| The double shell closure at N = Z = 28 in 56Ni has been investigated through the measurement of the magnetic moment of a nucleus one neutron removed from this core. Nuclear moments are fundamental, measurable properties that provide information on the structure of nuclei. The magnetic moments of doubly closed nuclei +/- 1 nucleon are of particular importance, since the properties of each of these nuclei are determined by the orbit occupied by that last nucleon. Any deviation from theory indicates the presence of higher order effects such as configuration mixing, meson exchange currents, isobar excitation, and/or even a breakdown of the shell closure.;The 56Ni core has been shown to be soft, attributed to the strong proton-neutron interaction, in comparison to the 48Ca core. The small magnetic dipole moment of 57Cu, with Tz = --1/2 and residing one proton outside 56Ni, suggests the double shell closure at proton and neutron numbers 28 is broken. However, the experimental ground state magnetic moments of the Tz = +1/2 nuclides 57Ni and 55Co agree well with shell model predictions, albeit with a "soft" 56Ni core. The ground state magnetic moment of 55Ni, also with Tz = --1/2 but with one neutron removed from the 56Ni core can provide critical insight on the nature of the 56Ni core, and can be a basis to understand how the structure of doubly-magic nuclei may change away from stability.;The nuclear magnetic moment of the ground state of 55Ni (Ipi = 3/2--, T1/2 = 204 ms) has been deduced in this work to be |mu( 55Ni)| = (0.976 + 0.026) muN using the beta-NMR technique. A polarized beam of 55Ni was produced by fragmentation of a 58Ni primary beam at energy 160 MeV/nucleon in a Be target. The A1900 and RF Fragment separators were used to eliminate all other beam contaminants. Results of a shell model calculation using the GXPF1 interaction in the full fp shell model space was found to reproduce the experimental value and support a softness of the 56Ni core. Together with the known magnetic moment of the mirror partner 55Co, the isoscalar spin expectation value ⟨sumsigmaz⟩ = 0.91 + 0.07 shows a similar trend to that established in the sd shell. Effective g factors for the A = 28 system obtained from a fit to isoscalar magnetic moments, isovector moments, and M1 decay matrix elements were applied to matrix elements for A = 55 calculated with the GXPF1 interaction to obtain ⟨sumsigma z⟩ for A = 55. The results of the calculation show the best agreement with the experimental value for both mu and ⟨sumsigma z⟩ and imply that a universal operator can be applied to both the sd and fp shells. |
