neutron, uncharged elementary particle of slightly greater mass than the proton. It was discovered by James Chadwick in 1932. The stable isotopes of all elements except hydrogen and helium contain a number of neutrons equal to or greater than the number of protons. The preponderance of neutrons becomes more marked for very heavy nuclei. A nucleus with an excess of neutrons is radioactive; the extra neutrons convert to protons by beta decay (see radioactivity). In a nucleus the neutron can be stable, but a free neutron decays with a half-life of about 17 min (1,013 sec), into a proton, an electron, and an antineutrino. The fact that the neutron possesses a magnetic moment suggests that it has an internal structure of electric charge, although the net charge is zero. The electron-scattering experiments of Robert Hofstadter indicate that the neutron, like the proton, is surrounded by a cloud of pions; protons and neutrons are bound together in nuclei by the exchange of virtual pions. The neutron and the proton are regarded by physicists as two aspects or states of a single entity, the nucleon. The antineutron, the neutron's antiparticle, was discovered in 1956. The neutron, like other particles, also possesses certain wave properties, as explained by the quantum theory. The field of neutron optics is concerned with such topics as the diffraction and polarization of beams of neutrons. The formation of images using the techniques of neutron optics is known as neutrography.