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The magnetic dipole moments and changes in mean-square charge radii of the neutron-rich 218m,219,229,231Fr 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 7s²S1/2 to 8p²P3/2 atomic transition. The δr²A,221 values for 218m,219Fr and 229,231Fr 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 ²²⁰Fr has a weakly inverted odd-even staggering while ²²⁸Fr 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(²¹⁹Fr)=+0.690.16em0ex(1) value supports a π1h9/2 shell-model configuration for the ground state. The g(229,231Fr) values support the tentative I^π(229,231Fr)=(1/2⁺) spin and point to a πs1/2^-1 intruder ground-state configuration.
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Budinčević et al. (2014) studied this question.
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