Kyle Brown, Michigan State University (MSU), East Lansing, USA, and colleagues have discovered a new magnesium isotope, magnesium-18. With 12 protons and only six neutrons in its nucleus, it is the lightest isotope of magnesium ever found. It is radically unstable and very short-lived, with a half-life of less than 10^–21 seconds.

The researchers accelerated a beam of magnesium-24 nuclei (the most common stable magnesium isotope) to about half the speed of light at MSU’s National Superconducting Cyclotron Laboratory, a circular ultrahigh-energy particle accelerator. Then they fired the high-speed beam of magnesium nuclei at a target made of beryllium. The collision yielded lighter magnesium isotopes, including the unstable isotope magnesium-20, which contains only eight neutrons per nucleus and decays radioactively in a few tenths of a second. The researchers fired the magnesium-20 nuclei at about half the speed of light at a beryllium target. One of the products of the resulting collision was magnesium-18.

The isotope is so short-lived that it decays inside the beryllium target. Therefore, the researchers deduced its presence from the telltale products of its decay: stray protons and the isotopes neon-16 and oxygen-14.

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• First Observation of the Four-Proton Unbound Nucleus ¹⁸Mg,
  Y. Jin, C. Y. Niu, K. W. Brown, Z. H. Li, H. Hua, A. K. Anthony, J. Barney, R. J. Charity, J. Crosby, D. Dell’Aquila, J. M. Elson, J. Estee, M. Ghazali, G. Jhang, J. G. Li, W. G. Lynch, N. Michel, L. G. Sobotka, S. Sweany, F. C. E. Teh, A. Thomas, C. Y. Tsang, M. B. Tsang, S. M. Wang, H. Y. Wu, C. X. Yuan, K. Zhu,
  Physical Review Letters 2021.
  https://doi.org/10.1103/PhysRevLett.127.262502