Speaker
Description
The evolution of nuclear shapes and the phenomenon of shape coexistence lie at the heart of our understanding of nuclear structure and the effective nuclear interaction. While dramatic examples of shape coexistence have long been established near closed shells, semi-magic nuclei have traditionally been regarded as structurally simple systems, dominated by pairing correlations and spherical mean fields. In this context, the tin isotopic chain, anchored by the robust $Z=50$ shell closure, has served for decades as the textbook paradigm of sphericity and seniority-driven structure.
This contribution will present compelling experimental evidence that fundamentally challenges this long-standing picture. Using a high-precision Coulomb-excitation experiment on $^{116}$Sn, located at the midpoint of the neutron mid-shell, an extensive and internally consistent set of electromagnetic matrix elements has been extracted. These data enable, for the first time in this nucleus, a fully model-independent determination of intrinsic quadrupole deformations for the ground and excited $0^+$ states, as well as spectroscopic quadrupole moments for multiple low-lying $2^+$ states.
The results reveal a remarkably rich and unexpected structural landscape. Rather than a spherical ground state weakly perturbed by a single intruder configuration, $^{116}$Sn exhibits multiple coexisting shapes at low excitation energy. The ground state itself is shown to possess a small but finite oblate deformation, incompatible with spherical symmetry and characterized by limited shape fluctuations. In addition, two excited $0^+$ states display distinct intrinsic deformations, providing unambiguous evidence for multiple-shape coexistence -a phenomenon observed in only a handful of nuclei across the entire nuclear chart.
These findings are discussed in the context of the experiment-driven Three-Band Mixing (3BM) model developed specifically for this work, as well as the Projected Generator Coordinate Method (PGCM) framework. Together, these approaches reveal strongly fragmented wave functions for the $0^+$ states, in stark contrast to the largely unperturbed nature of the $2^+$ states, and provide a coherent explanation of longstanding anomalies observed in nucleon-transfer reactions.
These results demonstrate that even semi-magic nuclei can host complex collective dynamics and multiple competing shapes. The case of $^{116}$Sn thus emerges not as an exception, but as a key benchmark for testing modern nuclear structure theories beyond the traditional limits of shell closures.