Precision Mass Measurements of Atomic Nuclei Unveil Proton Halo Structures

 

Overview of Proton Halo Discovery

Recent research led by the Institute of Modern Physics (IMP) at the Chinese Academy of Sciences, in collaboration with international experts, has achieved groundbreaking results in nuclear physics. Precise mass measurements of exotic atomic nuclei have uncovered proton halo structures and refined the proton dripline for aluminum, phosphorus, sulfur, and argon. This study introduces an innovative method to explore the elusive proton halo phenomenon.

Understanding Proton Halos
The atomic nucleus, a quantum many-body system of protons and neutrons, generally maintains a size similar to nearby nuclei. However, weakly bound nuclei sometimes exhibit an unusual configuration called a halo, where valence nucleons (protons or neutrons) extend far beyond the typical nuclear radius. While neutron halos are frequently observed, proton halos are much rarer due to the repelling force of the Coulomb barrier.

Yu Yue, a Ph.D. student at IMP and co-first author of the study, explained, “Proton halo nuclei are challenging to detect experimentally because the Coulomb barrier hampers their formation. However, precise mass measurements allow us to uncover hints of proton halo structures.”

Advanced Techniques and Discoveries
The research utilized the Cooler Storage Ring (CSRe) at the Heavy Ion Research Facility in Lanzhou (HIRFL). A cutting-edge Bρ-defined isochronous mass spectroscopy method enabled first-time precise mass measurements of rare isotopes like silicon-23, phosphorus-26, sulfur-27, and argon-31. This technique also improved the mass precision of sulfur-28 by 11 times.

The refined mass data allowed scientists to establish the proton dripline for several elements and analyze mirror energy differences—a quantity derived exclusively from atomic masses. Associate Prof. Xing Yuanming, another co-first author, highlighted, “Mirror energy differences serve as a powerful tool to detect proton halo structures and study isospin symmetry breaking in exotic nuclei.”

Key Findings and Implications
The research demonstrated isospin symmetry breaking in some nuclei near the proton dripline, linked to proton halo structures. Key findings include:

• Proton halos in phosphorus-26, phosphorus-27, sulfur-27, and sulfur-28.
• The discovery of argon-31 as a potential double-proton halo nucleus.
• Confirmation that aluminum-22 does not exhibit a proton halo in its ground state.

These discoveries are backed by theoretical calculations, solidifying the evidence for proton halo structures in specific nuclei.

Impact on Future Research
This innovative approach using mirror energy differences has broad implications for nuclear physics. It offers a new lens to study isospin symmetry breaking and proton halos, paving the way for further experimental and theoretical advancements.

By improving our understanding of exotic nuclear structures, this research not only deepens scientific knowledge but also sets the stage for future explorations in quantum systems.

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