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{{short description|Hypothetical substance in nuclear physics}}

'''Neutronium''' is a term used in [[science fiction]] and popular literature to refer to an extremely [[dense]] [[phases of matter|phase of matter]] composed primarily of [[neutron]]s. The word was coined by [[Andreas von Antropoff]] in 1926 for the 'element of atomic number zero' that he placed at the head of the periodic table. The meaning of the term changed over time, and from the last half of the [[20th century]] onwards it was used to refer to extremely dense phases of matter resembling the [[degenerate matter|neutron-degenerate matter]] postulated to exist in the cores of [[neutron star]]s.

'''Neutronium''' (or '''neutrium''',<ref name=InglisArkell>{{cite web | last = Inglis-Arkell | first = Esther | url = http://io9.com/5899961/neutrium-the-most-neutral-hypothetical-state-of-matter-ever | title = Neutrium: The Most Neutral Hypothetical State of Matter Ever | work = [[io9.com]] | date = 2012-04-14 | access-date = 2013-02-11 | archive-date = 2014-11-12 | archive-url = https://web.archive.org/web/20141112052011/http://io9.com/5899961/neutrium-the-most-neutral-hypothetical-state-of-matter-ever | url-status = live }}</ref> '''neutrite,'''<ref name="Zhuravleva">{{Cite book|url=https://books.google.com/books?id=HpttCzNiB6wC&pg=PA75|title=Ballad of the Stars: Stories of Science Fiction, Ultraimagination, and TRIZ|last=Zhuravleva|first=Valentina|date=2005|publisher=Technical Innovation Center, Inc.|isbn=978-0-9640740-6-4|page=75|access-date=2019-04-25|archive-date=2022-04-12|archive-url=https://web.archive.org/web/20220412033116/https://books.google.com/books?id=HpttCzNiB6wC&pg=PA75|url-status=live}}</ref> or '''element zero''') is a hypothetical substance made purely of [[neutron]]s. The word was coined by scientist [[Andreas von Antropoff]] in 1926 (before the 1932 [[discovery of the neutron]]) for the hypothetical "element of atomic number zero" (with no protons in its nucleus) that he placed at the head of the [[periodic table]] (denoted by -).<ref name='Antropoff 1926'>{{cite journal | last = von Antropoff | first = A. | title = Eine neue Form des periodischen Systems der Elementen | journal = [[Zeitschrift für Angewandte Chemie]] | date = 1926 | volume = 39 | issue = 23 | pages = 722–725 | doi = 10.1002/ange.19260392303| bibcode = 1926AngCh..39..722V |lang=de}}</ref><ref name='Stewart 2007'>{{cite journal | last = Stewart | first = P. J. | title = A century on from Dmitrii Mendeleev: Tables and spirals, noble gases and Nobel prizes | journal = [[Foundations of Chemistry]] | date = 2007 | volume = 9 | issue = 3 | pages = 235–245 | doi = 10.1007/s10698-007-9038-x | s2cid = 97131841 }}</ref> However, the meaning of the term has [[semantic change|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 star]]s.

== Neutronium and neutron stars ==

== Neutronium andIn neutron stars ==

The term '''neutronium''' is used in popular literature to refer to the material present in the cores of [[neutron star]]s (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 two reasons:

* [[{{Main|Neutron star]]}}

* There is no universally agreed-upon definition for the term "neutronium".

*[[Image:Neutron Therestar iscross very considerable uncertainty over the compositionsection.svg|thumb|Cross-section of theneutron materialstar. inHere, the corescore ofhas [[neutron starNeutron|neutrons]]s (it could beor [[Neutron degenerate matter|neutron-degenerate matter]], [[strange matter]],and [[quark matter]], or a variant or combination of the above).]]

Neutronium is used in popular physics literature<ref name=InglisArkell/><ref name=Zhuravleva/> 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). In scientific literature the term "neutron-degenerate matter"<ref>{{cite book | last1 = Angelo | first1 = J. A. | date = 2006 | title = Encyclopedia of Space and Astronomy | url = https://books.google.com/books?id=VUWno1sOwnUC&pg=PA178 | page = 178 | publisher = [[Infobase Publishing]] | isbn = 978-0-8160-5330-8 | access-date = 2016-10-28 | archive-date = 2019-12-15 | archive-url = https://web.archive.org/web/20191215113119/https://books.google.com/books?id=VUWno1sOwnUC&pg=PA178 | url-status = live }}</ref> or simply [[neutron matter]] is used for this material.<ref>{{Cite journal |last1=Gandolfi |first1=Stefano |last2=Gezerlis |first2=Alexandros |last3=Carlson |first3=J. |date=2015-10-19 |title=Neutron Matter from Low to High Density |url=https://www.annualreviews.org/doi/10.1146/annurev-nucl-102014-021957 |journal=Annual Review of Nuclear and Particle Science |language=en |volume=65 |issue=1 |pages=303–328 |doi=10.1146/annurev-nucl-102014-021957 |issn=0163-8998|arxiv=1501.05675 |bibcode=2015ARNPS..65..303G }}</ref>

A more detailed discussion of the structure of [[neutron star]]s is presented in the [[neutron star]] article. When [[neutron star]] core material is presumed to consist mostly of free neutrons, it is typically referred to as [[degenerate matter|neutron-degenerate matter]] in scientific literature.

==Hypothetical multi-neutrons==

== 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 [[proton]]s or [[electron]]s, which he placed as the [[chemical element]] of [[atomic number]] zero at the head of his new version of the [[periodic table]].<ref name='Antropoff 1926'/> 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), [[Edgar Emerson]] (1944),<ref>{{cite journal |title=A new spiral form of the periodic table |date=1944 |last1=Emerson |first1=Edgar I. |journal=Journal of Chemical Education |volume=21 |issue=3 |page=111 |bibcode=1944JChEd..21..111E |doi=10.1021/ed021p111 }}</ref><ref>{{cite journal| title=A chart based on atomic numbers showing the electronic structure of the elements| date=1944| last1=Emerson| first1=Edgar I.| journal=Journal of Chemical Education| volume=21| issue=5| page=254| bibcode=1944JChEd..21..254E| doi=10.1021/ed021p254}}</ref> and [[John Drury Clark|John D. Clark]] (1950).

{{anchor|Isotopes}}The term is not used in the scientific literature either for a condensed form of matter, or as an element, and theoretical analysis expects no bound forms of neutrons without protons.<ref>{{cite journal | last = Timofeyuk | first = N. K. | title = Do multineutrons exist? | date = 2003 | volume = 29 | issue = 2 | page = L9 | journal = [[Journal of Physics G]] | arxiv = nucl-th/0301020 | bibcode = 2003JPhG...29L...9T | doi = 10.1088/0954-3899/29/2/102 | s2cid = 2847145 }}</ref>

The term '''neutronium''' was coined in 1926 by Professor Andreas von Antropoff for a form of matter made up of [[neutrons]] with no [[protons]], which he placed as the [[element]] of [[atomic number]] zero at the head of his new version of the [[periodic table]]. It was subsequently placed as a [[noble gas]] in the middle of several spiral representations of the periodic system for classifying the [[chemical elements]]. It is at the centre of the [[Chemical Galaxy]] (2005).

===Scattering resonances with multiple neutrons===

Although the term is still not widely used in the scientific literature for a condensed form of matter, there are indications that, besides the [[free neutron]], there may exist two [[isotopes]] without protons. Further information can be found in the following articles:

* [[Free neutron]]: Isolated [[neutron]]s undergo [[beta decay]] with a [[half-life]] of approximately 10 [[minute]]s, becoming [[proton]]s (the [[atomic_nucleus|nucleus]] of [[hydrogen]]).

* [[Dineutron]]: The [[dineutron]], containing two bound neutrons, is proposed as an extremely short-lived state produced by nuclear reactions involving [[tritium]].

* [[Tetraneutron]]: A [[tetraneutron]] is a hypothetical particle consisting of four bound neutrons. It is expected to have a half-life considerably longer than that of the [[dineutron]].

The dineutron, containing two neutrons, is not a stable bound particle, but an extremely short-lived resonance state produced by nuclear reactions in the decay of beryllium-16. Evidence reported in 2012 for the resonance<ref>{{cite journal | author = Schirber, M. | title = Nuclei Emit Paired-up Neutrons | journal = [[Physics (American Physical Society magazine)|Physics]] | date = 2012 | volume = 5 | article-number = 30 | doi = 10.1103/Physics.5.30 | bibcode = 2012PhyOJ...5...30S }}</ref><ref>{{cite journal | author = Spyrou, A. | display-authors = 4 | author2 = Kohley, Z. | author3 = Baumann, T. | author4 = Bazin, D. | author5 = Brown, B. A. | author6 = Christian, G. | author7 = DeYoung, P. A. | author8 = Finck, J. E. | author9 = Frank, N. | author10 = Lunderberg, E. | author11 = Mosby, S. | author12 = Peters, W. A. | author13 = Schiller, A. | author14 = Smith, J. K. | author15 = Snyder, J. | author16 = Strongman, M. J. | author17 = Thoennessen, M. | author18 = Volya, A. | title = First Observation of Ground State Dineutron Decay: ¹⁶Be | journal = [[Physical Review Letters]] | date = 2012 | volume = 108 | issue = 10 | page = 102501 | doi = 10.1103/PhysRevLett.108.102501 | pmid = 22463404 | bibcode = 2012PhRvL.108j2501S | osti = 1104191 | doi-access = free }}</ref> was disputed,<ref>Marqués, F. M., Orr, N. A., Achouri, N. L., Delaunay, F., & Gibelin, J. (2012). Comment on “First Observation of Ground State Dineutron Decay: Be 16”. Physical Review Letters, 109(23), 239201.</ref> but new work reportedly clears up the issues.<ref>{{Cite journal |last1=Monteagudo |first1=B. |last2=Marqués |first2=F. M. |last3=Gibelin |first3=J. |last4=Orr |first4=N. A. |last5=Corsi |first5=A. |last6=Kubota |first6=Y. |last7=Casal |first7=J. |last8=Gómez-Camacho |first8=J. |last9=Authelet |first9=G. |last10=Baba |first10=H. |last11=Caesar |first11=C. |last12=Calvet |first12=D. |last13=Delbart |first13=A. |last14=Dozono |first14=M. |last15=Feng |first15=J. |date=2024-02-23 |title=Mass, Spectroscopy, and Two-Neutron Decay of $^{16}\mathrm{Be}$ |url=https://eprints.whiterose.ac.uk/211204/1/2401.16817.pdf |journal=Physical Review Letters |volume=132 |issue=8 |pages=082501 |doi=10.1103/PhysRevLett.132.082501 |pmid=38457706 |osti=2473832 }}</ref>

A '''trineutron''' state consisting of three bound neutrons has not been detected, and is not expected to be stable even for a short time. Calculations indicate that the hypothetical '''pentaneutron''' state, consisting of a cluster of five neutrons, would not be bound. [http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TVN-472K3HG-2K1&_coverDate=06%2F11%2F1981&_alid=349075295&_rdoc=1&_fmt=&_orig=search&_qd=1&_cdi=5539&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=f052b79209dd914c85a1bc0d32f774ab]

The dineutron hypothesis had been used in theoretical studies of the structure of [[exotic nuclei]]. For example ¹¹Li is modeled as a dineutron bound to a ⁹Li core.<ref name="Bertulani 1993 281–376">{{cite journal | last1 = Bertulani | first1 = C. A. | last2 = Canto | first2 = L. F. | last3 = Hussein | first3 = M. S. | title = The Structure And Reactions Of Neutron-Rich Nuclei | journal = [[Physics Reports]] | date = 1993 | volume = 226 | issue = 6 | pages = 281–376 | bibcode = 1993PhR...226..281B | doi = 10.1016/0370-1573(93)90128-Z | url = http://www.tamu-commerce.edu/physics/carlos/papers/PRep226_1993_281.pdf | archive-url = https://web.archive.org/web/20110928120249/http://www.tamu-commerce.edu/physics/carlos/papers/PRep226_1993_281.pdf | url-status = dead | archive-date = 2011-09-28 }}</ref><ref>{{cite journal | last1 = Hagino | first1 = K. | last2 = Sagawa | first2 = H. | last3 = Nakamura | first3 = T. | last4 = Shimoura | first4 = S. | title = Two-particle correlations in continuum dipole transitions in Borromean nuclei | journal = [[Physical Review C]] | date = 2009 | volume = 80 | issue = 3 | page = 1301 | arxiv = 0904.4775 | bibcode = 2009PhRvC..80c1301H | doi = 10.1103/PhysRevC.80.031301 | s2cid = 119293335 }}</ref> A system made up of only two neutrons is not bound, though the attraction between them is very nearly enough to make them so.<ref>{{cite journal | last1 = MacDonald | first1 = J. | last2 = Mullan | first2 = D. J. | date = 2009 | title = Big Bang Nucleosynthesis: The Strong Nuclear Force meets the Weak Anthropic Principle | journal = [[Physical Review D]] | volume = 80 | issue = 4 | page = 3507 | arxiv = 0904.1807 | bibcode = 2009PhRvD..80d3507M | doi = 10.1103/PhysRevD.80.043507 | s2cid = 119203730 }}</ref> This has some consequences on [[nucleosynthesis]] and the [[abundance of the chemical elements]].<ref name="Bertulani 1993 281–376"/><ref>{{cite journal | last1 = Kneller | first1 = J. P. | last2 = McLaughlin | first2 = G. C. | author2-link = Gail McLaughlin | title = The Effect of Bound Dineutrons upon BBN | journal = [[Physical Review D]] | date = 2004 | volume = 70 | issue = 4 | page = 3512 | arxiv = astro-ph/0312388 | bibcode = 2004PhRvD..70d3512K | doi = 10.1103/PhysRevD.70.043512 | s2cid = 119060865 }}</ref>

If one were to somehow allow the definition of an element without protons, that hypothetical 'element zero' could be systematically named nilium. However the systematic symbol (N), even though similar to the symbol for a [[neutron]] (n), conflicts with [[nitrogen]]. Most prefer the trivial name ''neutronium''. If the systematic names require three letters, then this would be "Nnn" (nilnilnilium). One alternative is to state that nil (n) is not written in leading positions and is never capitalized. That makes the symbol for nilium (n) exactly that of the neutron (n).

A trineutron state consisting of three bound neutrons has not been detected, and is not expected to be bound.<ref>{{cite journal |title=Ab initio no-core Gamow shell-model calculations of multineutron systems |first1=J. G. |last1=Li |first2=N. |last2=Michel |first3=B. S. |last3=Hu |first4=W. |last4=Zuo |first5=F. R. |last5=Xu |journal=Physical Review C |volume=100 |issue= 5|date=2019 |page=054313 |doi=10.1103/PhysRevC.100.054313 |arxiv=1911.06485|bibcode=2019PhRvC.100e4313L }}</ref>

== Neutronium 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 [[degenerate matter|neutron-degenerate matter]] in the cores of [[neutron star]]s, the material used in fiction bears at most only a superficial resemblance (usually depicted as an extremely strong solid under [[Earth|Earthlike]] conditions, while all proposed forms of [[neutron star]] core material are [[fluid]]s and are extremely unstable at [[pressure]]s lower than that found in [[star|stellar]] cores).

A [[tetraneutron]] is a hypothetical particle consisting of four bound neutrons. Reports of its existence have not been replicated.<ref>{{cite journal | last1 = Bertulani | first1 = C. A. | last2 = Zelevinsky | first2 = V. | year = 2003 | title = Is the tetraneutron a bound dineutron-dineutron molecule? | journal = [[Journal of Physics G]] | volume = 29 | issue = 10 | pages = 2431–2437 | arxiv = nucl-th/0212060 | bibcode = 2003JPhG...29.2431B | doi = 10.1088/0954-3899/29/10/309 | s2cid = 55535943 }}</ref><ref>[https://scitechdaily.com/tetra-neutron-experiment-understanding-of-nuclear-forces-might-have-to-be-significantly-changed/ "Tetra-Neutron Experiment: Understanding of Nuclear Forces Might Have To Be Significantly Changed"]. {{Webarchive|url=https://web.archive.org/web/20211213090609/https://scitechdaily.com/tetra-neutron-experiment-understanding-of-nuclear-forces-might-have-to-be-significantly-changed/ |date=2021-12-13 }}. SciTechDaily, December 12, 2021. Technical University of Munich (TUM)</ref>

Noteworthy appearances of neutronium in fiction include the following:

* In [[Star Trek]], neutronium is an extermely hard and durable substance, often used as armor, 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]], neutronium is a component of a [[superweapon]].

* 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.

* In the [[Known Space]] [[fictional universe]] of [[Larry Niven]], neutronium is actual [[neutron star]] core material. Niven does not make assumptions about its strength, but imagines that small blobs of it would remain stable (and inevitably spherical) under their own gravity.

* In [[Greg Bear]]'s ''The Planet-Killers'' (or ''The Law'') [[duology]], neutronium and [[anti-neutronium]] are used to destroy planet Earth.

Calculations indicate that the hypothetical pentaneutron state, consisting of a cluster of five neutrons, would not be bound.<ref>{{cite journal | last1 = Bevelacqua | first1 = J. J. | title = Particle stability of the pentaneutron | journal = [[Physics Letters B]] | date = 1981 | volume = 102 | issue = 2–3 | pages = 79–80 | bibcode = 1981PhLB..102...79B | doi = 10.1016/0370-2693(81)91033-9 }}</ref>

* In [[Sid Meyer's Aplha Centauri]], neutronium is one of the possible armor types to be used to equip units,

==See also==

Note that if neutronium were to be released of the immense pressures of gravity within the neutron star, it would instantly explode with immense force. This makes the aforementioned uses impossible without altering the element in some way, but does leave open the obvious possiblity of a superbomb. However, even if it would able to be harvested released of the immense pressures of gravity within the neutron star, its mass would most likely prevent this.

* [[{{annotated link|Compact star]]}}

== See also References==

{{reflist|25em}}

{{Periodic table (navbox)}}

* [[Neutron star]]

{{Neutron star}}

* [[Degenerate matter]]

{{Portal bar|Physics|Chemistry|Astronomy|Stars|Science}}

* [[Compact star]]

* [[Antineutron]]

[[Category:PhasesConcepts ofin matterastrophysics]]

== References ==

[[Category:CondensedExotic matter physics]]

[[Category:AstrophysicsNeutron]]

* {{Book reference | Title=Compact Stars | Author=Norman K. Glendenning, R. Kippenhahn, I. Appenzeller, G. Borner, M. Harwit | Year=2000 | Edition=2nd ed }}

[[Category:Fictional materials]]

{{Template:Phase_of_matter}}

[[Category:Astrophysics]]

[[Category:Condensed matter physics]]

[[Category:Phases of matter]]

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