Nuclear Charge Radius of He 8

The root-mean-square (rms) nuclear charge radius of $^{8}\mathrm{He}$, the most neutron-rich of all particle-stable nuclei, has been determined for the first time to be 1.93(3) fm. In addition, the rms charge radius of $^{6}\mathrm{He}$ was measured to be 2.068(11) fm, in excellent agreement with a previous result. The significant reduction in charge radius from $^{6}\mathrm{He}$ to $^{8}\mathrm{He}$ is an indication of the change in the correlations of the excess neutrons and is consistent with the $^{8}\mathrm{He}$ neutron halo structure. The experiment was based on laser spectroscopy of individual helium atoms cooled and confined in a magneto-optical trap. Charge radii were extracted from the measured isotope shifts with the help of precision atomic theory calculations.