Onset of collective motion in neutron-deficient Po nuclei

An open question in nuclear structure physics is a detailed understanding of the onset of collective motion in nuclei with relatively few particles outside of closed shells. To study the onset of collectivity in polonium nuclei with two protons above the Z=82 shell closure, the authors have measured the in-beam transitions in neutron-deficient Po isotopes with Si-induced heavy-ion fusion evaporation reactions on enriched Yb targets. A clear signature of collective, vibrational motion would be an energy ratio R(4/2) > 2.0; for no known Po isotope has R(4/2)≥2.0 been observed. However, in 196Po the 21+ and 61+ states have come down dramatically in energy, and the yrast levels up to the 8+ show a relatively even energy spacing, again consistent with the onset of vibrational motion. In addition, low-lying 22+ and 42+ states are populated in both 196,198Po, which could be members of phonon multiplets or indicate a more collective 4p-2h excitation across the Z=82 shell gap. This paper presents the systematics of even-A Po isotopes, gives semi-classical arguments for the role of specific orbitals in the onset of collectivity, and discusses what evidence, if any, supports the existence of 4p-2h excitations in neutron-deficient Po nuclei