Ca48+Bk249 Fusion Reaction Leading to Element Z =117: Long-Lived α-Decaying Db270 and Discovery of Lr266

The superheavy element with atomic number Z =117 was produced as an evaporation residue in the Ca48+Bk249 fusion reaction at the gas-filled recoil separator TASCA at GSI Darmstadt, Germany. The radioactive decay of evaporation residues and their α-decay products was studied using a detection setup that allowed measuring decays of single atomic nuclei with half-lives between sub-μs and a few days. Two decay chains comprising seven α decays and a spontaneous fission each were identified and are assigned to the isotope 117294 and its decay products. A hitherto unknown α-decay branch in Db270 (Z =105) was observed, which populated the new isotope Lr266 (Z =103). The identification of the long-lived (T_1/2=1.0-0.4+1.9 h) α-emitter Db270 marks an important step towards the observation of even more long-lived nuclei of superheavy elements located on an "island of stability."