Abstract
Radioactive 129Sb, which can be treated as a proton plus semimagic 128Sn core within the particle-core coupling scheme, was studied by Coulomb excitation. Reduced electric quadrupole transition probabilities, B(E2), for the 2+⊗πg7/2 multiplet members and candidate πd5/2 state were measured. The results indicate that the total electric quadrupole strength of 129Sb is a factor of 1.39(11) larger than the 128Sn core, which is in stark contrast to the expectations of the empirically successful particle-core coupling scheme. Shell-model calculations performed with two different sets of nucleon-nucleon interactions suggest that this enhanced collectivity is due to constructive quadrupole coherence in the wave functions stemming from the proton-neutron residual interactions, where adding one nucleon to a core near a double-shell closure can have a pronounced effect. The enhanced electric quadrupole strength is an early signal of the emerging nuclear collectivity that becomes dominant away from the shell closure.
