The English physicist James Chadwick is primarily remembered for his discovery of the neutron, which other physicists referred to as the historical beginning of nuclear physics. Using the neutron as a tool for investigating the atom, physicists were able to create a wide variety of new radioisotopes, split atoms and molecules, and initiate the nuclear chain reactions that led to the atomic bomb.
James Chadwick, the son of John Joseph Chadwick and Anne Mary Knowles, was born in Cheshire, an English mill town, and later moved with his family to Manchester. While enrolling at the University of Manchester, Chadwick accidentally found himself in the line for those hoping to major in physics. Chadwick, who had intended to be a mathematician, was too shy to seek out the proper line. He graduated from the Honours School of Physics in 1911 and spent the next two years at Manchester working with Ernest Rutherford (1871-1937), winner of the 1908 Nobel Prize for his theoretical work on the radioactive transformation of atoms. In 1913 Chadwick earned his Master in Science degree and a scholarship to work in Berlin with Hans Geiger (1882-1945), creator of the radiation counter. When World War I broke out, Chadwick was imprisoned as an enemy alien and confined to a Berlin stable. There he attempted to continue his research, improvising apparatus from teacups and beer glasses and a popular brand of radioactive toothpaste as his radiation source.
As soon as the war ended, Chadwick returned to England to continue his research in Rutherford's laboratory. In 1919, he accepted the Wollaston Studentship at Gonville and Caius College, Cambridge, in order to continue work with Rutherford, who had moved from Manchester to the Cavendish Laboratory, Cambridge. In 1923 Chadwick became Assistant Director of Research in the Cavendish Laboratory. Two years later he married Aileen Stewart-Brown of Liverpool. They had twin daughters.
In the 1920s Rutherford carried out the first artificial nuclear transformation. Chadwick and Rutherford were able to induce the transmutation of other light elements. Their studies of the nature of the atomic nucleus led to the identification of the proton. Rutherford had postulated the existence of the neutron, a subatomic particle with no charge. Chadwick began a series of experiments to demonstrate the existence of such a particle, but the neutron was elusive. Then, in 1930, Walther Bothe (1891-1957) and Herbert Becker described an unusual type of gamma ray produced by bombarding beryllium with alpha particles. Chadwick recognized that the properties of this radiation were more consistent with Rutherford's hypothetical neutron. Chadwick finally discovered the neutron after a famous three-week, round-the-clock, research marathon in February 1932. When Chadwick bombarded beryllium atoms with alpha particles, they released an unknown radiation that in turn ejected protons from the nuclei of various substances. Chadwick concluded that this radiation was composed of neutral particles that had a mass approximately equal to the proton. In 1934 in collaboration with Maurice Goldhaber, Chadwick discovered the nuclear photoelectric effect. These investigations provided evidence that the neutron is heavier than the proton. While working with slow neutrons, Chadwick and Goldhaber discovered that neutron could induce disintegration of lithium, boron, and nitrogen nuclei.
The neutron is one of the particles found in every atomic nucleus except for ordinary hydrogen. Its mass is about 1,840 times that of the electron. Because the electrically uncharged neutron is not deflected by charged atomic constituents, it could penetrate the atomic nucleus. Thus, the neutron could be used as a tool to induce atomic disintegration. Subsequently, many subatomic particles were discovered. Most of these are shortlived particles that were created in accelerators that produce high-energy collisions between particles. Chadwick's discovery pointed the way towards the fission of uranium 235 and the creation of the atomic bomb. The first self-sustaining chain reaction was achieved in a nuclear reactor in 1942. Three years later, American scientists produced the first atomic bomb.
In 1935 Chadwick was elected to the Lyon Jones Chair of Physics at the University of Liverpool. From 1943 to 1946 he worked in the United States as Head of the British Mission attached to the Manhattan Project for the development of the atomic bomb. In 1945, he witnessed the first nuclear explosion at Trinity. In 1946 he served on the U.N.'s Atomic Energy Commission, before he was able to retreat back to the sanctuary of his Liverpool laboratory. After World War II, Chadwick suffered from episodes of depression. Unlike some of his colleagues, he avoided politics and debates about the consequences of nuclear physics. In 1948, he retired from his professorship at Liverpool on his election as Master of Gonville and Caius College, Cambridge. He retired from this Mastership in 1959. From 1957 to 1962 he served as a member of the United Kingdom Atomic Energy Authority.
Chadwick was knighted in 1945. In addition to the 1935 Nobel Prize for Physics, he received many honors, including the Hughes Medal, the Copley Medal, the Franklin Medal, and many honorary doctorate degrees.
LOIS N. MAGNER