James Chadwick, at the Cavendish Laboratory of the University of Cambridge, under the direction of Ernest Rutherford, discovers in 1932 a subatomic particle with a mass similar to that of the proton but with no electric charge: the neutron. Rutherford had theoretically predicted the existence of such a particle as early as 1920, necessary to explain why the atomic nucleus does not disintegrate due to electrostatic repulsion between protons, but the lack of electric charge made the neutron extraordinarily difficult to detect with the experimental methods then available, which relied on the ionization produced by charged particles. Chadwick bombards beryllium with alpha particles and detects a neutral radiation of great penetrating power, which he correctly interprets — unlike earlier experiments by other physicists who had observed the same phenomenon without identifying it — as a new neutral particle of mass comparable to the proton. The discovery completes the modern model of the atomic nucleus as composed of protons and neutrons, and is also the discovery that makes possible, just six years later, nuclear fission: the neutron turns out to be the ideal particle for bombarding heavy nuclei without the electrostatic repulsion that affects charged particles, opening the path toward nuclear energy and nuclear weapons.