Nuclear Charge Radii of ${}^{10,11}$B

The first determination of the nuclear charge radius by laser spectroscopy for a five-electron system is reported. This is achieved by combining high-accuracy ab initio mass-shift calculations and a high-resolution measurement of the isotope shift in the $2s^2 2p, ^2$\mathrm{P}${1/2} \rightarrow 2s^2 3s, ^2$\mathrm{S}${1/2}$ ground state transition in boron atoms. Accuracy is increased by orders of magnitude for the stable isotopes ${}^{10,11}$B and the results are used to extract their difference in the mean-square charge radius $\langle r^2_$\mathrm{c}$\rangle^{11} - \langle r^2_$\mathrm{c}$\rangle^{10} = -0.49,(12),$\mathrm{f}\mathrm{m}$^2$. The result is qualitatively explained by a possible cluster structure of the boron nuclei and quantitatively used as a benchmark for new ab initio nuclear structure calculations using the no-core shell model and Green’s function Monte Carlo approaches.