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

Abstract

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 $2s^{2}2p{}^2{\mathrm{P}}_{1/2}\to 2s^{2}3s{}^2{\mathrm{S}}_{1/2}$ 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 $⟨r_{\mathrm{c}}^2{⟩}^{11}-⟨r_{\mathrm{c}}^2{⟩}^{10}=-0.49(12){\mathrm{fm}}^2$. 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.