Collective levels in ${\mathrm{Ni}}^{58,60,62}$ and ${\mathrm{Zn}}^{64,66,68}$ have been studied by inelastic scattering of 43-MeV alpha particles. The relative strengths of the $2^{+}$ first excited level are proportional to values of ${{\beta }_2}^2$ (derived from Coulomb excitation) for isotopes of the same element. The ratio of strength to ${{\beta }_2}^2$ shows no significant variation with $N$ for the nuclei studied, but is strongly dependent upon $Z$. Probable two-phonon triplets can be identified in ${\mathrm{Ni}}^{60}$ and ${\mathrm{Ni}}^{62}$. The two-phonon groups in ${\mathrm{Ni}}^{58,60,62}$ reach maxima in their angular distributions about 0.2 cycle later than in elastic scattering, while the two-phonon groups in ${\mathrm{Zn}}^{64,66,68}$ reach the maxima 0.2 cycle earlier than in elastic scattering. Thus, the two-phonon groups from Ni are out of phase with those from Zn. A proposed scheme of two-phonon levels is given. Levels with three-phonon properties are found near 3.24 MeV in ${\mathrm{Ni}}^{62}$ and probably near this energy in ${\mathrm{Ni}}^{58}$ and ${\mathrm{Ni}}^{60}$. The cross section for the $3^{-}$ collective level is about 1.8 times as large in Zn as in Ni, and varies rather slowly with neutron number. It is likely that the $3^{-}$ collective levels in Zn contain a large proportion of configurations with two unpaired protons. In all six targets groups were seen in the energy range expected for combination frequencies of a $3^{-}$ phonon and a $2^{+}$ phonon. A possible second $3^{-}$ level was seen in all six targets at slightly more than 1.5 times the energy of the first $3^{-}$ collective level.

• Received 10 January 1963

DOI:https://doi.org/10.1103/PhysRev.130.1914