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

Abstract

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, probing the $7s{}^2{S}_{1/2}$ to $8p{}^2{P}_{3/2}$ atomic transition. The $\delta ⟨r^2{⟩}^{A,221}$ 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.

Accepted for publication with Physical Review C

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