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 8₁⁺ 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 8₁⁺ states in 216,218Po, respectively. The measured B(E2;8₁⁺ → 6₁⁺) 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.