48Ca+249Bk Fusion Reaction Leading to Element Z=117: Long-Lived α-Decaying 270Db and Discovery of 266Lr

The superheavy element with atomic number Z = 117 was produced as an evaporation residue in the Ca-48 + Bk-249 fusion reaction at the gas-filled recoil separator TASCA at GSI Darmstadt, Germany. The radioactive decay of evaporation residues and their alpha-decay products was studied using a detection setup that allowed measuring decays of single atomic nuclei with half-lives between sub-mu s and a few days. Two decay chains comprising seven a decays and a spontaneous fission each were identified and are assigned to the isotope (294)117 and its decay products. A hitherto unknown alpha-decay branch in (270)Db (Z = 105) was observed, which populated the new isotope Lr-266 (Z = 103). The identification of the long-lived (T-1/2 = 1.0(-0.4)(+1.9) h) alpha-emitter (270)Db marks an important step towards the observation of even more long-lived nuclei of superheavy elements located on an island of stability.