Two short-lived isotopes ${}^{221}\mathrm{U}$ and ${}^{222}\mathrm{U}$ were produced as evaporation residues in the fusion reaction ${}^{50}\mathrm{Ti}+{}^{176}\mathrm{Yb}$ at the gas-filled recoil separator TASCA. An $\alpha$ decay with an energy of $E_{\alpha }=9.31(5)\mathrm{MeV}$ and half-life $T_{1/2}=4.7(7)\mu \mathrm{s}$ was attributed to ${}^{222}\mathrm{U}$. The new isotope ${}^{221}\mathrm{U}$ was identified in $\alpha$-decay chains starting with $E_{\alpha }=9.71(5)\mathrm{MeV}$ and $T_{1/2}=0.66(14)\mu \mathrm{s}$ leading to known daughters. Synthesis and detection of these unstable heavy nuclei and their descendants were achieved thanks to a fast data readout system. The evolution of the $N=126$ shell closure and its influence on the stability of uranium isotopes are discussed within the framework of $\alpha$-decay reduced width.

• Received 14 July 2015

DOI:https://doi.org/10.1103/PhysRevLett.115.242502