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 172 Hg . 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 170 Pt exhibits a predominantly prolate ground state , accompanied by triaxial and oblate isomeric configurations . In 172 Hg , 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 172 Hg—prolate, triaxial, and oblate—demonstrate a rare prosperous shape coexistence in a single nucleus. Meanwhile, 174 Pb 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 170 Pt to 180 Pt
The team extended their analysis across the even-even Pt isotopes from 170 Pt to 180 Pt. 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 180 Pt. 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.
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On shape coexistence and possible shape isomers of nuclei around 172Hg
26-May-2025