High-resolution study of excited $0^+$ states in mercury isotopes*

In recent years, much effort was invested in systematic studies of low-lying $0^+$ excitations in medium- to heavy-mass nuclei, ranging from $^{152}$Gd to $^{194}$Pt. This region is particularly interesting, as the structure of these nuclei changes from transitional nuclei in the Gd region, over well-deformed nuclei in the Yb region, to $\gamma$-soft nuclei in the Pt region. Recently, we moved further towards the $^{208}$Pb proton-neutron shell closure by investigating $0^+$ excitations in $^{198}$Hg, $^{200}$Hg, and $^{202}$Hg at the Q3D magnetic spectrograph in Munich. This allows us to test if the $0^+$ density can be used as a signature for the prolate-oblate shape-phase transition in the Hf-Hg region. We present and discuss the results of our high-resolution study on excited $0^+$ states in the mercury isotopes.