Early Signal of Emerging Nuclear Collectivity in Neutron-Rich ~(129)Sb

摘要

Radioactive Sb-129, which can be treated as a proton plus semimagic Sn-128 core within the particle-core coupling scheme, was studied by Coulomb excitation. Reduced electric quadrupole transition probabilities, B(E2), for the 2(+) circle times pi g(7/2) multiplet members and candidate pi d(5)(/2) state were measured. The results indicate that the total electric quadrupole strength of Sb-129 is a factor of 1.39(11) larger than the Sn-128 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.