| One of the fundamental questions in nuclear structure science is how the nucleon single-particle energies evolve with changing proton-to-neutron ratio. The nucleon magic numbers, correctly described by the shell model near the valley of beta stability, do not appear to be static across the nuclear landscape. The shifting energies of single-particle orbitals, resulting from variations in nucleon-nucleon interactions such as the tensor monopole interaction, lead to the erosion of some magic numbers, and the appearance of new subshell closures as the driplines are approached.;Near the borders of, and within the fp shell, relatively low single-particle level densities lead to a number of distinct regions of changing shell structure. The low-energy structure of nuclei near N=32 and Z=20 are stabilized by the presence of a subshell closure at N=32, a result of an energy gap between the nu2p3/2 level and the higher-lying nu2 p1/2 and nu1f5/2 levels. An open question, however, is whether or not the continued upward shift of the nu1f5/2 orbital in the Ca isotopes leads to another subshell closure at N =34. At slightly higher masses, the migration of the nu1g9/2 orbital leads to the erosion of the expected N=40 subshell closure, and the apparent development of a new region of deformation in the Cr, Mn and Fe isotopes. The question in this region is how quickly collectivity develops as a function of Z, as the nu1g9/2 orbital drops in energy with decreased occupancy of the proton nu1 f7/2 orbital below Z=28.;The beta decay and isomeric properties of nuclei in and on the border of the fp shell have been studied at the National Superconducting Cyclotron Laboratory (NSCL) using the combined experimental set-up of the NSCL beta Counting System and 16 detectors from the Segmented Germanium Array. The nuclei studied, 53,54Ca, 54,56Sc, 50K, and 61Cr, were produced in two separate experiments, through the fragmentation of a 76Ge primary beam by a 9Be target. Nuclei were implanted into a double-sided Si microstrip detector, and correlated with subsequent beta decays on an event-by-event basis. Detection of gamma rays in coincidence with the implant events permitted observation of mus isomeric states, while those detected in coincidence with decay events permitted elucidation of the populated levels in daughter nuclei.;The low-energy level structures of the neutron-rich 53,54,56Sc isotopes were investigated and compared with the expectations of the extreme single-particle model, as well as with more advanced shell-model calculations using the GXPF1, GXPF1A and KB3G effective interactions. The results confirm the N=32 subshell closure, but suggest a compression of the nu2 p1/2-nu1f5/2 spacing relative to that assumed in current effective interactions, which may preclude formation of a N=34 subshell closure in the Ca isotopes. The low-energy structure of 61Mn was probed through the beta decay of 61Cr. The structure was investigated for signs of developing deformation, but results suggest that the effects of the approaching deformation-driving nu1 g9/2 orbital are not yet significant at N=36 for the Mn isotopes. The observation of an isomer in 50K permitted investigation of the changing shell structure just below the fp shell. The isomeric structure in 50K suggests that the inversion of the pi1d3/2 and pi2s 1/2 orbitals which is known in 47K may persist above N > 28, which is at present unexpected in sd-fp cross-shell interactions. |
