SIR JAMES CHADWICK (1891 – 1974)

  September 15, 2021   Read time 5 min
SIR JAMES CHADWICK (1891 – 1974)
By discovering the neutron in 1932, the British physicist Sir James Chadwick enabled the rise of modern nuclear technology, because the neutron was the “magic bullet” that readily released the energy within the atomic nucleus. Chadwick was born on October 20, 1891, in Cheshire, England.

He enrolled at the University of Manchester in 1908 and graduated from the Honors School of Physics in 1911. He spent the next two years working under Ernest Rutherford (1871–1937) in his Physical Laboratory at Manchester. Chadwick had the opportunity to explore various problems in radioactivity and the emerging field of nuclear science. He received his M.S. degree in physics in 1913. That same year, he earned a scholarship to work under Hans Geiger (1882–1945) in Berlin. Unfortunately, shortly after Chadwick arrived in Germany, World War I broke out and he was interned as an enemy alien in the Zivilgefangenenlager at Ruhleben for the next four years. While Chadwick remained imprisoned in a horse stall at this former racetrack, the German physical chemist H. Walter Nernst (1864–1941) helped him set up a crude laboratory and conduct research.

Chadwick returned to England following World War I and rejoined Rutherford, who by that time had become director of the Cavendish Laboratory at Cambridge University. In 1919, Rutherford performed the first artificial nuclear transmutation experiment by bombarding nitrogen with alpha particles. The particle emitted in this experiment was identified by Rutherford as a nucleus of the hydrogen atom, which he named the proton. Rutherford then suggested the possible existence of a neutral particle inside the nucleus about the mass of a proton and called this suspected particle the neutron.

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At the Cavendish Laboratory, Chadwick collaborated with Rutherford in accomplishing the transmutation of other light elements and in investigating the properties and structure of atomic nuclei. However, tracking down and identifying the neutron also remained a major research objective for Chadwick. In 1921, he was elected a fellow of Gonville and Caius College at Cambridge, and he kept that position until 1935. From 1923 to 1935, Chadwick was also assistant director of research for the Cavendish Laboratory. He assumed more and more responsibility for its operation as Rutherford advanced in age.

It was at Cavendish Laboratory that Chadwick made his greatest discovery. A dedicated researcher, he spent much of the 1920s unsuccessfully pursuing the neutron through a series of experiments in which he bombarded aluminum nuclei with alpha particles in hopes of detecting the suspected, but thus far elusive, neutral subatomic particle. Chadwick’s quest for the neutron rested upon previous experiments done in Germany and France, the results of which had been mistakenly interpreted by each research team.

In 1930, the German physicist Walter Bothe (1891–1957) and his student Herbert Becker bombarded beryllium with alpha particles and observed that the ensuing nuclear reaction gave off a strange, penetrating, but electrically neutral radiation, which they mistakenly interpreted as being high-energy gamma rays. In 1932, Irène (1897–1956) and Frédérick (1900–1958) Joliot-Curie repeated Bothe’s experiment using a more intense polonium source of alpha particles and adding a paraffin (wax) target behind the beryllium metal target. They observed protons emerging from the paraffin but they incorrectly assumed that Bothe’s high-energy gamma rays were simply causing some type of Compton effect in the wax that knocked protons out of the paraffin. In fact, Marie Curie’s older daughter and her husband were capable scientists who would win the 1935 Nobel Prize in chemistry “for synthesis of new radioactive elements.” They just hastily reached the wrong conclusion about the interesting results of their important experiment.

In 1932, Chadwick received news about the recent experiment by the Joliot-Curies and did not share their conclusions. He immediately repeated the same experiment at the Cavendish Laboratory, but with the specific goal of looking for Rutherford’s long-postulated neutral particle. Chadwick’s experiments were successful, and he was able to not only prove the existence of the neutron but prove that this neutral particle had a mass of about 0.1 percent more than that of the proton. He announced his great discovery in a modestly titled paper, “Possible Existence of a Neutron,” published in the February 27, 1932, issue of Nature. Chadwick was awarded the 1935 Nobel Prize in physics for his discovery of the neutron. His great achievement marks the beginning of modern nuclear science because the neutron became the magic bullet that further unlocked the secrets of the atomic nucleus and supported the rise of modern nuclear technology.

Chadwick departed the Cavendish Laboratory in 1935 to accept the chair of physics at the University of Liverpool. There, he constructed the first cyclotron in the United Kingdom and supported the preliminary study by the Austrian-British physicist Otto Frisch (1904–1979) and the German-British physicist Rudolph Peirls (1907–1955) concerning the feasibility of making an atomic bomb. Chadwick then spent a good portion of World War II in the United States as the head of the British mission that supported the Manhattan Project.

King George VI of Britain knighted Chadwick in 1945 in recognition of his wartime scientific service, and Sir James Chadwick then became an influential advisor on nuclear energy matters to the British government. Since British scientists were being cut off from further cooperative interactions with the U.S. nuclear weapons program after the Manhattan Project, Chadwick encouraged development of a British atomic bomb. The first British atomic bomb, code-named Hurricane, was tested on October 3, 1952, at Monte Bello Islands, off the coast of Australia.

In 1948, Chadwick returned to Cambridge University as master of Gonville and Caius College. He retained that position until he retired in 1959. From 1957 to 1962, he also served as a part-time member of the United Kingdom Atomic Energy Authority. He died on July 24, 1974, in Cambridge, England.