James Chadwick

[b. Manchester, England, October 20, 1891, d. Cambridge, England, July 24, 1974]

Chadwick's early experiments established that atomic number is determined by the number of protons in an atom (now the definition). In 1932 he discovered the fourth known subatomic particle, the neutron (following the electron, photon, and proton); its existence had been suspected since 1924. Chadwick's later experiments with particle accelerators contributed to the invention of the nuclear fission bomb.

The English physicist Sir James Chadwick (1891-1974) made his most outstanding contribution to modern physics by demonstrating the existence of the neutron.

James Chadwick was born in Manchester on Oct. 20, 1891, the eldest son of John Joseph and Anne Mary Knowles Chadwick. In 1908 he enrolled at Victoria University in Manchester. Although his intention was to study mathematics, Chadwick was admitted to the physics programs and was too shy to correct the error. He graduated from the Honours School of Physics in 1911. During the next 2 years his education was continued in Ernest Rutherford's laboratory at the same university. It was there that Rutherford outlined his planetary theory of the atom. Chadwick's acquaintances in the physics department included Hans Geiger and Niels Bohr… After Chadwick received his master's degree in 1913, he was awarded the 1851 Exhibition Scholarship, which he used to finance his studies under Geiger in the foremost German research institute, the Physikalisch-Technische Reichsanstalt in Charlottenburg near Berlin. An early result of his work there was the establishment of the first energy spectrum of beta particles. Years later, subsequent developments along these lines prompted Wolfgang Pauli to postulate the existence of the neutrino.

Discovered the Neutron

After spending the years of World War I in a civilian internment camp in Ruhleben, Chadwick returned to England and used his fellowship at Gonville and Caius College to work with Rutherford at Cambridge University's Cavendish Laboratory. In 1920 he became the first to use a direct method in determining the electric charge on the nucleus. In 1922, he became assistant director of research under Rutherford. Together they spent much of their time experimenting with the transmutation of elements, attempting to break up the nucleus of one element so that different elements could be formed.

Throughout the years of work, Chadwick and Rutherford struggled with an inconsistency. They saw that almost every element had an atomic number that was less than its atomic mass. Rutherford suggested this might be due to the existence of a particle with the mass of a proton but with a neutral charge. However, their attempts to find such a particle were in vain. But in 1932 Chadwick found the answer in the work of the Joliot-Curies, who observed that beryllium had become radioactive after being exposed to alpha particles. Chadwick showed, by using a cloud chamber filled with nitrogen, that the radiation caused the nitrogen atoms to recoil with such energy as could be imparted only by collisions with uncharged particles having approximately the mass of protons. Chadwick had proven the existence of the neutron and received the Nobel Prize in physics in 1935.

From 1935 until 1948 Chadwick held the Lyon Jones chair of physics at the University of Liverpool. From 1943-1946, he also served as head of the British mission to the Manhattan Project and was present at the first atomic test in the New Mexico desert. He was knighted in 1945 and in 1948 was elected master of Gonville and Caius College, a post from which he retired in 1959. Three years later he retired also from the United Kingdom Atomic Energy Authority, on which he had served as part-time member from 1957. Sir James Chadwick died in Cambridge, England, on July 24, 1974.

Further Reading

Pais, Abraham, Inward Bound, Oxford University Press, 1986.

Rhodes, Richard, The Making of the Bomb Simon & Schuster, 1986.

Chadwick, Sir James, 1891–1974, English physicist, grad. Manchester Univ., 1908. He worked at Manchester under Ernest Rutherford on radioactivity. He was assistant director of radioactive research in the Cavendish Laboratory, Cambridge (1923–35), professor at the Univ. of Liverpool (1935–48), and master of Gonville and Caius College, Cambridge (1948–58). For his discovery of the neutron in 1932 he received the 1935 Nobel Prize in Physics. He was knighted in 1945.

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