Nuclear Charge Radii of B10,11

The first laser spectroscopic determination of the change in the nuclear charge radius for a five-electron system is reported. This is achieved by combining high-accuracy ab initio mass-shift calculations and a high-accuracy measurement of the isotope shift in the 2s22p P21/2→2s23s S21/2 ground state transition in boron atoms. Accuracy is increased by orders of magnitude for the stable isotopes B10,11 and the results are used to extract their difference in the mean-square charge radius ⟨rc2⟩11−⟨rc2⟩10=−0.49(12) fm2. The result is qualitatively explained by a possible cluster structure of the boron nuclei and quantitatively used to benchmark new ab initio nuclear structure calculations using the no-core shell model and Green's function Monte Carlo approaches. These results are the foundation for a laser spectroscopic determination of the charge radius of the proton-halo candidate B8.