We have measured the hyperfine structure of the ${7P}_{1/2}$ level for ${}^{208-212}\mathrm{Fr}$ 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.

• Received 5 April 1999

DOI:https://doi.org/10.1103/PhysRevLett.83.935