Halo nucleus

Halo nuclei form at the extreme edges of the table of nuclides — the neutron drip line and proton drip line — and have short half-lives, measured in milliseconds.

Typically, an atomic nucleus is a tightly bound group of protons and neutrons.

Often, this property may be detected in scattering experiments, which show the nucleus to be much larger than the otherwise expected value.

Normally, the cross-section (corresponding to the classical radius) of the nucleus is proportional to the cube root of its mass, as would be the case for a sphere of constant density.

For example, the two-neutron halo nucleus 6He (which can be taken as a three-body system consisting of an alpha particle and two neutrons) is bound, but neither 5He nor the dineutron is.