Nanosecond isomers and the evolution of collectivity in stable, even- A Hg isotopes

Isomeric states and associated collective structures have been studied up to high spin in $^{198,200,202}\mathrm{Hg}$ using multinucleon transfer reactions and the Gammasphere array. A coupled rotational band, with possible four-quasiparticle character, is established in $^{198}\mathrm{Hg}$. Sequences built on two-quasiparticle, positive- and negative-parity levels are assigned to $^{202}\mathrm{Hg}$. New isomers in $^{202}\mathrm{Hg}$ with ${I}^{\ensuremath{\pi}}=({7}^{\ensuremath{-}})$ and $({9}^{\ensuremath{-}})$, and ${T}_{1/2}$ = 10.4(4) ns and 1.4(3) ns, respectively, have been identified. A half-life of 1.0(3) ns is established for the ${I}^{\ensuremath{\pi}}={12}^{+}$ state in $^{200}\mathrm{Hg}$. $B(E2)$ values deduced from isomeric transitions in Hg isotopes indicate that, while collectivity near the ground state gradually diminishes from $N$ = 112 to $N$ = 124, it is found to increase for the ${12}^{+}$ and ${9}^{\ensuremath{-}}$ states up to $N$ = 118, followed by a reduction for higher neutron numbers. Calculations using the ultimate cranker code provide insight into the variation of deformation with spin and allow for an understanding of observed band crossings. The evolution of collectivity with spin, and along the isotopic chain, is described.