Vibrational levels in the 86-neutron nucleusGd150

The decay of 3.6-h ${}_{}{}^{150}{}_{}{}^{}\mathrm{Tb}$ to levels in ${}_{}{}^{150}{}_{}{}^{}\mathrm{Gd}$ has been studied by $\gamma$-ray and conversion-electron spectroscopy. A level scheme is proposed which accounts for all but 35 of the 256 $\gamma$ rays assigned to this decay. Most of the low-lying levels in ${}_{}{}^{150}{}_{}{}^{}\mathrm{Gd}$ show a remarkably close correspondence with the predictions of the simple vibrator model. Possible candidates are observed for three-phonon quadrupole states and for the states coupling quadrupole and octupole phonons. The low-lying positive-parity levels can be adequately described by the phenomenological collective model of Gneuss and Greiner. The microscopic boson-expansion model of Kishimoto and Tamura, however, appears to predict ${}_{}{}^{150}{}_{}{}^{}\mathrm{Gd}$ to be more transitional than the data imply. Systematics of the 86-neutron isotones show increased stability at 64 protons. This can be correlated with the filling of the $g_{\frac{7}{2}}$ and $d_{\frac{5}{2}}$ spherical proton orbitals.