Neutron

The neutron is a subatomic particle, a charge-neutral baryon that is a constituent of atomic nuclei with the exception of hydrogen-1. It has a rest mass of about 1.675×10^-27 kilograms (roughly 1.008665 atomic mass units) and a spin of 1/2. Its electric charge is zero and it has a magnetic moment of −1.913 μN. In the quark model, a neutron is composed of two down quarks and one up quark (udd).

In nuclei, neutrons and protons interact via the strong nuclear force, which binds nucleons together. A free neutron is unstable, decaying by beta decay into a proton, an electron, and an electron antineutrino with a mean lifetime of about 14.5 minutes. Within stable nuclei the neutron remains bound and does not spontaneously decay on observable timescales.

The neutron was discovered in 1932 by James Chadwick. Its discovery explained missing energy and radiation in certain nuclear reactions and established the neutron as a key component of atomic nuclei. The name derives from its lack of electric charge.

Neutrons are produced in a variety of ways, including radioactive decay, nuclear fission, and spallation in accelerators. They are used as probes in materials science through neutron diffraction and scattering, in neutron activation analysis, and in medical and industrial imaging. In nuclear reactors and fusion devices, neutrons sustain chain reactions or aid plasma heating.

In astrophysics, neutrons play a central role in stellar nucleosynthesis, where neutron capture processes such as the s-process and r-process build heavier elements. The properties and interactions of neutrons thus underpin both fundamental physics and many practical technologies.