Neutron

I

Introduction

Neutron, uncharged particle, one of the fundamental particles of which matter is composed. The mass of a neutron is 1.675 × 10^-27 kg, about one eighth of one per cent heavier than the proton. The existence of the neutron was predicted in 1920 by the British physicist Ernest Rutherford and by Australian and American scientists, but experimental verification of its existence was difficult because the net electrical charge on the neutron is zero. Most particle detectors register charged particles only.

II

Discovery

The neutron was first identified in 1932 by the British physicist James Chadwick, who correctly interpreted the results of experiments conducted at that time by the French physicists Irène and Frédéric Joliot-Curie and other scientists. The Joliot-Curies had produced a previously unknown kind of radiation by the interaction of alpha particles with beryllium nuclei. When this radiation was passed through paraffin wax, collisions between the neutrons and the hydrogen atoms in the wax produced readily detectable protons. Chadwick recognized that the radiation consisted of neutrons.

III

Behaviour

The neutron is a constituent particle of all nuclei of mass number greater than 1; that is, of all nuclei except ordinary hydrogen (see Atom). Free neutrons—those outside atomic nuclei—are produced in nuclear reactions. They can be ejected from atomic nuclei at various speeds or energies and are readily slowed down to very low energy by a series of collisions with light nuclei, such as those of hydrogen, deuterium, or carbon. (For the role of neutrons in the production of atomic energy, see Nuclear Energy.) When expelled from the nucleus, the neutron is unstable and decays to form a proton, an electron, and a neutrino. Like the proton and the electron, the neutron possesses angular momentum, or spin (see Mechanics). Neutrons act as small, individual magnets; this property enables beams of polarized neutrons to be created. The neutron has a negative magnetic moment of -1.913141 nuclear magnetons or approximately a thousandth of a Bohr magneton. The currently accepted value of its half-life is 615 s +/- 1.4 s. The corresponding value of the mean life, which is now more commonly used, is 887 s +/- 2s. See Radioactivity.

The antiparticle of a neutron, known as an antineutron, has the same mass, spin, and rate of beta decay. These particles are sometimes produced in the collisions of antiprotons with protons, and they possess a magnetic moment equal and opposite to that of the neutron. According to current particle theory, the neutron and the antineutron—and other nuclear particles—are themselves composed of quarks.

IV

Neutron Radiography

An increasingly important application of reactor-generated neutrons is neutron radiography, in which information is obtained by determining the absorption of a beam of neutrons emanating from a nuclear reactor or a powerful radioisotope source. The technique resembles X-ray radiography. Many substances, however, such as metals that are opaque to X-rays, will transmit neutrons; other substances (particularly hydrogen compounds) that transmit X-rays are opaque to neutrons. A neutron radiograph is made by exposing a thin foil to a beam of neutrons that has penetrated the test object. The neutrons leave an invisible radioactive “picture” of the object on the foil. A visible picture is made by placing a photographic film in contact with the foil. A direct, television-like technique for viewing images has also been developed.

First used in Europe in the 1930s, neutron radiography has been employed widely since the 1950s for examining nuclear fuel and other components of reactors. More recently it has been used in examining explosive devices and components of space vehicles. Beams of neutrons are widely used now in the physical and biological sciences and in technology, and neutron activation analysis is an important tool in such diverse fields as palaeontology, archaeology, and art history.