Physicists have just announced a new magic number that could change the way we understand the formation of everything in the Universe, opening up a promising research direction for modern physics.
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 the isotope silicon-22 – an extremely short-lived, neutron-deficient nucleus. The results, published in the journal Physical Review Letters , show that the proton number 14 in silicon-22 acts as a new 'magic number' in nuclear physics.
Atomic nuclei are made up of protons and neutrons. When their numbers reach special values – called 'magic numbers' – the nucleus becomes unusually stable. The classic magic numbers for stable isotopes have long been known, including 2, 8, 20, 28, 50, 82 and 126. This concept was explained by Maria Goeppert Mayer and J. Hans D. Jensen in the mid-20th century with the nuclear shell model, a work that earned them the Nobel Prize in Physics in 1963. However, for short-lived 'exotic' isotopes, new magic numbers are still being discovered. Studying them helps scientists test the nuclear force under extreme conditions and better understand how the elements in the early Universe formed.
In recent years, researchers have discovered more magic numbers associated with neutrons, such as 14, 16, 32, and 34, in nuclei far from the stable region. But clear evidence for the magic number of protons has been scarce. Previously, experiments showed that oxygen-22 (8 protons, 14 neutrons) behaved like a 'magic' nucleus in neutron number 14. Because of mirror symmetry in nuclear physics, theorists predicted that proton number 14 would also be 'magic' in its mirror nucleus: silicon-22 (14 protons, 8 neutrons).
The problem is that silicon-22 is very difficult to produce in large enough quantities and decays extremely quickly, making this hypothesis unconfirmed until now.
Using improved IMS mass spectrometry at the heavy ion storage ring in Lanzhou, the IMP team measured the ground state mass of silicon-22 – the first time a nucleus at the 'proton barrier' boundary has been massed. These are nuclei so rich in protons that adding just one more proton would cause them to decay.
In addition, the team also remeasured the mass of silicon-23 with seven times greater precision than before. The results showed that silicon-22 has a positive two-proton binding energy, meaning it does not spontaneously emit two protons as some long-standing theories. This ended a long-standing debate in nuclear physics.
From 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 the proton number 14 is indeed a new magic number, and the results are consistent with the Gamow shell model – an extension of the traditional nuclear shell model, which is used to explain strange, weakly bound and resonance nuclei.
Both silicon-22 and oxygen-22 exhibit the 'magic double' property, but the study found subtle differences in their internal structure: protons in silicon-22 are more widely distributed than neutrons in oxygen-22, suggesting a slight breaking of mirror symmetry.
This discovery not only expands our understanding of the structure of exotic nuclei, but also contributes to deciphering the 'source code' of nuclear forces under extreme conditions – the foundation for explaining how matter formed in the early Universe.