0gs+→21+Transition Strengths inSn106andSn108

The reduced transition probabilities, $B(E2;{0}_{\mathrm{gs}}^{+}\ensuremath{\rightarrow}{2}_{1}^{+})$, have been measured in the radioactive isotopes $^{108,106}\mathrm{Sn}$ using subbarrier Coulomb excitation at the REX-ISOLDE facility at CERN. Deexcitation $\ensuremath{\gamma}$ rays were detected by the highly segmented MINIBALL Ge-detector array. The results, $B(E2;{0}_{\mathrm{gs}}^{+}\ensuremath{\rightarrow}{2}_{1}^{+})=0.222(19){e}^{2}{b}^{2}$ for $^{108}\mathrm{Sn}$ and $B(E2;{0}_{\mathrm{gs}}^{+}\ensuremath{\rightarrow}{2}_{1}^{+})=0.195(39){e}^{2}{b}^{2}$ for $^{106}\mathrm{Sn}$ were determined relative to a stable $^{58}\mathrm{Ni}$ target. The resulting $B(E2)$ values are $\ensuremath{\sim}30%$ larger than shell-model predictions and deviate from the generalized seniority model. This experimental result may point towards a weakening of the $N=Z=50$ shell closure.