The magnetic dipole moments and changes in mean-square charge radii of the neutron-rich ${}^{218m,219,229,231}\mathrm{Fr}$ 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 $\delta {\langle r^2\rangle }^{A,221}$ values for ${}^{218m,219}\mathrm{Fr}$ and ${}^{229,231}\mathrm{Fr}$ 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 ${}^{220}\mathrm{Fr}$ has a weakly inverted odd-even staggering while ${}^{228}\mathrm{Fr}$ 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 $g{(}^{219}\mathrm{Fr})=+0.69\left(1\right)$ value supports a $\pi 1h_{9/2}$ shell-model configuration for the ground state. The $g{(}^{229,231}\mathrm{Fr})$ values support the tentative $I^{\pi }{(}^{229,231}\mathrm{Fr})=(1/2^{+})$ spin and point to a $\pi s_{1/2}^{-1}$ intruder ground-state configuration.

• Received 26 May 2014
• Revised 26 June 2014

DOI:https://doi.org/10.1103/PhysRevC.90.014317