An atomic nucleus is a collection of neutrons and protons. These come in roughly equal numbers, although a heavy nucleus is likely to choose more neutrons, because they are unaffected by the repulsive electric charges of the protons. But predicting the most extreme combinations of neutrons and protons that will bind together is not so simple. The rules that determine nuclear existence also influence the course of astrophysical processes, such as supernova explosions, that serve to enrich the Universe with heavy elements including those necessary for life on Earth. So, it is an achievement indeed for a team of nuclear physicists to have discovered a new extreme of nuclear matter, as reported by Bertram Blank et al.1 in Physical Review Letters. Working at the Grand Accélérateur National d'Ions Lourds laboratory in France, they have created a new isotope of nickel, nickel-48, which is made up of 28 protons and 20 neutrons. This represents the most severe shortage of neutrons for any known isotope.
The reason that 28 protons can support such a paucity of neutrons hinges on the shell structure of nuclei. Just as full electron shells in atoms provide extra stability for the ‘inert gases’ helium, neon and argon, so full nuclear shells give additional stability to nuclei. Both the proton number (28) and the neutron number (20) of nickel-48 correspond to full shells, called ‘magic’ numbers by nuclear physicists, so nickel-48 is ‘doubly magic’. The new results come after many years of technical development, with a mixture of competition and collaboration between laboratories. The neighbouring isotope nickel-49, for example, was created at the Gesellschaft für Schwerionenforschung laboratory in Germany, with leadership from the same Bertram Blank2.
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