nuclear physics Nuclear physics is the field of physics that studies atomic nuclei and their constituents and interactions, in addition to the study of other forms of nuclear matter. Nuclear physics should not be confused with atomic physics, which studies th ...

, a Borromean nucleus is an

atomic nucleus The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford at the Department_of_Physics_and_Astronomy,_University_of_Manchester , University of Manchester ...

comprising three bound components in which any subsystem of two components is unbound. This has the consequence that if one component is removed, the remaining two comprise an unbound

resonance Resonance is a phenomenon that occurs when an object or system is subjected to an external force or vibration whose frequency matches a resonant frequency (or resonance frequency) of the system, defined as a frequency that generates a maximu ...

, so that the original nucleus is split into three parts. The name is derived from the

Borromean rings In mathematics, the Borromean rings are three simple closed curves in three-dimensional space that are link (knot theory), topologically linked and cannot be separated from each other, but that break apart into two unknotted and unlinked loops wh ...

, a system of three linked rings in which no pair of rings is linked.

Examples of Borromean nuclei

Many Borromean nuclei are light nuclei near the

nuclear drip line The nuclear drip line is the boundary beyond which atomic nuclei are unbound with respect to the emission of a proton or neutron. An arbitrary combination of protons and neutrons does not necessarily yield a stable atomic nucleus, nucleus. One ...

s that have a nuclear halo and low

nuclear binding energy Nuclear binding energy in experimental physics is the minimum energy that is required to disassemble the nucleus of an atom into its constituent protons and neutrons, known collectively as nucleons. The binding energy for stable nuclei is alwa ...

. For example, the nuclei He, Li, and C each have a two-

neutron The neutron is a subatomic particle, symbol or , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...

halo surrounding a core containing the remaining nucleons. These are Borromean nuclei because the removal of either neutron from the halo will result in a resonance unbound to one-

neutron emission Neutron emission is a mode of radioactive decay in which one or more neutrons are ejected from a Atomic nucleus, nucleus. It occurs in the most neutron-rich/proton-deficient nuclides, and also from excited states of other nuclides as in photodisin ...

, whereas the dineutron (the particles in the halo) is itself an unbound system. Similarly, Ne is a Borromean nucleus with a two-proton halo; both the

diproton Helium (He) ( standard atomic weight: ) has nine known isotopes, but only helium-3 (He) and helium-4 (He) are stable. All radioisotopes are short-lived; the longest-lived is He with half-life . The least stable is He, with half-life (), though He ...

and F are unbound. Also, stable Be is a Borromean nucleus comprising two

alpha particle Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay but may also be produce ...

s and a neutron; the removal of any one component would produce one of the unbound resonances He or Be. Several Borromean nuclei such as Be and the

Hoyle state Carbon-12 (12C) is the most abundant of the two Stable isotope, stable isotopes of carbon (carbon-13 being the other), amounting to 98.93% of Periodic table, element carbon on Earth; its abundance is due to the triple-alpha process by which it i ...

(an excited resonance in C) play an important role in

nuclear astrophysics Nuclear astrophysics studies the origin of the chemical elements and isotopes, and the role of nuclear energy generation, in cosmic sources such as stars, supernovae, novae, and violent binary-star interactions. It is an interdisciplinary part ...

. Namely, these are three-body systems whose unbound components (formed from He) are intermediate steps in the

triple-alpha process The triple-alpha process is a set of nuclear fusion reactions by which three helium-4 nuclei (alpha particles) are transformed into carbon. In stars Helium accumulates in the cores of stars as a result of the proton–proton chain reaction a ...

; this limits the rate of production of heavier elements, for three bodies must react nearly simultaneously. Borromean nuclei made of more than three components can also exist. These also lie along the drip lines; for instance, He and Be are five-body Borromean systems with a four-neutron halo. It is also possible that nuclides produced in the

alpha process The alpha process, also known as alpha capture or the alpha ladder, is one of two classes of nuclear fusion reactions by which stars convert helium into heavier elements. The other class is a cycle of reactions called the triple-alpha process, w ...

(such as C and O) may be clusters of alpha particles, having a similar structure to Borromean nuclei. , the heaviest known Borromean nucleus was F. 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.

References

{{reflist Nuclear physics

Examples of Borromean nuclei

See also

References