Nuclear structure evolves as a progression is made from stable to exotic nuclei. The energy spacings between the single-particle orbitals that protons and neutrons occupy are not constant and change based on the ratio of protons to neutrons in a nucleus. The promotion of nucleons across an energy gap into a higher shell can drive nuclei from spherical to deformedshapes. When a deformed state arising from the filling of a higher orbital has a similar energy to a spherical state in which there is no occupation of the higher orbital, the situation is known as shape coexistence. Due to nuclear-structure evolution, in radioactive nuclei, the energy of an intruder orbital that originates from a higher shell near stability can become close to the energy of a lower-shell orbital. Therefore, the filling of intruder orbitals is often associated with nuclear deformation and low-energy shape coexistence. Demonstrating the evolution of nuclear structure, rapid changes have been observed in the properties of neutron-rich nuclei with neutron numbers near 40. A subshell closure has been proposed at N = 40 in 6828Ni40 based on the energy gap between the upper fp shell and the 0g9/2 orbital. However, collectivity rapidly develops in the 6626Fe40 and 6424Cr40 isotopes. Ground-state deformation, largely interpreted in terms of the occupation of the intruder ν0g9/2 orbital, has accordingly been inferred for 66Fe and 64Cr. Two separate experiments were performed at the National Superconducting Cyclotron Laboratory that provided insight about the excitation of neutrons out of the fp shell in theneutron-rich nuclei with approximately 40 neutrons and 28 or fewer protons. The nuclei studied were populated primarily through β decay. Isomeric decays of metastable excited states were also observed. In each experiment, the radioactive ion beam was deposited into a semiconductor detector that was used to detect both the implantation of the ions and their subsequent β decays. The β-decay detector was surrounded by an ancillary array of high-purity Ge detectors for gamma-ray detection. The results of both experiments complement prior work and emphasize the important role of the ν0g9/2 orbital in the low-energy levelschemes of the neutron-rich nuclei near N = 40. One experiment investigated the decay of the first excited state in 68Ni populated through the β decay of 68Co, and the analysis of decay events relied heavily on pulse-shape analysis. The ν0g9/2 orbital has an impact on the low-energy level scheme of 68Ni, and the first excited state is due to the excitation of a pair of neutrons across the N = 40 gap. The energy of the first excited state in 68Ni was measured precisely and determined to be significantly lower than the value reported previously. The decay strength of the excited state was also determined precisely. Interpreting within a simple two-level mixing model, the experimental results are consistent with Monte Carlo shell-model calculations that predict shape coexistence between a spherical 68Ni ground state and an oblate first excited state. A second experiment involved the study of the β decay of 61,63V to the odd-A 61,63Cr37,39 isotopes, where data were previously scarce. The inferred level scheme of 61Cr has anincreased low-energy level density relative to the neighboring lower mass odd-A Cr isotopes. The change in the level scheme of 61Cr was attributed to the onset of deformation resulting from the excitation of neutrons out of the fp shell. The isomeric decays of metastable states in 64,6625Mn39,41 were also investigated. Based on the inferred low-energy level schemes, shape coexistence was proposed, with the ground states of 64,66Mn having deformed shapes butthe isomeric states being spherical.
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