In an atomic nucleus, it's usually crowded: protons and neutrons are packed like commuters on a rush-hour train. But beryllium-11 is different. One neutron keeps its distance from the rest, forming a diffuse cloud — a nuclear halo. This nucleus resembles a planet with a satellite on a highly elongated orbit.
To figure out the structure of this exotic nucleus, physicists ran an experiment at the AT-TPC setup inside the SOLARIS solenoid. They bombarded beryllium-10 nuclei with deuterons (nuclei of heavy hydrogen) and tracked the production of beryllium-11. By measuring the angles of the outgoing particles, they could reconstruct which states of the nucleus are excited and what energies they carry. Of particular interest was a state at 3.40 MeV. Its properties are best explained if it has positive parity and spin 3/2. This means the halo neutron moves in an orbit with a certain symmetry, like an electron in an atom.
Comparison with state-of-the-art calculations, where the nucleus is modeled from first principles, showed good agreement. This bolsters confidence that the 3.40 MeV state is the second excited state of the rotational band built on the halo ground state. In other words, the halo nucleus can rotate as a whole, and the satellite neutron joins in the dance.
Studying beryllium-11 is a step toward understanding how nuclei behave at the edge of existence. Such nuclei live for fractions of a second, but they are born in stars and shape the pathways of element synthesis across the universe.
The halo nucleus of beryllium-11 is comparable in size to a lead nucleus, even though it contains twenty times fewer particles.