James Chadwick was a student of Ernest Rutherford, who over saw his Ph.D. and made him assistant director of the lab in Cambridge university. His own research was on radioactivity. Then in 1919, Rutherford hypothesised the existence of a neutral particle within the nucleus, however, Chadwick and the other scientists couldn't find it.
Later, Chadwick used the same technique that Fredrick and Irene Joliot-Curie used to track radiation, except he was looking for a particle without a charge and with a similar mass of a proton. He found the neutron and received the Nobel Prize for his published piece, The Existence of a Neutron in 1932.