α-decay studies of the francium isotopes 198Fr and 199Fr

Abstract

Very neutron deficient francium isotopes have been produced in fusion evaporation reactions using 60Ni ions on 141Pr 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 α-particle activities, with Eα=7613(15) keV and T1/2 = (15-5+12) ms and Eα=7684(15) keV and T1/2 = (16-5+13) ms were identified in the new isotope 198Fr. In addition, the half-life and α-particle energy of 199Fr were measured with improved precision. The measured decay properties deduced for 199Fr and 198Fr suggest that there is an onset of ground-state deformation at N=112 in the Fr isotopes. © 2013 American Physical Society.