${}^{48}\mathrm{Ca}+{}^{249}\mathrm{Bk}$ Fusion Reaction Leading to Element $Z=117$: Long-Lived $\alpha$-Decaying ${}^{270}\mathrm{Db}$ and Discovery of ${}^{266}\mathrm{Lr}$

The superheavy element with atomic number $Z=117$ was produced as an evaporation residue in the ${}^{48}\mathrm{Ca}+{}^{249}\mathrm{Bk}$ 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-$\mu \mathrm{s}$ and a few days. Two decay chains comprising seven $\alpha$ 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 ${}^{270}\mathrm{Db}$ ($Z=105$) was observed, which populated the new isotope ${}^{266}\mathrm{Lr}$ ($Z=103$). The identification of the long-lived ($T_{1/2}={1.0}_{-0.4}^{+1.9}\mathrm{h}$) $\alpha$-emitter ${}^{270}\mathrm{Db}$ marks an important step towards the observation of even more long-lived nuclei of superheavy elements located on an “island of stability.”

Figure 1

Decay chains assigned to ${}^{294}117$. Experimental energies in MeV and $\mathrm{\Delta }t$ of all events together with their digitized traces are shown. Energies of reconstructed $\alpha$ particles are marked by an asterisk ($*$). Boxes with black triangles are corresponding to events observed during beam-off periods. The measured fission fragment energies are from $\text{stop}+\text{box}$ detectors. Theoretically predicted [38] $\alpha$-decay energies (in MeV), partial $\alpha$, and unhindered SF half-lives are given on the left.