Stepping into the Sea of Instability: The New Sub-us Superheavy Nucleus 252Rf

We report the discovery of the new isotope ${}^{252}$Rf. With its extremely short half-life of $60_{-30}^{+90}$~ns, it expands the range of half-lives of the known superheavy nuclei by about two orders of magnitude. This nucleus was synthesized in its high-$K$ isomeric state, for which we measured a half-life of $13_{-3}^{+4}$~$\mu$s. Our results confirm a smooth onset of decreasing ground-state spontaneous fission half-lives in the neutron-deficient Rf isotopes towards the isotopic border of ${10}^{-14}$~s, which is the time needed to form an atomic shell. Our findings set a new benchmark for further exploration of phenomena associated with high-$K$ states and inverted fission-stability in the heaviest nuclei.