Level structure of $^{67}$Ni and the nature of yrast excitations near N=40

The region of nuclei around $^{68}$Ni$_{40}$, with its closed proton shell and closed neutron harmonic-oscillator sub-shell, continues to be of much interest as recent data indicate the stabilizing effects of the N=40 gap are rather localized in N and Z. With a single neutron hole in $^{68}$Ni, the structure of $^{67}$Ni provides an opportunity for detailed comparisons with calculations using different effective interactions and for learning about the nature of the yrast excitations. A level scheme built on the previously known 13-$\mu$s isomer has been delineated up to an excitation energy of ~5.3 MeV and a spin of 21/2. Shell model calculations have been carried out using two effective interactions in the $f_{5/2}pg_{9/2}$ model space with $^{56}$Ni as a core. Satisfactory agreement between experiment and theory is achieved. Comparisons between experiment and calculations indicate that the yrast and near-yrast excitations are dominated by complex multi-particle-hole excitations. This work is supported in part by the U.S. DOE, Office of Nuclear Physics under contracts DE-AC02-06CH11357 and DE-FG02-94-ER40834.