James Chadwick

James Chadwick the British physicist (1891-1974) who discovered the neutron in 1932 and was awarded the 1935 Nobel Prize in Physics for the achievement. His discovery completed the picture of the atomic nucleus, explained the existence of isotopes, and made possible the later development of nuclear fission and nuclear reactors.

Early Life and Education

James Chadwick was born on 20 October 1891 in Bollington, Cheshire, the son of a cotton spinner. He won a scholarship to the University of Manchester, where he graduated in physics in 1911 and stayed on to work under Ernest Rutherford. In 1913 he went to Berlin to study with Hans Geiger, intending to learn the technique of counting alpha particles.

When the First World War broke out in 1914, Chadwick was interned in Germany as an enemy alien and spent four years in a camp near Berlin, where he organised scientific lectures for his fellow prisoners with the help of others. After the war he returned to Britain and in 1919 joined Rutherford at the Cavendish Laboratory in Cambridge, where he worked for many years on radioactivity and the structure of the nucleus. In 1923 he moved to a research post at Gonville and Caius College and continued to collaborate closely with Rutherford.

The Search for the Neutron

For years physicists had puzzled over a discrepancy: the atomic number of an element, equal to its nuclear charge, was roughly half its atomic weight, which suggested that nuclei contained more positive charge than the number of protons could account for. Rutherford had suggested in 1920 that a neutral particle of about the mass of a proton might exist, but no one had found it.

In 1930 Walther Bothe and Herbert Becker in Germany observed that bombarding beryllium with alpha particles produced a penetrating radiation that could knock protons out of paraffin wax. The discoverers thought it was high-energy gamma radiation. Working in Cambridge, Chadwick repeated the experiments and showed that the radiation had about the mass of a proton and no electric charge. The large energies involved could not be reconciled with the gamma-ray hypothesis, since the particles ejected protons with far more energy than any available gamma ray could supply.

The Discovery Announced

Chadwick's decisive experiments in early 1932 demonstrated that the radiation consisted of neutral particles with a mass close to that of the proton. He worked intensively for several weeks, using a polonium source, a beryllium target and an ionisation chamber, and he established that the new particle had a mass slightly greater than that of the proton. He named it the neutron and published his results in a brief paper that had an immediate and profound impact on nuclear physics. For this work he received the Nobel Prize in Physics in 1935, three years after the discovery.

The discovery had sweeping consequences. Because the neutron carries no charge, it can penetrate the electrical barrier that repels charged particles from the nucleus, and it can be used to induce nuclear reactions in even the heaviest elements. The neutron supplied the missing constituent of the nucleus, which is now understood to consist of protons and neutrons, collectively called nucleons. The existence of isotopes, atoms of the same element with different masses, followed naturally once nuclei were seen to differ only in their number of neutrons, and the notion of nuclear isomers soon followed.

Nuclear Fission and the War Years

The discovery of the neutron led directly to Enrico Fermi's experiments with neutron bombardment and, in 1938, to the discovery of nuclear fission by Otto Hahn and Fritz Strassmann, interpreted by Lise Meitner and Otto Frisch. Chadwick, now at Liverpool, confirmed that fission released a large amount of energy and that more neutrons were emitted, making a chain reaction conceivable. He built a small cyclotron at Liverpool and prepared the ground for Britain's contribution to the atomic project.

During the Second World War Chadwick became a leading figure in the British atomic project and a member of the joint policy committee that coordinated British and American efforts. He moved to the United States and worked on the Manhattan Project, where his calm judgement earned him wide respect. He was knighted in 1945 for his services. After witnessing the first test of a nuclear weapon he is said to have remarked on its terrifying power, a reaction that coloured his later reflections on the uses of science.

Later Life and Legacy

After the war Chadwick returned to Britain, served as Master of Gonville and Caius College at Cambridge and took part in the early attempts to build a British nuclear programme. He was troubled by the destructive potential of the weapon that his fundamental discovery had helped to make possible, and he wrote frankly in later years about the responsibility of scientists. He also served on committees concerned with the international control of atomic energy and with the training of a new generation of nuclear physicists.

The neutron is now recognised as one of the basic building blocks of matter and an essential tool of both pure and applied science. It is used in neutron scattering to study the structure of materials, in medical therapy, and in the production of isotopes. Chadwick's identification of the particle stands as one of the pivotal discoveries of twentieth-century physics. It also gave the nucleus a clear structure of protons and neutrons held together by a strong force, a picture that shaped the whole subsequent development of nuclear theory.

See also

• Neutron
• Nuclear fission
• Atomic nucleus
• Manhattan Project