The quark content of the neutron. The color assignment of individual quarks is arbitrary, but all three colors must be present. Forces between quarks are mediated by gluons.
Composition1 up quark, 2 down quarks
InteractionsGravity, weak, strong, electromagnetic
TheorizedErnest Rutherford[1] (1920)
DiscoveredJames Chadwick[2] (1932)
Mass1.67492749804(95)×10−27 kg[3]
939.56542052(54) MeV/c2[3]
1.00866491588(49) Da[4]
Mean lifetime878.4(5) s (free)[5]
Electric chargee
(−2±8)×10−22 e (experimental limits)[6]
Electric dipole moment< 1.8×10−26 e⋅cm (experimental upper limit)
Electric polarizability1.16(15)×10−3 fm3
Magnetic moment−0.96623650(23)×10−26 J·T−1[4]
−1.04187563(25)×10−3 μB[4]
−1.91304273(45) μN[4]
Magnetic polarizability3.7(20)×10−4 fm3
Spin1/2 ħ
CondensedI(JP) = 1/2(1/2+)

The neutron is a subatomic particle, symbol
, 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 behave similarly within the nucleus, they are both referred to as nucleons. Nucleons have a mass of approximately one atomic mass unit, or dalton (symbol: Da). Their properties and interactions are described by nuclear physics. Protons and neutrons are not elementary particles; each is composed of three quarks.

The chemical properties of an atom are mostly determined by the configuration of electrons that orbit the atom's heavy nucleus. The electron configuration is determined by the charge of the nucleus, which is determined by the number of protons, or atomic number. The number of neutrons is the neutron number. Neutrons do not affect the electron configuration.

Atoms of a chemical element that differ only in neutron number are called isotopes. For example, carbon, with atomic number 6, has an abundant isotope carbon-12 with 6 neutrons and a rare isotope carbon-13 with 7 neutrons. Some elements occur in nature with only one stable isotope, such as fluorine. Other elements occur with many stable isotopes, such as tin with ten stable isotopes, or with no stable isotope, such as technetium.

The properties of an atomic nucleus depend on both atomic and neutron numbers. With their positive charge, the protons within the nucleus are repelled by the long-range electromagnetic force, but the much stronger, but short-range, nuclear force binds the nucleons closely together. Neutrons are required for the stability of nuclei, with the exception of the single-proton hydrogen nucleus. Neutrons are produced copiously in nuclear fission and fusion. They are a primary contributor to the nucleosynthesis of chemical elements within stars through fission, fusion, and neutron capture processes.

The neutron is essential to the production of nuclear power. In the decade after the neutron was discovered by James Chadwick in 1932, neutrons were used to induce many different types of nuclear transmutations. With the discovery of nuclear fission in 1938, it was quickly realized that, if a fission event produced neutrons, each of these neutrons might cause further fission events, in a cascade known as a nuclear chain reaction. These events and findings led to the first self-sustaining nuclear reactor (Chicago Pile-1, 1942) and the first nuclear weapon (Trinity, 1945).

Dedicated neutron sources like neutron generators, research reactors and spallation sources produce free neutrons for use in irradiation and in neutron scattering experiments. A free neutron spontaneously decays to a proton, an electron, and an antineutrino, with a mean lifetime of about 15 minutes. Free neutrons do not directly ionize atoms, but they do indirectly cause ionizing radiation, so they can be a biological hazard, depending on dose. A small natural "neutron background" flux of free neutrons exists on Earth, caused by cosmic ray showers, and by the natural radioactivity of spontaneously fissionable elements in the Earth's crust.

  1. ^ Ernest Rutherford Archived 2011-08-03 at the Wayback Machine. Retrieved on 2012-08-16.
  2. ^ 1935 Nobel Prize in Physics Archived 2017-10-03 at the Wayback Machine. Retrieved on 2012-08-16.
  3. ^ a b "2018 CODATA recommended values" Archived 2018-01-22 at the Wayback Machine
  4. ^ a b c d Mohr, P.J.; Taylor, B.N. and Newell, D.B. (2014), "The 2014 CODATA Recommended Values of the Fundamental Physical Constants" Archived 2013-10-09 at the Wayback Machine (Web Version 7.0). The database was developed by J. Baker, M. Douma, and S. Kotochigova. (2014). National Institute of Standards and Technology, Gaithersburg, Maryland 20899.
  5. ^ Zyla, P. A. (2020). "n MEAN LIFE". PDG Live: 2020 Review of Particle Physics. Particle Data Group. Archived from the original on 17 January 2021. Retrieved 25 February 2021.
  6. ^ Cite error: The named reference PDGLIVE was invoked but never defined (see the help page).

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