Seniority, collectivity, andB(E2)enhancement in72Ni

Gamma rays assigned to ${}_{28}^{72}$Ni${}_{44}^{}$ have been identified with Gammasphere in deep-inelastic reactions involving a 450-MeV ${}^{76}$Ge beam and a ${}^{198}$Pt target. Using a combination of spectra produced by double gates on the known 454-, 843-, and 1095-keV members of the ground-state cascade, a coincident line at 199 keV has been identified and is tentatively assigned as the ${8}^{+}\ensuremath{\rightarrow}{6}^{+}$ transition. These $\ensuremath{\gamma}$-ray coincidences have been observed only in prompt events, indicating an ${8}^{+}$ half-life below 20 ns and requiring a large $B(E2)$ enhancement compared to that expected from a seniority scheme. This value is consistent with models showing decay to a seniority $\ensuremath{\nu}=4,$ ${6}^{+}$ level that is depressed by the same two-body interaction responsible for the rather low 1095-keV ${2}_{1}^{+}$ energy, as compared to the valence-symmetry counterpart ${}_{44}^{94}$Ru${}_{50}^{}$.