The structure of low-lying states in ${}^{140}$Sm studied by Coulomb excitation

TL;DR

This study investigates the electromagnetic structure of $^{140}$Sm using Coulomb excitation with radioactive beams, providing insights into its shape, deformation, and collectivity through experimental data and theoretical models.

Contribution

It offers new experimental data on transition probabilities and quadrupole moments for $^{140}$Sm, and compares these with advanced shell model and mean-field calculations.

Findings

$^{140}$Sm exhibits significant $b3$ softness.

No evidence of low-energy shape coexistence was found.

Results enhance understanding of deformation onset in this mass region.

Abstract

The electromagnetic structure of $^{140}$Sm was studied in a low-energy Coulomb excitation experiment with a radioactive ion beam from the REX-ISOLDE facility at CERN. The $2^+$ and $4^+$ states of the ground-state band and a second $2^+$ state were populated by multi-step excitation. The analysis of the differential Coulomb excitation cross sections yielded reduced transition probabilities between all observed states and the spectroscopic quadrupole moment for the $2_1^+$ state. The experimental results are compared to large-scale shell model calculations and beyond-mean-field calculations based on the Gogny D1S interaction with a five-dimensional collective Hamiltonian formalism. Simpler geometric and algebraic models are also employed to interpret the experimental data. The results indicate that $^{140}$Sm shows considerable $\gamma$ softness, but in contrast to earlier speculation no signs of shape coexistence at low excitation energy. This work sheds more light on the onset of deformation and collectivity in this mass region.