Model-independent determination of nuclear charge radii from Li-like ions

We demonstrate that recent advances in QED theory of Li-like ions [V. A. Yerokhin et al., Phys. Rev. A 112, 042801 (2025)] enable determinations of absolute nuclear charge radii for heavy elements. By incorporating constraints derived from electron-scattering data, we obtain radii that are independent of the assumed model of the nuclear charge distribution. Our approach is validated for ${}^{208}$Pb, a well-studied spherical nucleus, and is then applied to ${}^{209}$Bi, where low-lying nuclear excitations complicate the interpretation of muonic-atom data.