James Chadwick discovered the neutron. With Ernest Rutherford's help, Chadwick discovered the neutron by demonstrating a neutral particle with a mass the same as a proton through a reaction between gamma rays and a wax sample.
Prior to James Chadwick's discovery, it was believed that atoms contained electrons within the nucleus as well as outside of it. However, Chadwick and his colleagues believed that particles containing no charge were contained in the nucleus.
After nearly a decade of research, Chadwick's team had no proof of the neutral particle until an experiment by Frederic and Irene Joliot-Curie was able to eject protons from wax by using gamma rays. The light bulb turned on when Chadwick learned that gamma rays contained enough energy to eject protons. Upon this revelation, Chadwick returned to his laboratory to further examine the Joliot-Curie experiment.
Chadwick discovered the neutron in 1932. Upon his discovery, he submitted his findings to the science journal, Nature, to announce the possibility of the existence of the neutron. Chadwick was not convinced he had discovered a new fundamental particle until physicist Werner Heisenberg showed him otherwise.
Within three years of his discovery, he was awarded the Nobel Prize. Moreover, several years later, due to his extraordinary findings, he was granted the position of the British team leader in the Manhattan Project, which supported the United States' efforts to build the first nuclear bomb in World War II.