Neutronium

Neutronium (sometimes shortened to neutrium^[1]) is a proposed name for a substance composed purely of neutrons. The word was coined by scientist Andreas von Antropoff in 1926 (before the discovery of the neutron) for the conjectured "element of atomic number zero" that he placed at the head of the periodic table.^[2][3] However, the meaning of the term has changed over time, and from the last half of the 20th century onward it has been also used legitimately to refer to extremely dense substances resembling the neutron-degenerate matter theorized to exist in the cores of neutron stars; hereinafter "degenerate neutronium" will refer to this. Science fiction and popular literature frequently use the term "neutronium" to refer to a highly dense phase of matter composed primarily of neutrons.

Neutronium and neutron stars

Neutronium is used in popular literature to refer to the material present in the cores of neutron stars (stars which are too massive to be supported by electron degeneracy pressure and which collapse into a denser phase of matter). This term is very rarely used in scientific literature, for three reasons:

• There are multiple definitions for the term "neutronium".
• There is considerable uncertainty over the composition of the material in the cores of neutron stars (it could be neutron-degenerate matter, strange matter, quark matter, or a variant or combination of the above).
• The properties of neutron star material should depend on depth due to changing pressure (see below), and no sharp boundary between the crust (consisting primarily of atomic nuclei) and almost protonless inner layer is expected to exist.

When neutron star core material is presumed to consist mostly of free neutrons, it is typically referred to as neutron-degenerate matter in scientific literature.^[4]

Neutronium and the periodic table

The term "neutronium" was coined in 1926 by Andreas von Antropoff for a conjectured form of matter made up of neutrons with no protons or electrons, which he placed as the chemical element of atomic number zero at the head of his new version of the periodic table. It was subsequently placed in the middle of several spiral representations of the periodic system for classifying the chemical elements, such as those of Charles Janet (1928), E. I. Emerson (1944), John D. Clark (1950) and in Philip Stewart's Chemical Galaxy (2005).

Although the term is not used in the scientific literature either for a condensed form of matter, or as an element, there have been reports that, besides the free neutron, there may exist two bound forms of neutrons without protons.^[5] If neutronium were considered to be an element, then these neutron clusters could be considered to be the isotopes of that element. However, these reports have not been further substantiated.

• Mononeutron: An isolated neutron undergoes beta decay with a mean lifetime of approximately 15 minutes (half-life of approximately 10 minutes), becoming a proton (the nucleus of hydrogen), an electron and an antineutrino.
• Dineutron: The dineutron, containing two neutrons was unambiguously observed in the decay of beryllium-16, in 2012 by researchers at Michigan State University.^[6][7] It is not a bound particle, but had been proposed as an extremely short-lived state produced by nuclear reactions involving tritium. It has been suggested to have a transitory existence in nuclear reactions produced by helions that result in the formation of a proton and a nucleus having the same atomic number as the target nucleus but a mass number two units greater. There had been evidence of dineutron emission from neutron-rich isotopes such as beryllium-16 where mononeutron decay would result in a less stable isotope. The dineutron hypothesis had been used in nuclear reactions with exotic nuclei for a long time.^[8] Several applications of the dineutron in nuclear reactions can be found in review papers.^[9] Its existence has been proven to be relevant for nuclear structure of exotic nuclei.^[10] A system made up of only two neutrons is not bound, though the attraction between them is very nearly enough to make them so.^[11] This has some consequences on nucleosynthesis and the abundance of the chemical elements.^[9][12]
• Trineutron: A trineutron state consisting of three bound neutrons has not been detected, and is not expected to exist even for a short time.
• Tetraneutron: A tetraneutron is a hypothetical particle consisting of four bound neutrons. Reports of its existence have not been replicated. If confirmed, it would require revision of current nuclear models.^[13][14]
• Pentaneutron: Calculations indicate that the hypothetical pentaneutron state, consisting of a cluster of five neutrons, would not be bound.^[15]

Although not called "neutronium", the National Nuclear Data Center's Nuclear Wallet Cards lists as its first "isotope" an "element" with the symbol n and atomic number Z = 0 and mass number A = 1. This isotope is described as decaying to element H with a half life of 7002614400000000000♠10.24±0.02 min.

Properties

Due to beta (β⁻) decay of mononeutron and extreme instability of aforementioned heavier "isotopes", degenerate neutronium is not expected to be stable under ordinary pressures. Free neutrons decay with a half-life of 10 minutes, 11 seconds. A teaspoon of degenerate neutronium gas would have a mass of two billion tonnes, and if moved to standard temperature and pressure, would emit 57 billion joules of β⁻ decay energy in the first half-life (average of 95 MW of power).^[16] This energy may be absorbed as the neutronium gas expands. Though, in the presence of atomic matter compressed to the state of electron degeneracy, the β⁻ decay may be inhibited due to Pauli exclusion principle, thus making free neutrons stable. Also, elevated pressures should make neutrons degenerate themselves. Compared to ordinary elements, neutronium should be more compressible due to the absence of electrically charged protons and electrons. This makes neutronium more energetically favorable than (positive-Z) atomic nuclei and leads to their conversion to (degenerate) neutronium through electron capture, a process which is believed to occur in stellar cores in the final seconds of the lifetime of massive stars, where it is facilitated by cooling via
ν
e emission. As a result, degenerate neutronium can have a density of 7017400000000000000♠4×10¹⁷ kg/m³,^[17] roughly 13 magnitudes denser than the densest known ordinary substances. It was theorized that extreme pressures of order 100 MeV/Fermi³ may deform the neutrons into a cubic symmetry, allowing tighter packing of neutrons,^[18] or cause a strange matter formation.

In fiction

The term "neutronium" has been popular in science fiction since at least the middle of the 20th century. It typically refers to an extremely dense, incredibly strong form of matter. While presumably inspired by the concept of neutron-degenerate matter in the cores of neutron stars, the material used in fiction bears at most only a superficial resemblance, usually depicted as an extremely strong solid under Earth-like conditions, or possessing exotic properties such as the ability to manipulate time and space. In contrast, all proposed forms of neutron star core material are fluids and are extremely unstable at pressures lower than that found in stellar cores. According to one analysis, a neutron star with a mass below about 0.2 solar masses will explode.^[19]

See also

• Compact star

References

Further reading

• Glendenning, N. K. (2000). Compact Stars: Nuclear Physics, Particle Physics, and General Relativity (2nd ed.). Springer. ISBN 978-0-387-98977-8.