James Chadwick (1891-1974), son of a businessman from Cheshire, England, applied to the University of Manchester at age 16. He planned to major in mathematics, but as he stood in the queue for his entrance inter­view he realized too late that he was in the wrong line. He was too embar­rassed to admit it and wound up in physics. His first year was miserable, as the physics classes were big and noisy, but when he heard a lecture by Ernest Rutherford he was converted to physics. He graduated in 1911 with a bachelor’s degree in physics and went on to Cambridge, where he earned

a master of science degree in physics in 1913. From there, he was awarded a scholarship to engage in nuclear research with Hans Geiger (1882-1945) at the University of Berlin, a valuable opportunity

Caught in Germany at the beginning of World War I, Chadwick was kept in a prisoner of war camp for nonmilitary aliens for the duration of the war. Released at the end of the war, he was hired by Rutherford to resume his research at the Cavendish Laboratory. His assignment was to look for the neutron, Rutherford’s theoretical particle, and it would be a long search. He started by studying what others in the field were doing. They were bombarding light elements with alpha particles to see what would happen.

The alpha particle, as discovered and named by Rutherford, is an extremely heavy particle of great energy. It consists of two protons and two neutrons stuck together, and it is literally the nucleus of a helium atom. In the 1920s, it was only known to be heavy and positively charged. Hit a light element, such as boron or aluminum, with an alpha particle and the nucleus disintegrates, throwing off a burst of gamma rays and proton debris. Oddly, beryllium emitted a rash of gamma rays 10 times that of other elements bombarded with alphas, and there was no proton debris from a supposed nuclear destruction. No one knew why.

Irene Joliot-Curie (1879-1956) and her husband, Frederic Joliot-Curie (1900-58), had reported detecting protons being knocked out of a sheet of paraffin by the gamma rays produced by the alpha-beryllium experi­ment. Chadwick was highly skeptical of the French findings. He believed their observations of the radiation were correct, but their explanation was questionable. They were saying that gamma rays from the beryllium were knocking protons out of solid wax. It was true that gamma rays of suf­ficient energy could deflect electrons, but protons are 1,836 times heavier than electrons. Saying that gamma rays were tossing protons into the ion chamber was like saying that a dump truck could be knocked into the oncoming lane by hitting it with a well-thrown baseball. Chadwick set up his own version of the Joliot-Curie experiment.

The protons were indeed knocked into the detector in Chadwick’s setup. The only thing that could exchange momentum with a stationary proton and send it flying at high speed into the ion chamber would be a particle of the same weight, hitting it hard. There were no protons coming out of the beryllium, or they would have shown up in the ion chamber. Chad­wick proposed a logical explanation. The particles coming out of the beryl­lium were neutrons. The neutrons made no impression on the radiation

RHODIA

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The toolbox of James Chadwick (1891-1974), ca. 1932, which he used to discover the neutron. These are silver and aluminum foils of various thicknesses used as barriers to assess the strength of radiation, which he kept in a cigarette box. (© SSPL/lmage Works)

detection apparatus simply because they were electrically neutral particles, but by hitting the paraffin a secondary effect could be seen, as the protons recoiled from billiard-ball collisions with the flying, invisible particles.

On February 17, 1932, after getting very little sleep over the previous week of intense work, James Chadwick published an announcement in the science journal Nature titled “Possible Existence of a Neutron,” and the productive era of nuclear physics began.