The magnetic dipole moments and changes in mean-square charge radii of the neutron-rich $^{218m,219,229,231}text{Fr}$ isotopes were measured with the newly-installed Collinear Resonance Ionization Spectroscopy (CRIS) beam line at ISOLDE, CERN, probin
g the $7s~^{2}S_{1/2}$ to $8p~^{2}P_{3/2}$ atomic transition. The $deltalangle r^{2}rangle^{A,221}$ values for $^{218m,219}text{Fr}$ and $^{229,231}text{Fr}$ 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 $^{220}text{Fr}$ has a weakly inverted odd-even staggering while $^{228}text{Fr}$ 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 $g(^{219}text{Fr}) = +0.69(1)$ value supports a $pi 1h_{9/2}$ shell model configuration for the ground state. The $g(^{229,231}text{Fr})$ values support the tentative $I^{pi}(^{229,231}text{Fr}) = (1/2^{+})$ spin, and point to a $pi s_{1/2}^{-1}$ intruder ground state configuration.
This paper reports on the hyperfine-structure and radioactive-decay studies of the neutron-deficient francium isotopes $^{202-206}$Fr performed with the Collinear Resonance Ionization Spectroscopy (CRIS) experiment at the ISOLDE facility, CERN. The h
igh resolution innate to collinear laser spectroscopy is combined with the high efficiency of ion detection to provide a highly-sensitive technique to probe the hyperfine structure of exotic isotopes. The technique of decay-assisted laser spectroscopy is presented, whereby the isomeric ion beam is deflected to a decay spectroscopy station for alpha-decay tagging of the hyperfine components. Here, we present the first hyperfine-structure measurements of the neutron-deficient francium isotopes $^{202-206}$Fr, in addition to the identification of the low-lying states of $^{202,204}$Fr performed at the CRIS experiment.
