Laser spectroscopy of francium isotopes at the borders of the region of reflection asymmetry

The magnetic dipole moments and changes in mean-square charge radii of the neutron-rich 218m,219,229,231 Fr isotopes were measured with the newly installed Collinear Resonance Ionization Spectroscopy (CRIS) beam line at the On-Line Isotope Mass Separator (ISOLDE), CERN, probing the 7s 2 S 1/2 to 8p 2 P 3/2 atomic transition. The δ⟨r 2 ⟩ A,221 values for 218m,219 Fr and 229,231 Fr follow the observed increasing slope of the charge radii beyond N=126 . The charge radii odd-even staggering in this neutron-rich region is discussed, showing that 220 Fr has a weakly inverted odd-even staggering while 228 Fr has normal staggering. This suggests that both isotopes reside at the borders of a region of inverted staggering, which has been associated with reflection-asymmetric shapes. The g( 219 Fr)=+0.69(1) value supports a π1h 9/2 shell-model configuration for the ground state. The g( 229,231 Fr) values support the tentative I π ( 229,231 Fr)=(1/2 + ) spin and point to a πs −1 1/2 intruder ground-state configuration.Accepted for publication with Physical Review Cstatus: publishe