Spectroscopic study of the64,66,68Ni isotopes populated in64Ni + 238U collisions

Excited states in ${}^{64}$Ni, ${}^{66}$Ni, and ${}^{68}$Ni were populated in quasielastic and deep-inelastic reactions of a 430-MeV ${}^{64}$Ni beam on a thick ${}^{238}$U target. Level schemes including many nonyrast states were established up to respective excitation energies of 6.8, 8.2, and 7.8 MeV on the basis of \ensuremath{\gamma}-ray coincidence events measured with the Gammasphere array. Spin-parity assignments were deduced from an angular-correlation analysis and from observed \ensuremath{\gamma}-decay patterns, but information from earlier \ensuremath{\gamma}-spectroscopy and nuclear-reaction studies was used as well. The spin assignments for nonyrast states were supported further by their observed population pattern in quasielastic reactions selected through a cross-coincidence technique. Previously established isomeric-state decays in ${}^{66}$Ni and ${}^{68}$Ni were verified and delineated more extensively through a delayed-coincidence analysis. A number of new states located above these long-lived states were identified. Shell-model calculations were carried out in the ${p}_{3/2}{f}_{5/2}{p}_{1/2}{g}_{9/2}$ model space with two effective interactions using a ${}^{56}$Ni core. Satisfactory agreement between experimental and computed level energies was achieved, even though the calculations indicate that all the states are associated with rather complex configurations. This complexity is illustrated through the discussion of the structure of the negative-parity states and of the M1 decays between them. The best agreement between data and calculations was achieved for ${}^{68}$Ni, the nucleus where the calculated states have the simplest structure. In this nucleus, the existence of two low-spin states reported recently was confirmed as well. Results of the present study do not indicate any involvement of collective degrees of freedom and confirm the validity of a shell-model description in terms of neutron excitations combined with a closed $Z$ $=$ 28 proton shell. Further improvements to the calculations are desirable.