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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
and ¹⁴
Be
are five-body Borromean systems 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]

References

^ ^a ^b Id Betan, R. M. (2017). "Cooper pairs in the Borromean nuclei ⁶He and ¹¹Li using continuum single particle level density". Nuclear Physics A. 959: 147–148. arXiv:1701.08099. Bibcode:2017NuPhA.959..147I. doi:10.1016/j.nuclphysa.2017.01.004. S2CID 119243017.
^ ^a ^b ^c ^d Manton, N.; Mee, N. (2017). "Nuclear Physics". The Physical World: An Inspirational Tour of Fundamental Physics. Oxford University Press. pp. 387–389. doi:10.1093/oso/9780198795933.003.0012. ISBN 978-0-19-879611-4. LCCN 2017934959.
^ ^a ^b ^c Vaagen, J. S.; Gridnev, D. K.; Heiberg-Andersen, H.; et al. (2000). "Borromean Halo Nuclei" (PDF). Physica Scripta. T88 (1): 209–213. Bibcode:2000PhST...88..209V. doi:10.1238/Physica.Topical.088a00209. S2CID 121095106.
^ Oishi, T.; Hagino, K.; Sagawa, H. (2010). "Diproton correlation in the proton-rich Borromean nucleus ¹⁷Ne". Physical Review C. 82 (6): 066901–1–066901–6. arXiv:1007.0835. doi:10.1103/PhysRevC.82.069901.
^ ^a ^b Riisager, K. (2013). "Halos and related structures". Physica Scripta. 2013 (14001): 014001. arXiv:1208.6415. Bibcode:2013PhST..152a4001R. doi:10.1088/0031-8949/2013/T152/014001. S2CID 119290542.
^ Gaudefroy, L.; Mittig, W.; Orr, N. A.; et al. (2012). "Direct Mass Measurements of ¹⁹B, ²²C, ²⁹F, ³¹Ne, ³⁴Na and Other Light Exotic Nuclei". Physical Review Letters. 109 (20): 202503–1–202503–5. arXiv:1211.3235. doi:10.1103/PhysRevLett.109.202503. PMID 23215476. S2CID 21166319.