The neutron, symbol n, is an electrically neutral subatomic particle found with protons in the atomic nuclei of most atoms. The nucleus of ordinary hydrogen, hydrogen-1, contains a proton but no neutron. Neutrons have slightly greater mass than protons and can exist outside nuclei as free particles, although free neutrons are unstable. They are important constituents of matter and participate in nuclear reactions, the formation of elements, and experimental investigations of materials. (energy.gov)
Discovery
James Chadwick established the neutron’s existence in 1932. His work followed experiments in which alpha particles striking beryllium produced unusually penetrating radiation. Initially interpreted as high-energy electromagnetic radiation, this radiation could eject fast-moving protons from hydrogen-rich substances such as paraffin wax. Chadwick examined the energies of particles recoiling from several targets and showed that the observations were consistent with collisions involving an uncharged particle of approximately proton mass. (nobelprize.org)
Ernest Rutherford had proposed a neutral nuclear constituent in 1920, although his suggested proton–electron combination was not the modern neutron. Chadwick received the 1935 Nobel Prize in Physics for the discovery. Identifying the neutron provided a physical basis for describing nuclei in terms of protons and neutrons rather than nuclear protons and electrons. (nobelprize.org)
Properties and internal structure
A neutron has zero net electric charge. Its rest mass is approximately kilograms, corresponding to a rest-energy equivalent of about 939.565 MeV. Despite its neutrality, it possesses a magnetic moment and therefore interacts with magnetic fields and magnetic structures in matter. Electrical neutrality does not mean that it lacks electromagnetic properties. (physics.nist.gov)
The neutron has spin quantum number , making it a fermion. It is not an elementary particle: it is a baryon whose valence-quark composition is one up quark and two down quarks, conventionally written udd. Their charges, , , and in units of the proton charge, sum to zero. Its internal structure also includes gluons and quark–antiquark contributions, so the three-valence-quark description is not a complete inventory of its contents. (misportal.jlab.org)
Quantum chromodynamics describes the interactions of quarks and gluons. At nuclear scales, the residual effects of the strong interaction bind neutrons and protons together; at shorter distances, experiments resolve aspects of their constituent quarks. These descriptions apply at different scales rather than representing incompatible pictures. (jlab.org)
Neutrons in nuclei
The proton number determines a chemical element, whereas differing neutron numbers distinguish its isotopes. Carbon-12 and carbon-14, for example, both contain six protons but have six and eight neutrons respectively. Changing neutron number can substantially alter nuclear stability without changing the element’s identity. (energy.gov)
Neutrons contribute to nuclear binding without adding proton–proton electrical repulsion. Nevertheless, adding neutrons does not invariably stabilize a nucleus. Stability depends on the complete proton–neutron configuration and the available decay pathways. An unstable nucleus may change its neutron-to-proton balance through beta decay, or release particles through other forms of radioactive decay. A neutron bound within a stable nucleus need not undergo the decay characteristic of an isolated neutron. (energy.gov)
Free-neutron decay
A free neutron undergoes beta-minus decay into a proton, an electron, and an electron antineutrino, the antiparticle associated with the electron neutrino:
Its mean lifetime is approximately 15 minutes. This is an average decay time, not a fixed duration after which every neutron disappears. For exponential decay, the half-life is the mean lifetime multiplied by , making it roughly ten minutes. (nist.gov)
Experiments determine the lifetime using complementary methods. Beam experiments measure decay products from a known neutron population passing through an apparatus; storage, or “bottle,” experiments count neutrons remaining after confinement. Precise measurements test the Standard Model and inform calculations of early-universe element formation. Different experimental approaches have produced a persistent discrepancy, motivating further investigation of their uncertainties and possible systematic effects. (nist.gov)
Nuclear reactions and production
Neutrons can initiate nuclear fission when absorbed by suitable heavy nuclei. Fission releases energy and additional neutrons, which can induce further reactions. A controlled chain reaction supplies the heat used in nuclear power generation. Reaction probabilities depend on both the target nucleus and the incoming neutron’s energy. (energy.gov)
Research neutrons are produced in reactors and accelerator facilities. In accelerator-driven spallation sources, energetic protons strike a heavy target and release neutrons. Moderators slow the resulting particles through collisions. Thermal neutrons have energies characteristic of the moderator’s temperature; colder moderators produce longer-wavelength neutrons suited to investigating larger structures. Neutrons also occur naturally through cosmic-ray interactions and radioactive processes. (ornl.gov)
Scientific and cosmic roles
Neutron scattering measures how neutron beams interact with samples. Neutrons probe atomic nuclei and magnetic structures, providing information complementary to X-ray methods. Their sensitivity to some light elements and differences between isotopes makes them valuable for studying hydrogen-containing materials and selectively highlighting components of complex samples. (info.ornl.gov)
Elastic scattering reveals structural arrangements, while inelastic scattering measures energy exchanged with atomic motions and other excitations. Neutron diffraction investigates atomic and magnetic ordering; imaging can examine internal features of engineering components. Some samples become radioactive through neutron irradiation and require controlled handling afterward. (ornl.gov)
Neutrons also participate in nucleosynthesis. Neutron capture, combined with subsequent nuclear decays, builds heavier nuclei in astrophysical environments. The identification of strontium in material expelled by the neutron-star merger GW170817 provided direct evidence that such mergers produce neutron-capture elements. (ntrs.nasa.gov)