Early signal of emerging nuclear collectivity in neutron-rich $^{129}$Sb

TL;DR

This study measures electric quadrupole transition probabilities in neutron-rich $^{129}$Sb, revealing early signs of nuclear collectivity emerging away from shell closures, challenging traditional particle-core coupling models.

Contribution

It provides the first experimental evidence of enhanced quadrupole collectivity in $^{129}$Sb, highlighting the role of proton-neutron interactions in nuclear structure.

Findings

$^{129}$Sb shows 1.39 times larger $B(E2)$ than $^{128}$Sn

Enhanced collectivity arises from constructive quadrupole coherence

Early signal of emerging nuclear collectivity away from shell closure

Abstract

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