Z=50 shell gap near $^{100}$Sn from intermediate-energy Coulomb   excitations in even-mass $^{106--112}$Sn isotopes
  Rare isotope beams of neutron-deficient $^{106,108,110}$Sn nuclei from the fragmentation of $^{124}$Xe were employed in an intermediate-energy Coulomb excitation experiment yielding $B(E2, 0^+_1 \to 2^+_1)$ transition strengths. The results indicate that these $B(E2,0^+_1 \to 2^+_1)$ values are much larger than predicted by current state-of-the-art shell model calculations. This discrepancy can be explained if protons from within the Z = 50 shell are contributing to the structure of low-energy excited states in this region. Such contributions imply a breaking of the doubly-magic $^{100}$Sn core in the light Sn isotopes. 
C.Vaman, C.Andreoiu, D.Bazin, A.Becerril, A.Brown, C.M.Campbell,   A.Chester, J.M.Cook, D.C.Dinca, A.Gade, D.Galaviz, T.Glas...