ground state, can be observed. This will allow a detailed picture of the structure of this nucleus to be built up. Similarly, nickel-68 is also neutron rich, with 40 neutrons and 28 protons. This nucleus is also particularly interesting as 40 and 28 are considered magic numbers on the line of stability; however, with increasing neutron excess it is suspected that 40 ceases to be a magic number. Some of the structure of nickel-68 is already known, including long-lived states called 'isomers'. While these states often present problems for spectroscopists, this proposal aims to use them to identify when a nickel-68 nucleus has decayed. This involves using a number of detectors looking for gamma rays at different times, but correlating their data with each other. Finally, this proposal aims to test a new method of producing neutron-rich nuclei. Isotopes have previously been created by fusing two lighter nuclei together, and waiting for 2 protons to evaporate. However, this work plans to push this method further by identifying when 3 protons evaporate from a fused system. This method relies on the observation of known beta decays, which accompany the gamma rays of interest. The benefits of this work for nuclear physics are widespread. There exists a two-way relationship between experiment and theory: whilst providing new testing ground for current theory, the results themselves may be used to constrain shell-model calculations and improve their predictive power for other nuclei. The applications of this work extend to other disciplines. In astrophysics, the understanding of the formation of heavy elements in stars is based on processes that run through very neutron-rich nuclei. Current models rely on the extrapolation of data from moderately neutron-rich nuclei to predict the pathways of these processes. As data become available on ever more neutron-rich species, these calculations improve and, consequently, so does our understanding of the origins of elements.