A Borromean nucleus is an atomic nucleus comprising three bound components in which any subsystem of two components is unbound.[1] This has the consequence that if one component is removed, the remaining two comprise an unbound resonance, so that the original nucleus is split into three parts.[2]
The name is derived from the Borromean rings, a system of three linked rings in which no pair of rings is linked.[2]
Many Borromean nuclei are light nuclei near the nuclear drip lines that have a nuclear halo and low nuclear binding energy. For example, the nuclei,, and each possess a two-neutron halo surrounding a core containing the remaining nucleons.[2] These are Borromean nuclei because the removal of either neutron from the halo will result in a resonance unbound to one-neutron emission, whereas the dineutron (the particles in the halo) is itself an unbound system.[1] Similarly, is a Borromean nucleus with a two-proton halo; both the diproton and are unbound.[3]
Additionally, is a Borromean nucleus comprising two alpha particles and a neutron; the removal of any one component would produce one of the unbound resonances or .
Several Borromean nuclei such as and the Hoyle state (an excited resonance in) play an important role in nuclear astrophysics. Namely, these are three-body systems whose unbound components (formed from) are intermediate steps in the triple-alpha process; this limits the rate of production of heavier elements, for three bodies must react nearly simultaneously.[4]
Borromean nuclei consisting of more than three components can also exist. These also lie along the drip lines; for instance, is a five-body Borromean system with a four-neutron halo.[5] It is also possible that nuclides produced in the alpha process (such as and) may be clusters of alpha particles, having a similar structure to Borromean nuclei.[2]
, the heaviest known Borromean nucleus was .[6] Heavier species along the neutron drip line have since been observed; these and undiscovered heavier nuclei along the drip line are also likely to be Borromean nuclei with varying numbers (3, 5, 7, or more) of bodies.[5]