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The neutron is a subatomic particle, symbol n or n0, with no net electric charge and a mass slightly larger than that of a proton. Protons and neutrons, each with mass approximately one atomic mass unit, constitute the nucleus of an atom, and they are collectively referred to as nucleons.<ref name="Nuc">{{#invoke:citation/CS1|citation |CitationClass=citation }}</ref> Their properties and interactions are described by nuclear physics.

The nucleus consists of Z protons, where Z is called the atomic number, and N neutrons, where N is the neutron number. The atomic number defines the chemical properties of the atom, and the neutron number determines the isotope or nuclide.<ref name="ENW">{{#invoke:citation/CS1|citation |CitationClass=citation }}</ref> The terms isotope and nuclide are often used synonymously, but they refer to chemical and nuclear properties, respectively. The atomic mass number, symbol A, equals Z+N. For example, carbon has atomic number 6, and its abundant carbon-12 isotope has 6 neutrons, whereas its rare carbon-13 isotope has 7 neutrons. Some elements occur in nature with only one stable isotope, such as fluorine (see stable nuclide). Other elements occur as many stable isotopes, such as tin with ten stable isotopes. Even though it is not a chemical element, the neutron is included in the table of nuclides.<ref>Nudat 2. Nndc.bnl.gov. Retrieved on 2010-12-04.</ref>

Within the nucleus, protons and neutrons are bound together through the nuclear force, and neutrons are required for the stability of nuclei. 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 in 1932,<ref name="Chad1932"> {{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> neutrons were used to effect many different types of nuclear transmutations. With the discovery of nuclear fission in 1938,<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}. The authors were identified as being at the Kaiser-Wilhelm-Institut für Chemie, Berlin-Dahlem. Received 22 December 1938.</ref> it was quickly realized that, if a fission event produced neutrons, each of these neutrons might cause further fission events, etc., in a cascade known as a nuclear chain reaction.<ref name="ENW"/> These events and findings led to the first self-sustaining nuclear reactor (Chicago Pile-1, 1942) and the first nuclear weapon (Trinity, 1945).

Free neutrons, or individual neutrons free of the nucleus, are effectively a form of ionizing radiation, and as such, are a biological hazard, depending upon dose.<ref name="ENW"/> A small natural "neutron background" flux of free neutrons exists on Earth, caused by cosmic ray muons, and by the natural radioactivity of spontaneously fissionable elements in the Earth's crust.<ref name="NatNeu"> {{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> Dedicated neutron sources like neutron generators, research reactors and spallation sources produce free neutrons for use in irradiation and in neutron scattering experiments.


Neutron sections
Intro  Description  Discovery   Beta decay and the stability of the nucleus    Intrinsic properties    Neutron compounds    Detection    Sources and production    Applications    Medical therapies   Protection   Neutron temperature   See also  References  Further reading  External links  

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