Stability of the heaviest elements: K isomer in 250No

Decay spectroscopy of No-250 has been performed using digital electronics and pulse-shape analysis of the fast nuclear decays for the first time. Previous studies of No-250 reported two distinct fission decay lifetimes, related to the direct fission of the ground state and to the decay of an isomeric state but without the possibility to determine if the isomeric state decayed directly via fission or via internal electromagnetic transitions to the ground state. The data obtained in the current experiment allowed the puzzle to finally be resolved, attributing the shorter half-life of t(1/2) = 3.8 +/- 0.3 mu s to the ground state and the longer half-life t(1/2) = 34.9(-3.2)(+3.9) mu s to the decay of an isomeric state. No-250 becomes, thus, one of a very few examples of very heavy nuclei with an isomeric state living considerably longer than its ground state. This phenomenon has important consequences for the nuclear-structure models aiming to determine the borders of the island of stability of superheavy elements.