The shape of exotic even-mass ${}^{182–190}\mathrm{Pb}$ isotopes was probed by measurement of optical isotope shifts providing mean square charge radii ($\delta ⟨r^2⟩$). The experiment was carried out at the isolde (cern) on-line mass separator, using in-source laser spectroscopy. Small deviations from the spherical droplet model are observed, but when compared to model calculations, those are explained by high sensitivity of $\delta ⟨r^2⟩$ to beyond mean-field correlations and small admixtures of intruder configurations in the ground state. The data support the predominantly spherical shape of the ground state of the proton-magic $Z=82$ lead isotopes near neutron midshell ($N=104$).

• Received 7 December 2006

DOI:https://doi.org/10.1103/PhysRevLett.98.112502