Coulomb excitation of 124,126,128Sn

High-precision measurements of $\ensuremath{\langle}{0}_{1}||M(E2)||{2}_{1}\ensuremath{\rangle}$ matrix elements from the Coulomb excitation of ${}^{124,126,128}$Sn on a ${}^{12}$C target are presented. The extracted $B(E2)$ values decrease monotonically from the neutron midshell toward the ${}^{132}$Sn double-shell closure, despite a near constancy in the first ${2}^{+}$ level energy. Furthermore, Coulomb excitation of ${}^{124,126,128}$Sn on an enriched ${}^{50}$Ti target, combined with the results from the ${}^{12}$C target, provide a measure of the static quadrupole moments, $Q({2}_{1}{}^{+})$ (expected to be zero for a spherical shape). These new results confirm that the unstable neutron-rich ${}^{126,128}$Sn isotopes have deformations consistent with zero. The present study marks the first report on measured ${2}_{1}{}^{+}$ static quadrupole moments for unstable closed-shell nuclei.