Quadrupole collectivity in neutron-deficient Sn nuclei: 104Sn and the role of proton excitations

We report on the experimental study of quadrupole collectivity in the neutron-deficient nucleus Sn-104 using intermediate-energy Coulomb excitation. The B(E2; 0(1)(+). -> 2(1)(+)) value for the excitation of the first 2(+) state in Sn-104 has been measured to be 0.180(37) e(2)b(2) relative to the well-known B(E2) value of Cd-102. This result disagrees by more than one sigma with a recently published measurement [G. Guastalla et al., Phys. Rev. Lett. 110, 172501 (2013)]. Our result indicates that the most modern many-body calculations remain unable to describe the enhanced collectivity below midshell in Sn approaching N = Z = 50. We attribute the enhanced collectivity to proton particle-hole configurations beyond the necessarily limited shell-model spaces and suggest the asymmetry of the B(E2)-value trend around midshell to originate from enhanced proton excitations across Z = 50 as N = Z is approached.