Tetraneutron
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The tetraneutron is considered an unbound isotope with a lifetime around 10−22 seconds. The stability of this cluster of four
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 ...
s is not supported by current models of nuclear forces. Recent
empirical Empirical evidence is evidence obtained through sense experience or experimental procedure. It is of central importance to the sciences and plays a role in various other fields, like epistemology and law. There is no general agreement on how t ...
evidence is "consistent with a quasi-bound tetraneutron state existing for a very short time".


Marqués' experiment

Francisco-Miguel Marqués and co-workers at the GANIL accelerator in
Caen Caen (; ; ) is a Communes of France, commune inland from the northwestern coast of France. It is the Prefectures in France, prefecture of the Departments of France, department of Calvados (department), Calvados. The city proper has 105,512 inha ...
used a
particle accelerator A particle accelerator is a machine that uses electromagnetic fields to propel electric charge, charged particles to very high speeds and energies to contain them in well-defined particle beam, beams. Small accelerators are used for fundamental ...
to fire atomic nuclei at carbon targets and observed the "spray" of particles from the resulting collisions. In this case the experiment involved firing beryllium-14, boron-15 and lithium-11 nuclei at a small
carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...
target, the most successful being beryllium-14. This isotope of beryllium has a nuclear halo that consists of four clustered neutrons; this allows it to be easily separated intact in the high-speed collision with the carbon target. Current nuclear models suggest that four separate neutrons should result when beryllium-10 is produced, but the single signal detected in the production of beryllium-10 suggested a multineutron cluster in the breakup products; most likely a beryllium-10 nucleus and four neutrons fused together into a tetraneutron. A later analysis of the method used in the Marqués' experiment suggested that the detection mechanism was unlikely but the suggestion was refuted, and attempts to reproduce these observations with different methods have not successfully detected any neutron clusters.


Consequences of hypothetical bound tetraneutrons

If, however, the existence of bound tetraneutrons were ever independently confirmed, considerable adjustments would have to be made to current nuclear models. Bertulani and Zelevinsky proposed that, if it existed, the tetraneutron could be formed by a bound state of two dineutron systems. However, attempts to model interactions that might give rise to multineutron clusters have failed, and it "does not seem possible to change modern nuclear Hamiltonians to bind a tetraneutron without destroying many other successful predictions of those Hamiltonians. This means that, should a recent experimental claim of a bound tetraneutron be confirmed, our understanding of nuclear forces will have to be significantly changed." Further work in 2019 suggests potentially observable consequences in
neutron star A neutron star is the gravitationally collapsed Stellar core, core of a massive supergiant star. It results from the supernova explosion of a stellar evolution#Massive star, massive star—combined with gravitational collapse—that compresses ...
crusts, if the tetraneutron exists.


Evidence for very short lived resonances

In 2016, researchers at RIKEN in Wakō, Japan observed evidence that the tetraneutron exists briefly as a
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 ...
. They fired a beam of neutron-rich helium-8 nuclei (two protons and six neutrons) at a liquid target composed of
helium-4 Helium-4 () is a stable isotope of the element helium. It is by far the more abundant of the two naturally occurring isotopes of helium, making up about 99.99986% of the helium on Earth. Its nucleus is identical to an alpha particle, and consi ...
(two protons and two neutrons). Occasionally, the reaction produced beryllium-8 nuclei with four protons and four neutrons, leaving four neutrons unaccounted for. If a four-neutron nucleus did occur, it lasted for about 10−21 seconds before decaying into other particles. Evidence for unbound clusters of 4 neutrons resonances in the disintegration of
beryllium Beryllium is a chemical element; it has Symbol (chemistry), symbol Be and atomic number 4. It is a steel-gray, hard, strong, lightweight and brittle alkaline earth metal. It is a divalent element that occurs naturally only in combination with ...
-14 nuclei, in 8He-8Be interactions, and collisions of 4He nuclei give an estimated lifetime around 10−22 seconds. These discoveries should deepen our understanding of the nuclear forces.


See also

*
Neutronium Neutronium (or neutrium, neutrite, or element zero) is a hypothetical substance made purely of neutrons. The word was coined by scientist Andreas von Antropoff in 1926 (before the 1932 discovery of the neutron) for the hypothetical "element of ...
* Tetraquark


Notes


External links


Announcement of possible tetraneutron observations


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Hypothetical nuclei Neutron Neutronium-4