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]
Examples of Borromean nuclei
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 6
He
, 11
Li
, and 22
C
each possess a two-neutron halo surrounding a core containing the remaining nucleons.[2][3] 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, 17
Ne
is a Borromean nucleus with a two-proton halo; both the diproton and 16
F
are unbound.[4]
Additionally, 9
Be
is a Borromean nucleus comprising two alpha particles and a neutron;[3] the removal of any one component would produce one of the unbound resonances 5
He
or 8
Be
.
Several Borromean nuclei such as 9
Be
and the Hoyle state (an excited resonance in 12
C
) play an important role in nuclear astrophysics. Namely, these are three-body systems whose unbound components (formed from 4
He
) are intermediate steps in the triple-alpha process; this limits the rate of production of heavier elements, for three bodies must react nearly simultaneously.[3]
Borromean nuclei consisting of more than three components can also exist. These also lie along the drip lines; for instance, 8
He
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 12
C
and 16
O
) may be clusters of alpha particles, having a similar structure to Borromean nuclei.[2]
As of 2012[update], the heaviest known Borromean nucleus was 29
F
.[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]