Seeing francium nuclei as tiny magnets:
The ratio of the hyperfine splittings of s and p states is not constant across isotopes due to the isotope-dependent distribution of nuclear magnetization, a phenomenon called the hyperfine anomaly. By carrying out measurements of the hyperfine splitting of the excited electronic 7p1/2 state at the 100-ppm level, and comparing to previously known ground state 7s splittings, the hyperfine anomaly in six isotopes of francium (Fr) was experimentally determined. As reported in Physical Review Letters (2015) the measured magnetic distributions behave regularly from 213Fr through 207Fr, but 206Fr stops behaving like a spherical nucleus with valence nucleons. The results are valuable input for future calculations of both the anapole moments and the neutron radii needed for small corrections to Francium Trapping Facility measurements of atomic-parity violation for 207−213Fr.