Unveiling shape coexistence near mercury-172: A new window into nuclear structure

Tracing the Shape Evolution of Mid-Shell Nuclei

A recent theoretical study has shed new light on shape coexistence—the ability of atomic nuclei to exist in multiple deformation states simultaneously—particularly in the neutron-deficient region near ¹⁷²Hg. Using a macro-microscopic framework, physicists have mapped several isotopes' potential energy surfaces (PESs), uncovering a range of ground-state and isomeric shapes that depend sensitively on the strength of nuclear pairing interactions.

From Prolate to Oblate: A Spectrum of Coexisting Shapes

The study reveals that ¹⁷⁰Pt exhibits a predominantly prolate ground state, accompanied by triaxial and oblate isomeric configurations. In ¹⁷²Hg, the ground-state shape evolves from triaxial to oblate as pairing interaction increases, highlighting its gamma-soft or $\gamma$-unstable nature. Three shape isomers in ¹⁷²Hg—prolate, triaxial, and oblate—demonstrate a rare prosperous shape coexistence in a single nucleus. Meanwhile, ¹⁷⁴Pb shows a trend toward spherical symmetry as pairing strengthens, with reduced evidence for stable isomerism.

Modeling Nuclear Deformation with Precision

The researchers employed the Lublin-Strasbourg Drop model combined with a Yukawa-Folded potential and Exact and BCS-type pairing interactions to produce detailed PES landscapes. A comparison between Exact pairing and the widely used BCS approximation indicated that BCS tends to smooth out shape coexistence and diminish the stability of isomeric minima. "Our approach captures the sensitivity of nuclear shape to pairing correlations, offering a more nuanced view of deformation mechanics," the lead author explained.

A Systematic View from ¹⁷⁰Pt to ¹⁸⁰Pt

The team extended their analysis across the even-even Pt isotopes from ¹⁷⁰Pt to ¹⁸⁰Pt. A clear progression emerged: from prolate ground-states in lighter isotopes to gamma-unstable and triaxial shapes in mid-range isotopes, eventually returning to dominant prolate deformation by ¹⁸⁰Pt. These findings suggest that pairing and shell effects play a pivotal role in governing nuclear shapes, particularly in regions far from closed shells.

Implications for Nuclear Theory and Experiment

This research enhances our understanding of nuclear deformation and structural transitions and refines the theoretical foundations used to interpret experimental data. Insights into shape coexistence and isomer formation could inform the design of future experiments in radioactive beam facilities and improve models relevant to astrophysical nucleosynthesis and nuclear energy systems.

"By systematically exploring how pairing interactions shape the energy landscape of mid-shell nuclei, we are uncovering new patterns that challenge traditional nuclear structure models," the research team concluded. "Our results contribute to a more accurate and predictive framework for understanding how atomic nuclei behave under various structural constraints."

The complete study is accessible via DOI：https://doi.org/10.1007/s41365-025-01737-w

Nuclear Science and Techniques (NST) is a peer-reviewed international journal sponsored by the Shanghai Institute of Applied Physics, Chinese Academy of Sciences. The journal publishes high-quality research across a broad range of nuclear science disciplines, including nuclear physics, nuclear energy, accelerator physics, and nuclear electronics. Its Editor-in-Chief is the renowned physicist, Professor Yu-Gang Ma.