Nuclear species consisting of a nucleus surrounded by 'halo' of neutrons. Discovered in short-lived isotopes of light elements with excess neutrons. The Nuclei appeared to have a large volume for their mass but have actually been found to consist of the nucleus surrounded by a neutron halo.

The halo is the result of neutrons with a low nuclear binding energy Quantum Tunnelling out of the nucleus, but remaining inside the atom. This is because the neutrons are 'linked' not bound to the nuclei as illustrated by Borromean rings. Therefore the species is able to exist as the neutron halo as it is not dependent on the strong nuclear force, which has no effect outside the nucleus.

The Neutron Halo has been found in single and double neutron forms. The two-neutron form was identifiable due to instances of Deuteron emission, since a double neutron halo cannot remain as a single halo after Beta decay. Berilyum-11 is an example of a single Neutron Halo. Lithium-11 has a double Neutron Halo.

A good source of information on Neutron Halos