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 discovered the particle in the nucleus of an atom that became known as the neutron because it has no electric charge.
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. Over the next two years he worked with Robert Oppenheimer, Edward Teller, Rudolf Peierls, Otto Frisch, Enrico Fermi, David Bohm, James Chadwick, James Franck, Felix Bloch, Emilio Segre, Eugene Wigner, Leo Szilard and Klaus Fuchs in developing the atom bombs dropped on Hiroshima and Nagasaki.