The previously unknown nucleus aluminum-20 has been observed for the first time by detecting its in-flight decays.
Three-proton emission from aluminium-20. Image credit: Xiaodong Xu.
Currently, more than 3,300 nuclides are known, yet fewer than 300 are stable and exist naturally. The remainder are unstable nuclides that undergo radioactive decay.
Common decay modes, such as α decay, β- decay, β+ decay, electron capture, γ radiation, and fission, were discovered by the mid-20th century.
Over the past several decades, due to the tremendous development in nuclear physics experimental facilities and detection technologies, scientists have discovered several exotic decay modes in the study of nuclei far from the stability, particularly in neutron-deficient nuclei.
In the 1970s, scientists discovered single-proton radioactivity, where nuclei decay by emitting a proton.
In the 21st century, two-proton radioactivity was found in the decays of some extremely neutron-deficient nuclei.
In recent years, even rarer decay phenomena such as three-, four-, and five-proton emission were observed.
“Aluminum-20 is the lightest aluminum isotope that has been discovered so far,” said Dr. Xiaodong Xu, a physicist with the Institute of Modern Physics at the Chinese Academy of Sciences.
“Located beyond the proton drip line, it has seven fewer neutrons than the stable aluminum isotope.”
Using an in-flight decay technique at the Fragment Separator of the GSI Helmholtz Center for Heavy Ion Research, the physicists measured angular correlations of aluminum-20’s decay products.
Through detailed analysis of angular correlations, they found that the aluminum-20 ground state first decays by emitting one proton to the intermediate ground state of magnesium-19, followed by subsequent decay of magnesium-19 ground state via simultaneous two-proton emission.
Aluminum-20 is the first observed three-proton emitter where its one-proton decay daughter nucleus is a two-proton radioactive nucleus.
The researchers also found that the decay energy of the aluminum-20 ground state is significantly smaller than the predictions inferred from the isospin symmetry, indicating a possible isospin symmetry breaking in aluminum-20 and its mirror partner neon-20.
This finding is supported by state-of-the-art theoretical calculations that predict that the spin-parity of the aluminum-20 ground state differs from the spin-parity of the neon-20 ground state.
“This study advances our understanding of the proton-emission phenomena, and provides insights into the structure and decay of nuclei beyond the proton drip line,” Dr. Xu said.
The team’s paper was published this month in the journal Physical Review Letters.
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X.-D. Xu et al. 2025. Isospin Symmetry Breaking Disclosed in the Decay of Three-Proton Emitter 20Al. Phys. Rev. Lett 135, 022502; doi: 10.1103/hkmy-yfdk