Very neutron deficient francium isotopes have been produced in fusion evaporation reactions using ${}^{60}$Ni ions on ${}^{141}$Pr targets. The gas-filled recoil separator RITU was employed to collect the fusion products and to separate them from the scattered beam. The activities were implanted into a position sensitive silicon detector after passing through a gas-counter system. The isotopes were identified using spatial and time correlations between the implants and the decays. Two $\alpha$-particle activities, with $E_{\alpha }=7613\left(15\right)$ keV and $T_{1/2}$ $=$ (${15}_{-5}^{+12}$) ms and $E_{\alpha }=7684\left(15\right)$ keV and $T_{1/2}$ $=$ (${16}_{-5}^{+13}$) ms were identified in the new isotope ${}^{198}$Fr. In addition, the half-life and $\alpha$-particle energy of ${}^{199}$Fr were measured with improved precision. The measured decay properties deduced for ${}^{199}$Fr and ${}^{198}$Fr suggest that there is an onset of ground-state deformation at $N=112$ in the Fr isotopes.

• Received 30 November 2012

DOI:https://doi.org/10.1103/PhysRevC.87.064304