Nuclear structure near N = Z = 28: Study of neutron-deficient nickel isotopes via one-neutron knockout and intermediate-energy Coulomb excitation

Two experiments were performed at the National Superconducting Cyclotron Laboratory to study nuclear structure in the vicinity of the N = Z = 28 doubly-magic shell closure. The 56Ni nucleus is the lightest radioactive doubly-closed-shell nucleus. Previous measurements of large reduced transition probabilities to the first excited state of 56Ni have suggested that the properties of radioactive doubly-magic nuclei may not mirror those of stable closed-shell nuclei.; The single-particle structure of the neighboring nucleus 57Ni was studied via one-neutron knockout. Spectroscopic factors of C 2S = 0.56(11) to the ground state and C 2S = 4.3(3) to excited states of the core fragment 56Ni were measured for the ground state of 57Ni. The measured spectroscopic factors only exhaust 50–60% of those predicted by the nuclear shell model. The measured reductions are consistent with those observed from other nucleon-knockout and electron scattering experiments on well-bound stable nuclei. In addition to other known excited states, an excited state in 56Ni was observed at 5661(72) keV with de-excitation gamma rays of 1726(10) keV and 3027(71) keV. This confirms the existence of a state previously suggested at 5668 keV, but due to angular momentum selection rules does not agree with the suggested spin and parity of J π = 6+.; The excited states and reduced transition probabilities for the nuclei 52Fe and 54,55,56,58Ni were measured in an intermediate-energy Coulomb excitation experiment. A B(E2 $\uparrow$) of 494(119) e2fm4 to the first excited state of 56Ni was measured, and a weighted average of 548(51) e2fm4 for the three previous measurements and the current measurement was determined. The average value was found to disagree with a theoretical prediction which assumed a structure for 56Ni like that for the doubly-magic stable nuclei. An energy of 1396(5) keV for the first excited state of 54Ni was measured for the first time. The transition probability to the first excited state was measured to be B(E2 $\uparrow$) = 626(169) e2fm4. The energy of the excited state and B(E2 $\uparrow$) for 54Ni are both comparable to the same quantities for the mirror nucleus 54Fe. A shell model calculation predicted that the transition probability for 54Ni would be lower than that for doubly-magic 56Ni, while the opposite trend was observed. The doubly-magic status of the 56Ni nucleus is reinforced, although the disagreement between the shell-model predictions and measured transition probabilities for 56Ni still leave the strength of the N = Z = 28 shell closure in question. (Abstract shortened by UMI.)...