Physics and Astronomy PhD Defense

David Little

“Nature of Low-Spin Excitations and Shape Coexistence in 64Ni”

The structure of 64Ni, the heaviest stable Ni isotope, has been investigated to search for shape coexistence, a phenomenon recently observed in neutron-rich 66Ni and 70Ni as well as in “doubly-magic”, N = 40, 68Ni. In the latter case, the ground state is associated with a spherical shape, the 0_2+ level was shown to be oblate, and the 0_3+ one prolate. Such a prolate 0_2+ level has also been reported in 70Ni and, within the last two years, four 0+ states have been established in 66Ni, with the 0_1+ and 0_3+ levels being spherical, and the 0_2+ and 0_4+ ones oblate and prolate, respectively. The present search for evidence of these shapes in 64Ni was stimulated by recent, state-of-the-art Monte Carlo shell-model calculations (MCSM), where the Hamiltonian includes effective interactions incorporating the monopole tensor force found to play an essential role in describing shell evolution in neutron-rich nuclei. In a first experiment, the wavefunctions of the low-spin excitations in 64Ni were probed through reduced transition probabilities, B(E2), obtained from a high-statistics Coulomb excitation measurement. The structure of the nucleus at higher spin and excitation energy was also explored further through high-fold coincidence data collected following deep-inelastic reactions. From comparisons between the new results and MCSM and other shell-model calculations, a clearer picture of the structure of 64Ni emerges. Specifically, most low-spin states are dominated by proton and neutron excitations within the fp shell, although a few involve the g_9/2 shape-driving neutron orbital. The agreement between data and MCSM results argues for a 0_2+ level of small oblate deformation, and a spherical 0_3+ state. The calculated lack of collective excitations along the yrast line has now been shown experimentally to persist in the spin 16-17 range up to 17.9 MeV excitation energy. Finally, the small upper limit determined for the B(E2) probability of a transition associated with the decay of a 3463-keV, 0_4+ state discovered recently agrees with its proposed assignment to a prolate shape, herewith providing first evidence for triple shape coexistence in a stable Ni isotope.

This defense will take place remotely over Zoom. Please see department listservs for details.