Exploring the role of high-j configurations in collective observables through the Coulomb excitation of ¹⁰⁶Cd

The shape and collectivity of Cd-106 was investigated via a sub-barrier-energy Coulomb excitation experiment performed at the National Superconducting Cyclotron Laboratory Re-accelerator facility using the JANUS setup. Transition matrix elements between low-lying states were found to agree with adopted values, and information on the shape and collectivity of higher-lying states was extracted for the first time. Locally optimized large-scale shell-model calculations were found to describe well the B(E2) transition strengths but failed to reproduce the spectroscopic quadrupole moments Q(s). An analysis of the E2 rotational invariants and the normalized quadrupole moment q(s) indicates that this may be due to a significant degree of triaxiality in Cd-106 which is not captured by the present shell-model calculations. Analogous calculations for the Fe isotopes (two protons below the Z = 28 magic number) reveal the critical role of high- j neutron configurations for the description of quadrupole moments in the heavy Fe and Cd isotopes (two protons below magic Z = 50), but this effect is insufficient to explain the shape of Cd-106, posing a puzzle for the understanding of nuclear structure towards N = 50.