In nuclear physics, an atomic nucleus is called a halo nucleus or is said to have a nuclear halo when it has a core nucleus surrounded by a halo of orbiting protons or neutrons. The electrons then orbit the core nucleus much further away than the orbit of the halo nucleons. The halo makes the nucleus radius appreciably larger than that predicted by the liquid drop model, wherein the nucleus is assumed to be a sphere of constant density. One example is 11Li where the nuclear radius is about the size of that of 208Pb.
For a nucleus of mass number A, the radius r is (approximately)
where is 1.2 fm.
Typically, an atomic nucleus is a tightly bound group of protons and neutrons. However, in some isotopes, there is an overabundance of one species of nucleon. In some of these cases, a nuclear core and a halo will form.
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. This is the same relation as would be seen with a solid sphere.
One example of a halo nucleus is 11Li which has a half life of 8.6 ms. It decays into 11Be by the emission of an antineutrino and an electron.  Its cross-section of 3.16 fm is close to that of 32S, or even more impressively that of 208Pb, both much heavier nuclei. It contains a core of 3 protons and 6 neutrons, and a halo of two independent and loosely bound neutrons.
Nuclei which have a neutron halo include 11Be and 19C. A two-neutron halo is exhibited by 6He, 11Li, 17B, 19B and 22C. Two-neutron halo nuclei break into three fragments and are called Borromean because of this behavior. 8He and 14Be both exhibit a four-neutron halo.
Nuclei which have a proton halo include 8B and 26P. A two-proton halo is exhibited by 17Ne and 27S. Proton halos are expected to be more rare and unstable than the neutron examples, because of the repulsive forces of the excess proton(s).
Halo nuclei form at the extreme edges of the chart of the nuclides — the neutron drip line and proton drip line — and have short half-lives, measured in milliseconds. These nuclei are studied shortly after their formation in an ion beam.
List of known isotopes with nuclear halo
|Name||# of nuclear
- Krieger, A.; K. Blaum, M. L. Bissell, N. Frömmgen, Ch. Geppert, M. Hammen, K. Kreim, M. Kowalska, J. Krämer, T. Neff, R. Neugart, G. Neyens, W. Nörtershäuser, Ch. Novotny, R. Sánchez, D. T. Yordanov (2012). "Nuclear Charge Radius of 12Be". Physical Review Letters 108 (14). doi:10.1103/PhysRevLett.108.142501. ISSN 0031-9007.