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Neutronium is an often misused term for an extremely dense phase of matter that occurs under the intense pressure found in the core of neutron stars and is currently not well understood. The word was coined by Andreas von Antropoff in 1926 for the 'element of atomic number zero' that he placed at the head of his periodic table. It has subsequently figured in the centre of several spiral versions of the periodic system. It is not an accepted term in astrophysics literature for reasons which will be explained below, but is used with some regularity in science fiction. Despite the very extreme instability of neutronium under Earthly pressures, science fiction authors and screenwriters often design useful materials, such as armor or structural supports, out of it.

Neutron stars

Main article: neutron star

When a massive star creates an iron core whose mass exceeds the Chandrasekhar limit, it will collapse and create a type II supernova. The core of the collapsing star is initially composed of iron supported by electron degeneracy pressure, since the nuclear fusion of iron doesn't release energy. When the core collapses, the densities and pressures in the core overcome even the electron degeneracy pressure and the iron atoms' electrons are compressed into their nuclei where they combine with protons to form neutrons.

{\hbox{p}}+{\hbox{e}}^{-}\to {\hbox{n}}+\nu _{\mathrm {e} }

proton + electron → neutron + neutrino

The neutrino is emitted from the core, leaving the neutron behind. The material that remains has a density of approximately 10¹⁴–10¹⁵ g/cm³.

Cross section of a neutron star

A teaspoon full of this matter would have a mass of 100 million metric tons. According to current theory, matter at the surface of a neutron star is composed of ordinary nuclei as well as electrons. The "atmosphere" of the star is roughly one metre thick, below which one encounters a solid "crust". Proceeding inward, one encounters nuclei with ever increasing numbers of neutrons; such nuclei would quickly decay on Earth, but are kept stable by tremendous pressures. Proceeding deeper, one comes to a point called neutron drip where free neutrons leak out of nuclei. In this region we have nuclei, free electrons, and free neutrons. The nuclei become smaller and smaller until the core is reached, by definition the point where they disappear altogether. The exact nature of the superdense matter in the core is still not well understood. Some researchers refer to this theoretical substance as neutronium, but because of the myriad of uncertainties, and its close relation with science fiction, the term "neutronium" is rarely found in the scientific literature. It could be a superfluid mixture of neutrons with a few protons and electrons, other high-energy particles like pions and kaons may be present, and even sub-atomic quark matter is possible. However so far observations have not indicated nor ruled out such exotic states of matter.

All of these uncertainties can be summarized in an equation of state which describes the pressure of neutron star material given a certain temperature and density. Calculating equations of state is an active and uncertain area of physics. Frequently in the literature, theoretical physicists will refer to a "stiff" equation of state or a "soft" equation of state. A "stiff" equation of state has a higher pressure than a "soft" equation at a given temperature and density.

There is a limit beyond which a neutron star can no longer support itself via neutron degeneracy pressure and would collapse all the way into a black hole; this limit occurs at about 3 solar masses. Current equations of state are considerably "softer" than the guesses for equations of state used in the 1970s which had limits of 7 or 8 solar masses.

Neutron matter

Neutron matter is matter composed solely of neutrons. It is featured on the Chemical Galaxy periodic table as element 0, using symbol "n" (the symbol for a neutron). This matter is commonly referred to as neutronium. Two hypothetical names using IUPAC element-naming conventions for neutron matter include the name and symbol nilnilnilium (Nnn); and (shunning leading zeroes and capital N) the name and symbol nilium (n). Though these names are incorrect, since neutronium is not a real element.

Free neutrons

Main article: free neutron

While neutrons can be stable when bound inside nuclei, free neutrons are unstable and decay with a half-life of about ten minutes. The only possible decay mode is into a proton, an electron, and an electron antineutrino:

{\hbox{n}}\to {\hbox{p}}+{\hbox{e}}^{-}+{\overline {\nu }}_{\mathrm {e} }

They are likely the most common form of neutronium.

Dineutrons

Main article: dineutron

Dineutrons are a fleeting state produced by nuclear reactions involving tritium.

Tetraneutrons

Main article: tetraneutron

A tetraneutron is a hypothetical relatively stable form of neutronium consisting of a tetrad of neutrons (4 neutrons).

Tetraneutronium is consists of a tetrad of neutrons. It is theorized that this radioactive isotope may have a measurable half life.

Neutronium in fiction

In Star Trek, neutronium is an extermely hard and durable substance, which conventional weapons cannot penetrate or even dent.
In Doctor Who, neutronium is a substance which can shield spaces from time-shear when used as shielding in time-vessels.
In Peter F. Hamilton's The Neutronium Alchemist, it plays a starring role.
In Stargate SG-1, neutronium is a substance which is the basis of the technology of the advanced Asgard race, as well as a primary component of human-form Replicators.