Coulomb excitation of 124^,126^,128Sn

High-precision measurements of 0₁||M(E2)||2₁ matrix elements from the Coulomb excitation of 124^,126^,128Sn on a 12C target are presented. The extracted B(E2) values decrease monotonically from the neutron midshell toward the 132Sn double-shell closure, despite a near constancy in the first 2⁺ level energy. Furthermore, Coulomb excitation of 124^,126^,128Sn on an enriched 50Ti target, combined with the results from the 12C target, provide a measure of the static quadrupole moments, Q(2₁⁺) (expected to be zero for a spherical shape). These new results confirm that the unstable neutron-rich 126^,128Sn isotopes have deformations consistent with zero. The present study marks the first report on measured 2₁⁺ static quadrupole moments for unstable closed-shell nuclei.