Neutron

A picture of a neutron. The 'u' stands for an up quark, and the 'd' stands for a down quark.

Neutrons, with protons and electrons, make up an atom. Neutrons and protons are found in the nucleus of an atom.^[1]^[2]^[3] Unlike protons, which have a positive charge, or electrons, which have a negative charge, neutrons have zero charge^[1]^[4] which means they are neutral particles. Neutrons bind with protons with the residual strong force.

Neutrons were predicted by Ernest Rutherford,^[5] and discovered by James Chadwick,^[6]^[7] in 1932.^[6] Atoms were fired at a thin pane of beryllium. Particles emerged which had no charge, and he called these 'neutrons'. They were later added to the modern image of the atom.

Neutrons have a mass of 1.675 × 10^-24g,^[8] which is a little heavier than the proton.^[8] Neutrons are 1839 times heavier than electrons.^[8]

Like all hadrons, neutrons are made of quarks. A neutron is made of two down quarks and one up quark.^[2]^[3] One up quark has a charge of +2/3, and the two down quarks each have a charge of -1/3. The fact that these charges cancel out is why neutrons have a neutral (0) charge. Quarks are held together by gluons.

Isotopes

Neutrons can be found in almost all atoms together with protons and electrons. Hydrogen-1 is the only exception. Atoms with the same number of protons but a different number of neutrons are called isotopes of the same element.

The number of neutrons in an atom does not affect its chemical properties. However it affects its half-life, a measure of its stability. An unstable isotope has a short half-life, in which half of it decays to lighter elements. By contrast, a stable isotope has a long half-life, much longer than that of an unstable isotope. The stability of an isotope is related to radioactivity: an unstable isotope can be highly radioactive.

Atomic reactions

Neutrons are the key to nuclear chain reactions, nuclear power and nuclear weapons.

Neutron Media

Neutron quark structure (ring-shape)
Nuclear fission caused by absorption of a neutron by uranium-235. The heavy nuclide fragments into lighter components and additional neutrons.
Models depicting the nucleus and electron energy levels in hydrogen, helium, lithium, and neon atoms. In reality, the diameter of the nucleus is about 100,000 times smaller than the diameter of the atom.
A schematic of the nucleus of an atom indicating radiation, the emission of a fast electron from the nucleus (the accompanying antineutrino is omitted). In the Rutherford model for the nucleus, red spheres were protons with positive charge and blue spheres were protons tightly bound to an electron with no net charge.*The inset shows beta decay of a free neutron as it is understood today; an electron and antineutrino are created in this process.
The Feynman diagram for beta decay of a neutron into a proton, electron, and electron antineutrino via an intermediate heavy W boson
The leading-order Feynman diagram for decay of a proton into a neutron, positron, and electron neutrino via an intermediate boson.
Institut Laue–Langevin (ILL) in Grenoble, France – a major neutron research facility.
Cold neutron source providing neutrons at about the temperature of liquid hydrogen
The fusion reaction rate increases rapidly with temperature until it maximizes and then gradually drops off. The D–T rate peaks at a lower temperature (about 70 keV, or 800 million kelvins) and at a higher value than other reactions commonly considered for fusion energy.
Transmutation flow in light water reactor, which is a thermal-spectrum reactor