Ph.D. Doctoral Dissertation Defense in Physics: Kartikeya Sharma 11/21

Brief Abstract: Very neutron-rich nuclei in the A≈190 region is of interest from both nuclear structure and nuclear astrophysics perspectives. Long-lived isomers at high angular momenta in this region, predicted for decades, e.g., in neutron-rich Hf (Z=72) isotopes, are of interest in probing the limits of approximate symmetries that lead to the conservation of the K quantum number, defined as the projection of the total angular momentum of a deformed nucleus on its symmetry axis. The nuclei in this region also border the r-process pathway, with their β-decays relevant for understanding heavy-element nucleosynthesis. Their spectroscopy, however, has remained elusive to date as they are difficult to populate with the more standard fusion and multi-nucleon transfer reactions.

An experiment was conducted at the National Superconducting Cyclotron Laboratory at Michigan State University to study neutron-rich nuclides in the Hf-Ta-W region through a first fragmentation reaction of a newly developed 198Pt primary beam, the analysis of which constitutes the entirety of this thesis. Additional motivations for this experiment included discovery of new isotopes, as well as comparing angular momenta imparted to the fragments with different production targets. The heavy fragments were selected with an electromagnetic separator, momentum-analyzed with beam optics specially designed for this experiment and implanted into a stack of Si detectors. This allowed for full event-by-event particle identification (A, Z, Q) using measured energy loss, magnetic rigidity, total kinetic energy and time-of-flight parameters.

The Si stack was surrounded by the GRETINA γ-ray tracking array, which provided efficient detection of γ-ray cascades following isomer decays. Multi-quasiparticle isomers with half-lives ranging from a few hundred nanoseconds to a few milliseconds, were identified, together with new neutron-rich isotopes observed for the first time.

The current work identified 33 isomers in 26 nuclear species between Hf(Z=72) and Os(Z=76), of which 14 were observed for the first time. Previously known isomers with angular momenta as high as 18 ̄h were populated, arguably some of the highest populated in fragmentation reactions at this facility. Level schemes could be deduced in a few cases where the production cross-sections allowed sufficient statistics to be accumulated for γ-γ coincidences. In the rest, the first spectra provide a base for future work to deduce level structures with higher statistics. This thesis focuses on the analysis of the half-lives and the decay spectroscopy of multi-quasiparticle isomers, primarily in Hf, Ta and W nuclei, and provides experimental data for testing existing theories of shape evolution in this neutron-rich landscape.