Discovery of new superheavy element isotopes

The first confirmation of element 114 production and decay was performed in 2009 with the Berkeley Gas-filled Separator at the Lawrence Berkeley National Laboratory 88-Inch Cyclotron. The $^{48}$Ca~+~$^{242}$Pu reaction was used. Compound nucleus evaporation residues were separated from beam and other reaction products with the Berkeley Gas-filled separator and implanted in the focal plane detector system. Production and decay of one atom each of $^{287}$114 (via the $^{242}$Pu($^{48}$Ca,~3$n)^{287}$114 reaction) and $^{286}$114(via the $^{242}$Pu($^{48}$Ca,~4$n)^{286}$114 reaction) were observed. Production cross sections, decay modes, decay energies, and half-lives and for these element 114 isotopes and their daughters were consistent with those reported by the Dubna Gas Filled Recoil Separator Group (Yuri Oganessian$,$ J. Phys. G: Nucl. Part. Phys. \textbf{34 }(2007) R165--R242). In 2010, the $^{48}$Ca~+~$^{242}$Pu reaction was used again, at an increased beam energy to optimize the production of new isotope, $^{285}$114, by the $^{242}$Pu($^{48}$Ca,~5$n)^{285}$114 reaction. The production and decay of one atom of $^{286}$114 (via the $^{242}$Pu($^{48}$Ca,~4$n)^{286}$114 reaction) was observed, re-confirming the properties of this isotope. In addition, a single event corresponding to the production and decay of $^{285}$114 (via the $^{242}$Pu($^{48}$Ca,~5$n)^{285}$114 reaction) was observed. The implantation of $^{285}$114 in the detector was followed by five \textit{$\alpha $}-decays and a spontaneous fission event, indicating the \textit{$\alpha $}-decays of new isotopes, $^{285}$114, $^{281}$Cp, $^{277}$Ds, $^{273}$Hs, $^{269}$Sg, and the spontaneous fission of new isotope, $^{265}$Rf. The decay properties of all these new isotopes match expectations based on microscopic-macroscopic mass models supplemented with extrapolations of previously reported superheavy element isotope decay properties. However, some systematic differences between observed and predicted \textit{$\alpha $}-decay $Q$-values may be used to refine models of nuclear shell effects in heavy element isotopes.