Neutrons

Neutrons are electrically neutral subatomic particles with a mass slightly larger than that of a proton. They are one of the basic constituents of atomic nuclei, along with protons. Each neutron has spin 1/2 and is a fermion. Its quark composition is udd, and its mass is about 1.675×10^-27 kg (approximately 1.0087 atomic mass units). The neutron carries no net electric charge, but it has a magnetic moment of about -1.913 μN, where μN is the nuclear magneton.

Free neutrons are unstable and decay by beta decay into a proton, an electron, and an electron antineutrino, with a mean lifetime of about 14.5 minutes (878 seconds). Within nuclei, neutrons can be bound by the strong nuclear force, contributing to binding energy; some nuclei are stable while others beta-decay.

Neutrons are produced in nuclear reactions, in stars, and by human-made sources such as nuclear reactors and spallation. They interact with matter mainly through the strong and weak nuclear forces and via elastic and inelastic scattering; they also undergo radiative capture. The strong sensitivity to neutron energy leads to different cross sections for thermal, fast, and epithermal neutrons, and neutron-induced fission is a key reaction in reactors and weapons.

Applications include nuclear power generation, materials research via neutron scattering and diffraction, neutron radiography, and medical therapies such as boron neutron capture therapy. Safety concerns include exposure to radiation and activation of materials.

Discovered in 1932 by James Chadwick, neutrons completed the picture of atomic structure and enabled the development of nuclear physics and reactor technology.