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Neutronium (sometimes shortened to neutrium, also referred to as neutrite) is a hypothetical substance composed purely of
neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behav ...
s. The word was coined by scientist
Andreas von Antropoff Andreas von Antropoff (russian: Андрей Романович Антропов; 16 August 1878, Reval, Russian empire — 2 June 1956, Bonn) — Russian (Estonian-born) and German scientist-chemist, professor at the Bonn University and is known t ...
in 1926 (before the 1932 discovery of the neutron) for the hypothetical "element of atomic number zero" (with zero protons in its nucleus) that he placed at the head of the periodic table (denoted by -, or Nu). 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 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.


In neutron stars

Neutronium is used in popular physics 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 Below may refer to: *Earth * Ground (disambiguation) * Soil * Floor * Bottom (disambiguation) * Less than *Temperatures below freezing * Hell or underworld People with the surname * Ernst von Below (1863–1955), German World War I general * Fr ...
), and no sharp boundary between the crust (consisting primarily of
atomic nuclei 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 based on the 1909 Geiger–Marsden gold foil experiment. After the discovery of the neutron in ...
) 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.


In 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 A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron m ...
or
electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary partic ...
s, which he placed as the
chemical element A chemical element is a species of atoms that have a given number of protons in their nuclei, including the pure substance consisting only of that species. Unlike chemical compounds, chemical elements cannot be broken down into simpler sub ...
of atomic number zero at the head of his new version of the
periodic table The periodic table, also known as the periodic table of the (chemical) elements, is a rows and columns arrangement of the chemical elements. It is widely used in chemistry, physics, and other sciences, and is generally seen as an icon of ...
. 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), and John D. Clark (1950). 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 The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons beha ...
, there may exist two bound forms of neutrons without protons. If neutronium were considered to be an element, then these neutron clusters could be considered to be the
isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers ( mass number ...
s of that element. However, these reports have not been further substantiated. *
Mononeutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons beha ...
: An isolated neutron undergoes beta decay with a mean lifetime of approximately 15
minute The minute is a unit of time usually equal to (the first sexagesimal fraction) of an hour, or 60 seconds. In the UTC time standard, a minute on rare occasions has 61 seconds, a consequence of leap seconds (there is a provision to insert a n ...
s ( half-life of approximately 10 minutes), becoming a proton (the nucleus of
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
), an
electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary partic ...
, and an antineutrino. * Dineutron: The dineutron, containing two neutrons, was unambiguously observed in 2012 in the decay of beryllium-16. It is not a bound particle, but had been proposed as an extremely short-lived resonance state produced by nuclear reactions involving
tritium Tritium ( or , ) or hydrogen-3 (symbol T or H) is a rare and radioactive isotope of hydrogen with half-life about 12 years. The nucleus of tritium (t, sometimes called a ''triton'') contains one proton and two neutrons, whereas the nucleus ...
. It has been suggested to have a transitory existence in
nuclear reaction In nuclear physics and nuclear chemistry, a nuclear reaction is a process in which two nuclei, or a nucleus and an external subatomic particle, collide to produce one or more new nuclides. Thus, a nuclear reaction must cause a transformatio ...
s produced by helions (completely ionized helium-3 nuclei) 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. The dineutron hypothesis had been used in nuclear reactions with exotic nuclei for a long time. Several applications of the dineutron in nuclear reactions can be found in review papers. Its existence has been proven to be relevant for nuclear structure of exotic nuclei. A system made up of only two neutrons is not bound, though the attraction between them is very nearly enough to make them so. This has some consequences on nucleosynthesis and the abundance of the chemical elements. * 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. * Pentaneutron: Calculations indicate that the hypothetical pentaneutron state, consisting of a cluster of five neutrons, would not be bound. Although not called "neutronium", the National Nuclear Data Center's ''
Nuclear Wallet Cards Nuclear may refer to: Physics Relating to the nucleus of the atom: *Nuclear engineering *Nuclear physics *Nuclear power *Nuclear reactor *Nuclear weapon *Nuclear medicine *Radiation therapy *Nuclear warfare Mathematics *Nuclear space * Nuclear ...
'' 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 hydrogen-1 with a half life of .


Properties

Neutron matter is equivalent to a
chemical element A chemical element is a species of atoms that have a given number of protons in their nuclei, including the pure substance consisting only of that species. Unlike chemical compounds, chemical elements cannot be broken down into simpler sub ...
with atomic number 0, which is to say that it is equivalent to a species of atoms having no
protons A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron m ...
in their atomic nuclei. It is extremely
radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consi ...
; its only legitimate equivalent isotope, the free neutron, has a half-life of 10 minutes, which is approximately half that of the most stable known isotope of
francium Francium is a chemical element with the symbol Fr and atomic number 87. It is extremely radioactive; its most stable isotope, francium-223 (originally called actinium K after the natural decay chain it appears in), has a half-life of only 22 ...
. Neutron matter decays quickly into
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
. Neutron matter has no electronic structure on account of its total lack of electrons. While this lifetime is long enough to permit the study of neutronium's chemical properties, there are serious practical problems. Having no charge or electrons, neutronium would not interact strongly with ordinary low-energy photons (visible light) and would feel no electrostatic forces, so it would diffuse into the walls of most containers made of ordinary matter. Certain materials are able to resist diffusion or absorption of ultracold neutrons due to nuclear-quantum effects, specifically reflection caused by the strong interaction. At ambient temperature and in the presence of other elements, thermal neutrons readily undergo neutron capture to form heavier (and often radioactive) isotopes of that element. Neutron matter at standard pressure and temperature is predicted by the ideal gas law to be less dense than even hydrogen, with a density of only (roughly 27 times less dense than air and half as dense as hydrogen gas). Neutron matter is expected to remain gaseous down to absolute zero at normal pressures, as the zero-point energy of the system is too high to allow condensation. However, neutron matter should in theory form a degenerate gaseous superfluid at these temperatures, composed of transient neutron-pairs called ''dineutrons''. Under extremely low pressure, this low temperature, gaseous superfluid should exhibit quantum coherence producing a Bose–Einstein condensate. At higher temperatures, neutron matter will only condense with sufficient pressure, and solidify with even greater pressure. Such pressures exist in neutron stars, where the extreme pressure causes the neutron matter to become degenerate. However, in the presence of atomic matter compressed to the state of
electron degeneracy Electron degeneracy pressure is a particular manifestation of the more general phenomenon of quantum degeneracy pressure. The Pauli exclusion principle disallows two identical half-integer spin particles (electrons and all other fermions) from sim ...
, β decay may be inhibited due to the Pauli exclusion principle, thus making free neutrons stable. Also, elevated pressures should make neutrons
degenerate Degeneracy, degenerate, or degeneration may refer to: Arts and entertainment * ''Degenerate'' (album), a 2010 album by the British band Trigger the Bloodshed * Degenerate art, a term adopted in the 1920s by the Nazi Party in Germany to descr ...
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 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 based on the 1909 Geiger–Marsden gold foil experiment. After the discovery of the neutron in ...
and leads to their conversion to (degenerate) neutronium through electron capture, a process that is believed to occur in stellar cores in the final seconds of the lifetime of massive stars, where it is facilitated by cooling via emission. As a result, degenerate neutronium can have a density of , roughly 14 orders of magnitude denser than the densest known ordinary substances. It was theorized that extreme pressures of order might deform the neutrons into a cubic symmetry, allowing tighter packing of neutrons, or cause a strange matter formation.


See also

* Compact star


References


Further reading

* {{Portal bar, Physics, Chemistry, Astronomy, Stars, Science Astrophysics Exotic matter Neutron Fictional materials