Electric Monopole Transition Strengths in Stable Nickel Isotopes

Electric monopole ($E0$) transition strengths are a sensitive probe for investigating nuclear structure and shape coexistence. There is a need for $E0$ transition strengths in closed shell nuclei in order to develop our understanding of the mechanisms responsible for the generation of electric monopole strength. Simultaneous detections of $\gamma$ rays and internal conversion electrons must be measured in order to determine an $E0$ transition strength. A series of measurements in the stable nickel isotopes were performed at the Australian National University. Excited states in $^{58,60,62}$Ni were populated via inelastic proton scattering. The CAESAR array of Compton-suppressed HPGe detectors was used to measure the ($E2/M1$) mixing ratio of transitions from angular distributions of $\gamma$ rays. The Super-e spectrometer was used to measure electron-gamma branching ratios in order to extract $E0$ transition strengths for a number of $J^\pi \rightarrow J^\pi$ transitions. An overview of the experiments will be presented, along with preliminary results for $E0$ transition strengths between $J^{\pi} \neq 0$ states in the semi-magic nuclei, $^{58, 60, 62}$Ni. A comparison with the matrix elements obtained from a new microscopic model for $E0$ transitions will be made.