Revealing 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 ^{210}Po  (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 ^{214,216,218}Po isotopes produced in the β^{-} decay of ^{214,216,218}Bi at ISOLDE, CERN. In particular, our new half-life value of 607(14) ps for the 8_{1}^{+} state in ^{214}Po is nearly 20 times shorter than the value available in the literature and comparable with the newly measured half-lives of 409(16) and 628(25) ps for the corresponding 8_{1}^{+} states in ^{216,218}Po, respectively. The measured B(E2;8_{1}^{+}→6_{1}^{+}) transition probability values follow an increasing trend relative to isotope mass, reaching a maximum for ^{216}Po. The increase contradicts the previous claims of isomerism for the 8^{+} yrast states in neutron-rich ^{214}Po 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 toward the heavier isotopes.