In these first experiments radioactive varieties of nitrogen, aluminum, and phosphorus were identified. Of the various processes resulting in the production of radioactive species, neutron-induced nuclear fission, achieved in 1939, has been the most fruitful. In 1941 it was learned that fission may also occur spontaneously. In this case, certain unstable nuclei of heavier elements split into nearly equal fragments without the introduction of outside energy. With such discoveries, modern theories of nuclear structure became possible, and the large-scale release of nuclear energy was achieved in 1942.
Radioactive substances emit energy in the form of ionizing radiations.
Such radiations dissipate their energy in passing through matter by producing ionization and other effects. If the radioactive source is a compact portion of matter, some of the energy of radiations is dissipated in the source itself. The source then tends to maintain a temperature higher than that of its surroundings. The emission is spontaneous, and its rate is uninfluenced by changes of pressure and temperature available to laboratory study. It is, however, not inexhaustible.