English physicist James Chadwick’s 1932 discovery of the neutron won him the 1935 Nobel Prize for Physics.
Photo: Corbis

1932: English physicist James Chadwick publishes a letter on the existence of the neutron. His discovery helps clear the way for splitting the nuclei of even the heaviest atomic elements, making possible the development of the atomic bomb.

Unlike the proton, the other large subatomic particle that helps form the nucleus of an atom, the neutron contains no electric charge. This enables it to pass through the electric barrier of heavy atoms to penetrate and split their nuclei, the basis of the nuclear chain reaction.

Chadwick studied various problems related to radioactivity under Nobel laureate (and proton-discoverer) Ernest Rutherford at the University of Manchester before going to Germany to work with Hans Geiger at the Physikalisch-Technische Reichanstalt (Imperial Physical-Technical Institution) in Berlin. He was in the German capital when World War I began.

As an enemy alien, Chadwick was interned by the Germans, but allowed to set up a laboratory in the stables of his civilian internee camp outside Berlin. He remained there throughout the war, doing his research, before returning to Britain in 1919.

Working again with Rutherford, who had by this time moved to Cambridge University, Chadwick helped his mentor achieve the first artificial nuclear transformation. They also achieved the transmutation of other light elements by bombarding them with alpha particles, while pressing ahead with research into the basic structures of the atomic nucleus.

Chadwick’s discovery of the neutron — posited by Rutherford 12 years earlier — was made while he was still at Cambridge. It led to the fission of uranium 235, the key element used in the development of the atomic bomb. Regarding his achievement, Chadwick remarked with some ambivalence that he now realized that the development of an atomic weapon was not only likely, but inevitable.

During World War II, Chadwick came to the United States as part of the British delegation working on the Manhattan Project.

For his discovery of the neutron, Chadwick was first awarded the Hughes Medal by the Royal Society in 1932 and, three years later, the Nobel Prize for Physics.