At a Glance
- Scientists have observed the previously unknown isotope aluminum-20 for the first time by detecting its unique in-flight decay at a heavy ion research facility in Germany.
- Aluminum-20 decays through a rare three-proton emission, first ejecting one proton before its daughter nucleus, magnesium-19, simultaneously emits two more protons in a sequential process.
- The measured decay energy is significantly lower than predicted, indicating a possible breakdown of isospin symmetry when compared to its mirror nucleus, neon-20, which has a similar mass.
- This symmetry breaking is supported by calculations showing that the aluminum-20 ground state has a different spin-parity (1−) than its mirror partner, neon-20 (2−), indicating structural differences.
- The discovery advances the understanding of exotic radioactive decay and provides crucial data on the structure of atomic nuclei located far from the region of stability.
Physicists have announced the discovery of aluminum-20, a previously unknown and extremely unstable form, or isotope, of aluminum. In a study published in Physical Review Letters, an international team of scientists reported observing this new isotope for the first time as it underwent a rare form of radioactive decay, emitting three protons to transform into a different element. Radioactive decay is the process by which unstable atomic nuclei release energy, and studying exotic forms, such as proton emission, helps scientists understand the fundamental forces that hold nuclei together.
The discovery was made using an in-flight decay technique at the GSI Helmholtz Center for Heavy Ion Research in Germany. “Aluminum-20 is the lightest aluminum isotope that has been discovered so far,” said Xu Xiaodong, the study’s first author and an associate professor at the Institute of Modern Physics of the Chinese Academy of Sciences. Located “beyond the proton drip line,” a theoretical boundary where nuclei become so rich in protons they can no longer hold them, aluminum-20 is highly unstable. Researchers tracked its decay products to confirm its existence and study its properties.
The team found that aluminum-20 decays in a unique, sequential manner. First, it ejects a single proton to become magnesium-19. This “daughter” nucleus is itself radioactive and immediately decays by emitting two protons at once. This is the first time scientists have observed a daughter nucleus undergoing two-proton radioactivity following a one-proton decay from its parent. This detailed observation allowed the team to measure the decay energy of aluminum-20’s ground state precisely.
This measurement revealed a significant discrepancy that challenges a core concept in nuclear physics. The observed energy was much lower than predicted by isospin symmetry, a principle that suggests specific pairs of “mirror nuclei”—which have their proton and neutron counts swapped—should have very similar structures and energies. The properties of aluminum-20 differ significantly from its mirror partner, neon-20, indicating a potential breaking of this fundamental symmetry. “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,” Xu said.
References
- Nannan, Z. & Chinese Academy of Sciences. (2025, July 21). Physicists discover aluminum-20, a new three-proton-emitting isotope. Phys.Org; Chinese Academy of Sciences. https://phys.org/news/2025-07-physicists-aluminum-proton-emitting-isotope.html
- Xu, X.-D., Mukha, I., Li, J. G., Wang, S. M., Acosta, L., Bajzek, M., Casarejos, E., Cortina-Gil, D., Espino, J. M., Fomichev, A., Geissel, H., Gómez-Camacho, J., Grigorenko, L. V., Kiselev, O., Korsheninnikov, A. A., Kostyleva, D., Kurz, N., Litvinov, Yu. A., Martel, I., … Zhukov, M. V. (2025). Isospin symmetry breaking disclosed in the decay of three-proton emitter al 20. Physical Review Letters, 135(2), 022502. https://doi.org/10.1103/hkmy-yfdk
