Scientists Discover a New “Magic Number” That Could Rewrite the Rules of Nuclear Physics

Physicists have discovered that silicon-22 reveals a new proton magic number offering critical insights into nuclear structure and the forces shaping the universe’s rarest atoms.

In nuclear physics, “magic numbers” refer to certain quantities of protons or neutrons that make an atomic nucleus significantly more stable. Identifying these special numbers plays a key role in helping scientists uncover how atomic nuclei are structured.

While the magic numbers linked to stable, long-lasting isotopes have been understood for decades, the situation is less clear for highly unstable, short-lived isotopes. Studying these rare and fleeting forms of matter gives scientists valuable clues about how nuclear structures behave under extreme conditions. These discoveries not only offer insight into how the elements in the Universe were formed but also enhance our knowledge of the forces that hold atomic nuclei together.

In a recent advancement, scientists at the Institute of Modern Physics (IMP) under the Chinese Academy of Sciences have achieved a first-of-its-kind measurement of the mass of silicon-22. This isotope is both extremely unstable and lacking in neutrons. Their work has revealed that in silicon-22, the proton count of 14 behaves as a newly confirmed magic number.

Their findings were recently published in the journal Physical Review Letters.

The Shell Model and Known Magic Numbers

Atomic nuclei are composed of protons and neutrons. When the number of protons or neutrons reaches a “magic number”, such as 2, 8, 20, 28, 50, 82, or 126, the nucleus becomes more stable. Maria Goeppert Mayer and J. Hans D. Jensen explained this phenomenon in the 1940s–1950s through the nuclear shell model, for which they were awarded the 1963 Nobel Prize in Physics.

In recent years, studies of exotic nuclei far from the valley of stability—the region on a nuclide chart where stable isotopes are found—have identified new neutron magic numbers, such as 14, 16, 32, and 34. However, new proton magic numbers remain rare in experimental observations.

Previously, scientists discovered that in oxygen-22 (14 neutrons and 8 protons), the neutron number 14 exhibits magic characteristics. Based on nuclear mirror symmetry, theorists predicted that proton number 14 should also be a magic number in its mirror nucleus, silicon-22 (8 neutrons and 14 protons). However, generating and measuring silicon-22 is extremely challenging due to its low yields and short half-life, leaving this theoretical prediction unverified until now.

Advanced Mass Spectroscopy at IMP

Using improved Bρ-defined isochronous mass spectroscopy, researchers at IMP successfully measured the ground-state mass of silicon-22 at the Cooling Storage Ring of the Heavy Ion Research Facility in Lanzhou. They also improved the mass precision of their previously measured silicon-23 by nearly sevenfold.

Their results show that silicon-22 has a positive two-proton separation energy—in other words, it doesn’t spontaneously lose two protons. This confirms its status as a proton drip-line nucleus without two-proton radioactivity, thereby resolving a long-standing debate in nuclear physics.

Using the new mass value, the team calculated the proton pairing energy of silicon-22 and compared it with the neutron pairing energy of its mirror nucleus oxygen-22, revealing the new proton magic number 14. This finding is supported by the Gamow shell model.

Although silicon-22 exhibits double-magic properties similar to oxygen-22, the study found that its proton spatial distribution is more spread compared with the neutron distribution of oxygen-22, exhibiting a slight symmetry breaking.

This study deepens our understanding of exotic nuclear structures and provides new insights into nucleon interactions and the existence of extremely exotic nuclei.

Reference: “Z=14 Magicity Revealed by the Mass of the Proton Dripline Nucleus Si22” by Y. M. Xing, Y. F. Luo, Y. H. Zhang, M. Wang, X. H. Zhou, J. G. Li, K. H. Li, Q. Yuan, Y. F. Niu, J. Y. Guo, J. C. Pei, F. R. Xu, G. de Angelis, Yu. A. Litvinov, K. Blaum, I. Tanihata, T. Yamaguchi, Y. Yu, X. Zhou, H. S. Xu, Z. Y. Chen, R. J. Chen, H. Y. Deng, C. Y. Fu, W. W. Ge, W. J. Huang, H. Y. Jiao, H. F. Li, T. Liao, J. Y. Shi, M. Si, M. Z. Sun, P. Shuai, X. L. Tu, Q. Wang, X. Xu, X. L. Yan, Y. J. Yuan and M. Zhang, 2 July 2025, Physical Review Letters.
DOI: 10.1103/ffwt-n7yc

This work was supported by the National Key R&D Program of China, the Strategic Priority Research Program of CAS, and the Youth Innovation Promotion Association of CAS, among others.

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