NEUTRON

one of the constituent particles of every atomic nucleus except ordinary hydrogen; it has no electrical charge and its mass is nearly 1,840 times that of the electron. Neutrons and protons constitute more than 99.9 percent of an atom's mass. These two types of nuclear particles, commonly called nucleons, interact through the so-called strong force and bond in various proportions to form the different atomic species of the chemical elements. The neutron possesses an intrinsic angular momentum (q.v.) and a magnetic momenti.e., it behaves like a minute magnet in ways that suggest that it is an entity of moving electric charges. Like the proton and other baryons, the neutron appears to consist of three quarks, extremely small subatomic particles that are thought to be among the fundamental components of matter (see also quark). A free neutronone that is not incorporated into a nucleusis subject to radioactive decay of a type called beta decay. It can break down into a proton, an electron, and an antineutrino (the antimatter counterpart of the neutrino, a particle with no charge and little or no mass). Because it readily disintegrates in this manner, the neutron does not exist in nature in its free state but must be produced artificially. Since free neutrons are electrically neutral, they pass unhindered through the electrical fields within atoms and so constitute a penetrating form of radiation, interacting with matter almost exclusively through relatively rare collisions with atomic nuclei. The neutron was discovered in 1932 by the English physicist James Chadwick. Within a few years after this discovery, many investigators throughout the world were studying the properties and interactions of the particle. It was found that various elements, when bombarded by neutrons, undergo fissioni.e., a type of nuclear reaction that occurs when the nucleus of a heavy element is split into two nearly equal fragments. During this reaction each fissioned nucleus gives off additional free neutrons, as well as those bound to the fission fragments. In 1942 a group of American researchers, under the leadership of the physicist Enrico Fermi, demonstrated that enough free neutrons are produced during the fission process to sustain a chain reaction. This development led to the construction of the atomic bomb. Subsequent technological breakthroughs resulted in the large-scale production of electric power from nuclear energy. The absorption of neutrons by nuclei exposed to the high neutron intensities available in nuclear reactors has also made it possible to produce large quantities of radioactive isotopes useful for a wide variety of purposes (see radioactive isotope). Furthermore, the neutron has become an important tool in pure research. Knowledge of its properties and structure is essential to an understanding of the structure of matter in general. Nuclear reactions induced by neutrons are valuable sources of information about the atomic nucleus and the force that binds it together.