Magnetic moments of 2 1 + states in 124 , 126 , 128 Sn

The $g$ factors of the first-excited states of stable ${}^{124}$Sn and radioactive ${}^{126,128}$Sn were measured by the recoil-in-vacuum method with comparatively high precision. The experiments were performed at the Holifield Radioactive Ion Beam Facility by Coulomb exciting $\ensuremath{\sim}3$ MeV/u beams in inverse kinematics on carbon and titanium targets. The results for ${}^{124}$Sn and ${}^{126}$Sn are in excellent agreement with recent shell-model calculations. For ${}^{128}$Sn the experiment suggests an increase in the magnitude of $g({2}_{1}^{+})$, as predicted by some models. The present results provide a sensitive probe of the valence orbitals that contribute to the ${2}_{1}^{+}$ wave functions as the double-shell closure at ${}^{132}$Sn is approached.