Isovector dipole and quadrupole excitations in66Zn

Abstract

The nucleus ${}^{66}\mathrm{Zn}$ is investigated using $\ensuremath{\gamma}\ensuremath{\gamma}$-coincidence studies following the $\ensuremath{\beta}$ decay of the ${0}^{+}$ ground state of ${}^{66}\mathrm{Ga}.$ The data survey short-lived ${J}^{\ensuremath{\pi}}{=1}^{+}{,2}^{+}$ states, $E2/M1$ mixing ratios, and, together with the previously known lifetimes, also absolute transition strengths $B(\ensuremath{\sigma}L).$ The proton-neutron mixed-symmetry states ${2}_{\mathrm{ms}}^{+}$ (isovector quadrupole excitation) and ${1}_{\mathrm{ms}}^{+}$ (``scissors mode'') were identified from their decay patterns and absolute transition strengths. A $\ensuremath{\gamma}$ transition between these mixed-symmetry states was observed and its rate is discussed. We additionally present a candidate for the two-phonon ${2}_{2,ms}^{+}$ state, whose structure is closely related to the ${1}_{\mathrm{ms}}^{+}$ excitation. Fifteen $M1$ and $E2$ strengths involving mixed-symmetry states show a reasonable agreement with the prediction of the pure O(6) limit of the proton-neutron extension of the interacting boson model (IBM-2) using the effective quadrupole boson charges ${e}_{\ensuremath{\pi}}{+e}_{\ensuremath{\nu}}$ and the g-factor difference ${g}_{\ensuremath{\pi}}\ensuremath{-}{g}_{\ensuremath{\nu}}$ as the only free parameters.