We have studied via in-beam γ-ray spectroscopy ${}_{}{}^{196}{}_{}{}^{}\mathrm{Po}$ and ${}_{}{}^{198}{}_{}{}^{}\mathrm{Po}$, which are the first neutron-deficient Po isotopes to exhibit a collective low-lying structure. The ratios of yrast state energies and the E2 branching ratios of transitions from non-yrast to yrast states are indicative of a low-lying vibrational structure. The onset of collective motion in these isotopes can be attributed to the opening of the neutron ${\mathit{i}}_{13/2}$ orbital at N≊112 and the resulting large overlap between the two valence protons in the ${\mathit{h}}_{9/2}$ orbital and the valence neutrons in the ${\mathit{i}}_{13/2}$ orbital.

• Received 28 February 1995

DOI:https://doi.org/10.1103/PhysRevC.52.621