neutronium

{{Main|Neutron star}}

Image:Neutron star cross section.svg or neutron-degenerate matter and quark matter.]]

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). In scientific literature the term "neutron-degenerate matter"{{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 }} or simply neutron matter is used for this material.{{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 }}

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), Edgar Emerson (1944),{{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 }}{{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}} and 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.{{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 }}

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{{cite journal | author = Schirber, M. | title = Nuclei Emit Paired-up Neutrons | journal = Physics | date = 2012 | volume = 5 | page = 30 | doi = 10.1103/Physics.5.30 | bibcode = 2012PhyOJ...5...30S }}{{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 | doi-access = free | url = https://www.osti.gov/biblio/1104191 }} was disputed,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. but new work reportedly clears up the issues.{{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}}

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.{{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 }}{{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 }} A system made up of only two neutrons is not bound, though the attraction between them is very nearly enough to make them so.{{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 }} This has some consequences on nucleosynthesis and the abundance of the chemical elements.{{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 }}

A trineutron state consisting of three bound neutrons has not been detected, and is not expected to be bound.{{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 }}

A tetraneutron is a hypothetical particle consisting of four bound neutrons. Reports of its existence have not been replicated.{{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 }}[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)

Calculations indicate that the hypothetical pentaneutron state, consisting of a cluster of five neutrons, would not be bound.{{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 }}

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