A team of physicists at the Institute of Modern Physics (IMP), part of the Chinese Academy of Sciences (CAS), has made the first precise measurement of the mass of silicon-22, a very short-lived and neutron-deficient nucleus. Their results, published in Physical Review Letters, show that proton number 14 in silicon-22 acts as a new “magic number” in nuclear physics.
Atomic nuclei are built from protons and neutrons. Certain counts of these particles, called “magic numbers,” make nuclei unusually stable. The classic magic numbers for stable isotopes are 2, 8, 20, 28, 50, 82, and 126. This idea was explained in the mid-20th century by Maria Goeppert Mayer and J. Hans D. Jensen through the nuclear shell model, work that won them the 1963 Nobel Prize in Physics. While the magic numbers for stable isotopes are well established, the ones for exotic, short-lived isotopes are still being uncovered. Studying these rare cases helps scientists test nuclear forces under extreme conditions and better understand how elements formed in the Universe, as nuclear forces are two of the four fundamental forces of nature.
In recent years, researchers have found new neutron magic numbers such as 14, 16, 32, and 34 in exotic nuclei far from stability. But clear evidence for new proton magic numbers has been much harder to find. Earlier studies showed that oxygen-22, which has 14 neutrons and 8 protons, behaves like a magic nucleus at neutron number 14. Based on nuclear mirror symmetry, theorists predicted that proton number 14 should also be magic in its mirror nucleus, silicon-22, which has 14 protons and 8 neutrons.
Mirror nuclei are pairs of atomic nuclei where the number of protons and neutrons are swapped between the two nuclei.
The problem was that silicon-22 is extremely hard to produce in large enough amounts and decays very quickly, so this prediction had never been confirmed until now.
Using improved Bρ-defined isochronous mass spectroscopy (IMS) technique at the Cooling Storage Ring of the Heavy Ion Research Facility in Lanzhou, the IMP team managed to measure the ground-state mass of silicon-22, the first mass measurement of the proton drip line nucleus.
A proton drip line nucleus is an exotic, proton-rich nucleus at the limit of nuclear stability, beyond which it cannot bind an extra proton and will decay via proton emission.
They also re-measured silicon-23 with nearly seven times better precision. The results showed that silicon-22 has a positive two-proton separation energy, meaning it does not naturally emit two protons. This confirms that silicon-22 sits at the proton drip line but does not undergo two-proton radioactivity, settling a long-standing debate in nuclear physics.
With the new mass value, the researchers calculated the proton pairing energy of silicon-22 and compared it with the neutron pairing energy of oxygen-22. The analysis confirmed that proton number 14 is indeed a new magic number, a conclusion also supported by the Gamow shell model.
The Gamow Shell Model (GSM) is an open-quantum system extension of the traditional nuclear shell model (SM) in the complex-energy plane to describe exotic weakly bound and resonant nuclei that the traditional SM cannot explain.
Both silicon-22 and oxygen-22 show “double-magic” properties, but the study found a difference in their internal structures. The protons in silicon-22 are more spread out than the neutrons in oxygen-22, showing a small breaking of mirror symmetry.
This discovery gives scientists fresh insight into the structure of exotic nuclei and how nucleons interact. It also pushes forward the broader effort to decode the nuclear “building code” under extreme conditions, helping us understand how matter itself was formed in the Universe under such conditions during its origin.
Source: IMP-CAS, APS | Image via Depositphotos
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