Place: United States of America
Subject: biography, physics
English physicist who discovered the neutron in 1932. For this achievement he was awarded the 1935 Nobel Prize for Physics.
Chadwick was born in Bollington, Cheshire, on 20 October 1891. He began his scientific career at Manchester University, graduating in physics in 1911. Chadwick then continued at Manchester and, under Ernest Rutherford, investigated the emission of gamma rays from radioactive materials. To gain further research experience, he went in 1913 to Berlin to work with Hans Geiger, the inventor of the geiger counter, where he discovered the continuous nature of the energy spectrum and investigated beta particles emitted by radioactive substances. Chadwick was then interned as an enemy alien on the outbreak of World War I, living and working in a stable for the duration of the war. He still managed to do original research, however, and investigated the ionization present during the oxidation of phosphorus and the photochemical reaction between chlorine and carbon monoxide.
At the end of the war, Rutherford invited Chadwick to Cambridge. During this period, he determined the atomic numbers of certain elements by the way in which alpha particles were scattered. He also established the equivalence of atomic number and atomic charge. With Rutherford, he produced artificial disintegration of some of the lighter elements by alpha-particle bombardment.
His most famous achievement, the discovery of the neutron, came in 1932 after its existence had been suspected by Rutherford as early as 1920. In experiments in which beryllium was bombarded by alpha particles, a usually energetic gamma radiation appeared to be emitted. It was more penetrating than gamma radiation from radioactive elements. Measurements of the energies involved and the conservation of energy and momentum suggested to Chadwick that a new kind of particle was being produced rather than radiation. The results pointed towards a neutral particle made up of a proton and an electron. Its mass should thus be slightly greater than that of the proton. Because the mass of the beryllium nucleus had not then been measured, Chadwick designed and carried out an experiment in which boron was bombarded with alpha particles. This produced neutrons, and from the mass of the boron nucleus and other elements and the energies involved, Chadwick determined the mass of the neutron to be 1.0067 atomic mass units, slightly greater than that of the proton.
In the same year, Chadwick became professor of physics at the University of Liverpool. He ordered the building of a cyclotron and, from 1939 onwards, used it to investigate the nuclear disintegration of the light elements. During World War II, he was closely involved with the atomic bomb, and much of the research and calculation for the British contribution to the Manhattan Project was carried out at Liverpool under his direction. From 1943, he led the British team with the project in the USA.
In 1945 Chadwick was knighted, and in the same year he returned to Liverpool to continue his own research and to develop a research school in nuclear physics. He returned to Cambridge as master of Gonville and Caius College in 1948, and stayed in this position until his retirement ten years later. He died on 24 July 1974.
The discovery of the neutron made by Chadwick led to a much deeper understanding of the nature of matter, explaining for example why isotopes of elements exist. It also inspired Enrico Fermi and other physicists to investigate nuclear reactions produced by neutrons, leading to the discovery of nuclear fission.
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