The negative parity energy levels of the even-even Ge isotopes with mass number between 64 and 76 are studied systematically by enlarging the model space of the interacting boson approximation model to include both collective and noncollective basis states. The basis states consist of ${\mathit{N}}_{\mathit{B}}$-1 sd-boson plus a f-boson configuration and ${\mathit{N}}_{\mathit{B}}$-1 sd boson plus a fermion pair configuration. The fermions are allowed to occupy the ${\mathit{f}}_{5/2}$ and ${\mathit{g}}_{9/2}$ single-particle orbitals, respectively. It was found that the negative parity energy levels of ${}_{}{}^{64\mathrm{-}76}{}_{}{}^{}\mathrm{Ge}$ nuclei can be described reasonably well. The intensities of the collective configuration in $3^{\mathrm{-}}$ states increase when going from nucleus ${}_{}{}^{64}{}_{}{}^{}\mathrm{Ge}$ to nucleus ${}_{}{}^{72}{}_{}{}^{}\mathrm{Ge}$ and decrease from nucleus ${}_{}{}^{72}{}_{}{}^{}\mathrm{Ge}$ to nucleus ${}_{}{}^{74}{}_{}{}^{}\mathrm{Ge}$. The B(E3;$3_1^{\mathrm{-}}$→$0_1^{+}$) values are calculated and compared with the available observed data.

• Received 11 December 1991

DOI:https://doi.org/10.1103/PhysRevC.47.183