Observation of Superheavy Nuclei Produced in the Reaction ofK86rwithP208b

Following a prediction by Smolańczuk [Phys. Rev. C 59, 2634 (1999)], we searched for superheavy element formation in the bombardment of ${}^{208}\mathrm{Pb}$ with 449-MeV ${}^{86}\mathrm{Kr}$ ions. We have observed three decay chains, each consisting of an implanted heavy atom and six subsequent $\alpha$ decays, correlated in time and position. In these decay chains, a rapid (ms) sequence of high energy $\alpha$ particles ( $E_{\alpha }\ge 10\mathrm{MeV}$) indicates the decay of a new high- $Z$ element. The observed chains are consistent with the formation of ${}_{}{}^{293}{}_{}{}^{}118$ and its decay by sequential $\alpha$-particle emission to ${}_{}{}^{289}{}_{}{}^{}116$, ${}_{}{}^{285}{}_{}{}^{}114$, ${}_{}{}^{281}{}_{}{}^{}112$, ${}_{}{}^{277}{}_{}{}^{}110$, ${}^{273}\mathrm{Hs}$ ( $Z=108\mathrm{}$) and ${}^{269}\mathrm{Sg}$ ( $Z=106\mathrm{}$). The production cross section is ${2.2}_{-0.8\mathrm{}}^{+2.6\mathrm{}}\mathrm{pb}$.