The atomic nucleus is usually pictured as a dense ball where protons and neutrons are mixed together. But in some isotopes, neutrons can stick out far beyond this ball, forming a diffuse shell — a neutron halo. This is especially common in nuclei with many more neutrons than protons.

Physicists took magnesium isotopes — from light magnesium-24 to heavy magnesium-38—and calculated how they scatter off carbon at an energy of 240 MeV per nucleon. By measuring how often the nuclei graze each other, they could determine the radius of the neutron cloud. It turned out that in magnesium-37, one neutron is so weakly bound that it forms a single halo.

Then the team applied the same approach to magnesium-40, which had not been studied before. The calculations showed that two neutrons are so loosely attached that they form a double halo. Such nuclei live for fractions of a second, but they are the perfect testing ground for our understanding of nuclear forces.

The double halo in magnesium-40 is a rare case where two neutrons pair up and together venture beyond the dense core. It's like two climbers roping together on the very edge of a cliff.