Revealing the nature of yrast states in neutron-rich polonium isotopes

Abstract

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 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 ${}^{216,218}$Po, respectively. The measured $B(E2;8_1^{+}\to 6_1^{+})$ 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 towards the heavier isotopes.

6 pages, 5 figures (main manuscript), 2 pages (supplemental material)