The neutron is a subatomic particle, symbol
n
or
n0
, 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, and each has a mass of approximately one dalton, they are both
referred to as nucleons.[7] 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, but the mass of the nucleus is
approximately equal to the sum of proton and neutron masses, minus a small amount due to nuclear
binding energy.[8]: 822
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,[9] neutrons were used to induce many different types
of nuclear transmutations. With the discovery of nuclear fission in 1938,[10] 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.[11] These events and findings led to the first selfsustaining 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.[12] Free neutrons do not directly ionize atoms, but they do indirectly cause ionizing
radiation, so they can be a biological hazard, depending on dose.[11] 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.[13]