The magnetic moments and isotope shifts of the neutron-deficient francium isotopes ${}^{202\mathrm{\text{−}}205}\mathrm{Fr}$ were measured at ISOLDE-CERN with use of collinear resonance ionization spectroscopy. A production-to-detection efficiency of 1% was measured for ${}^{202}\mathrm{Fr}$. The background from nonresonant and collisional ionization was maintained below one ion in ${10}^5$ beam particles. Through a comparison of the measured charge radii with predictions from the spherical droplet model, it is concluded that the ground-state wave function remains spherical down to ${}^{205}\mathrm{Fr}$, with a departure observed in ${}^{203}\mathrm{Fr}$ ($N=116$).

• Received 13 August 2013

DOI:https://doi.org/10.1103/PhysRevLett.111.212501