Neutronium (orneutrium,[1]neutrite,[2] orelement zero) is a hypothetical substance made purely ofneutrons. The word was coined by scientistAndreas von Antropoff in 1926 (before the 1932discovery of the neutron) for the hypothetical "element of atomic number zero" (with no protons in its nucleus) that he placed at the head of theperiodic table (denoted by -).[3][4] However, the meaning of the term haschanged over time, and from the last half of the 20th century onward it has been also used to refer to extremely dense substances resembling theneutron-degenerate matter theorized to exist in the cores ofneutron stars.
Neutronium is used in popular physics literature[1][2] to refer to the material present in the cores of neutron stars (stars which are too massive to be supported byelectron degeneracy pressure and which collapse into a denser phase of matter). In scientific literature the term "neutron-degenerate matter"[5] or simplyneutron matter is used for this material.[6]
The term "neutronium" was coined in 1926 by Andreas von Antropoff for a conjectured form of matter made up ofneutrons with noprotons orelectrons, which he placed as thechemical element ofatomic number zero at the head of his new version of theperiodic table.[3] It was subsequently placed in the middle of several spiral representations of the periodic system for classifying the chemical elements, such as those ofCharles Janet (1928),Edgar Emerson (1944),[7][8] andJohn D. Clark (1950).
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.[9]
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[10][11] was disputed,[12] but new work reportedly clears up the issues.[13]
The dineutron hypothesis had been used in theoretical studies of the structure ofexotic nuclei. For example11Li is modeled as a dineutron bound to a9Li core.[14][15] A system made up of only two neutrons is not bound, though the attraction between them is very nearly enough to make them so.[16] This has some consequences onnucleosynthesis and theabundance of the chemical elements.[14][17]
A trineutron state consisting of three bound neutrons has not been detected, and is not expected to be bound.[18]
Atetraneutron is a hypothetical particle consisting of four bound neutrons. Reports of its existence have not been replicated.[19][20]
Calculations indicate that the hypothetical pentaneutron state, consisting of a cluster of five neutrons, would not be bound.[21]
^Emerson, Edgar I. (1944). "A chart based on atomic numbers showing the electronic structure of the elements".Journal of Chemical Education.21 (5): 254.Bibcode:1944JChEd..21..254E.doi:10.1021/ed021p254.
^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.
