On the Nature of Yrast States in Neutron-Rich Polonium Isotopes

Polonium isotopes having two protons above the shell closure at Z=82 show a wide variety of low-lying high-spin isomeric states across the whole chain. The structure of neutron-deficient isotopes up to ^210Po (N=126) is well established as they are easily produced through various methods. However, there is not much information available for the neutron-rich counterparts for which only selective techniques can be used for their production. We report on the first fast-timing measurements of yrast states up to the 8^+ level in ^214,216,218Po isotopes produced in the β^- decay of ^214,216,218Bi at ISOLDE, CERN. In particular, our new half-life value of 607(14) ps for the 8_1^+ state in ^214Po is nearly 20 times shorter than the one available in literature and comparable with the newly measured half-lives of 409(16) and 628(25) ps for the corresponding 8_1^+ states in ^216,218Po, respectively. The measured B(E2;8_1^+ → 6_1^+) transition probability values follow an increasing trend relative to isotope mass, reaching a maximum for ^216Po. The increase contradicts the previous claims of isomerism for the 8^+ yrast states in neutron-rich ^214Po and beyond. Together with the other measured yrast transitions, the B(E2) values provide a crucial test of the different theoretical approaches describing the underlying configurations of the yrast band. The new experimental results are compared to shell-model calculations using the KHPE and H208 effective interactions and their pairing modified versions, showing an increase in configuration mixing when moving towards the heavier isotopes.