α-decay studies of the francium isotopes198Fr and199Fr

Abstract

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 $\ensuremath{\alpha}$-particle activities, with ${E}_{\ensuremath{\alpha}}=7613(15)$ keV and ${T}_{1/2}$ $=$ (${15}_{\ensuremath{-}5}^{+12}$) ms and ${E}_{\ensuremath{\alpha}}=7684(15)$ keV and ${T}_{1/2}$ $=$ (${16}_{\ensuremath{-}5}^{+13}$) ms were identified in the new isotope ${}^{198}$Fr. In addition, the half-life and $\ensuremath{\alpha}$-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.