Physicists' discovery of silicon-22 reveals a new proton magic number, providing crucial insights into understanding the structure of atomic nuclei and the power that shapes the rarest atoms in the universe.
In nuclear physics, the "magic number" refers to a specific number of protons or neutrons that make the atomic nucleus significantly more stable. Identifying these special numbers is crucial for helping scientists reveal the structure of atomic nuclei.
Although people have known about the magical numbers related to stable and long-lived isotopes for decades, the situation for highly unstable and short-lived isotopes is not as clear. Studying these rare and short-lived forms of matter provides scientists with valuable clues about how atomic nuclear structures behave under extreme conditions. These findings not only help us understand the formation process of elements in the universe, but also deepen our understanding of the nuclear holding force.
Scientists from the Institute of Modern Physics (IMP) of the Chinese Academy of Sciences recently made a progress, measuring the mass of silicon-22 for the first time. This isotope is extremely unstable and lacks neutrons. Their research indicates that in silicon-based materials, the proton number 14 appears as a newly confirmed magic number.

1. Shell model and known magic numbers
The atomic nucleus is composed of protons and neutrons. When the number of protons or neutrons reaches a certain 'magic number', such as 2, 8, 20, 28, 50, 82, or 126, the atomic nucleus becomes more stable. Maria Goeppert Mayer and J. Hans D. Jensen explained this phenomenon through the nuclear shell model in the 1940s and 1950s, for which they were awarded the Nobel Prize in Physics in 1963.


In recent years, research on strange nuclei far from the stable valley (the region where stable isotopes are found on the nuclide map) has discovered new neutron magic numbers, such as 14, 16, 32, and 34. However, new proton magic numbers are still rare in experimental observations.
Previously, scientists found that in oxygen-22 (14 neutrons and 8 protons), the neutron number 14 exhibits magic number characteristics. Based on nuclear mirror symmetry, theorists predict that in its mirror nucleus silicon-22 (8 neutrons and 14 protons), the number of protons 14 should also be a magic number. However, due to the low yield and short half-life of silicon-based, its generation and measurement are extremely challenging, resulting in this theoretical prediction still not being validated to this day.

2. Advanced mass spectrometry technology for IMP
Using an improved B ρ - defined isothermal mass spectrometry technique, researchers from the Institute of Heavy Ion Physics, Chinese Academy of Sciences, successfully measured the ground state mass of silicon-based on a cooling storage ring at a heavy ion research facility in Lanzhou, and improved the accuracy of the previously measured mass of silicon-based by nearly seven times.
Their research results indicate that silicon-22 has a positive biproton separation energy - in other words, it does not spontaneously lose two protons. This confirms its position as a proton drop line nucleus without biprotonic radioactivity, thus resolving a long-standing debate in nuclear physics.
Using the new mass values, the research team calculated the proton pairing energy of silicon-22 and compared it with the neutron pairing energy of its mirror nucleus oxygen-22, revealing a new proton magic number of 14. This discovery is supported by the Gamow shell model.
Although silicon-based exhibits similar dual magic properties as oxygen-22, research has found that its proton spatial distribution is more dispersed compared to the neutron distribution of oxygen-22, exhibiting slight symmetry breaking.
This study deepens our understanding of foreign nuclear structures and provides new insights into nuclear interactions and the existence of extremely foreign nuclei.