Hyperfine Anomaly Measurements in Francium Isotopes and the Radial Distribution of Neutrons

We have measured the hyperfine structure of the 7P1/2 level for ^208-212Fr to a precision of 300 ppm. These measurements along with previous ground state hyperfine structure measurements reveal a hyperfine anomaly. The hyperfine anomaly exhibits a strong sensitivity to the radial distribution of the neutron magnetization, providing a good way to probe this quantity. We use neutron radial distributions from recent theories to qualitatively explain the measurements.