In 1914 Chadwick went to study under Hans Geiger at the Technische Hochschule in Berlin (today the Technical University of Berlin). During the First World War Chadwick was interned in Germany as an enemy alien.
After the war Chadwick returned to Cambridge where he worked with Ernest Rutherford in investigating the emission of gamma rays from radioactive materials. They also studied the transmutation of elements by bombarding them with alpha particles and investigated the nature of the atomic nucleus.
In 1932 Chadwick made a fundamental discovery in the domain of nuclear science: he discovered the particle in the nucleus of an atom that became known as the neutron because it has no electric charge. In contrast with the helium nuclei (alpha particles) which are charged, and therefore repelled by the considerable electrical forces present in the nuclei of heavy atoms, this new tool in atomic disintegration need not overcome any electric barrier and is capable of penetrating and splitting the nuclei of even the heaviest elements. Chadwick in this way prepared the way towards the fission of uranium 235 and towards the creation of the atomic bomb. For this epoch-making discovery he was awarded the Hughes Medal of the Royal Society in 1932, and subsequently the Nobel Prize for Physics in 1935.
Chadwick became professor of Physics at Liverpool University in 1935 and during the Second World War he joined the Manhattan Project in the United States, developing the atomic bombs dropped on Hiroshima and Nagasaki.
He died in Cambridge.