Nuclear Fission

Nuclear fission is a process in which a heavy nucleus splits into two much lighter nuclei. For some very unstable nuclei fission can happen spontaneously, but that is a very rare event. Usually, the process is induced by the excitation of the nuclei by bombarding them with particles or with gamma rays. Heavy nuclei have a greater neutron/proton ratio than the lighter nuclei, and the fragments will therefore contain too many neutrons. To reduce the excess of neutrons, two or three neutrons will be emitted by the fragments immediately, and the fragments will then decay by β-decay until stable isotopes are reached.

Nuclear fission was discovered in the 1930s when nuclear physics was still a young research field. At this time, a completely new realm of phenomena opened up when researchers discovered that radioactivity could be induced in heavy elements when bombarding them with neutrons. Initially, it had been discovered by Irene Curie (1897–1956) and her husband Fréderic Joliot (1900–1958) in 1934 that when bombarding light elements with alpha particles, these would transmute into radioactive isotopes of near-by elements. Because of the positive charge of the alpha particles, Curie and Joliot could only induce radioactivity in light elements. However, Enrico Fermi (1901–1954) soon realized that neutron bombardment could be used to induce radioactivity in heavy elements. After a series of experiments, Fermi and his collaborators reported that for a large number of elements of any atomic weight, neutron bombardment would produce unstable elements which emitted β-particles. Fermi's team therefore concentrated on the heavy nuclei thorium and uranium, since their general instability might give rise to successive disintegrations. For uranium, the last element in the periodic table as it was then known, such a series of β-emissions would lead to elements that did not exist in nature, and it attracted the attention of scientists around the world when Fermi's group reported that they had identified the first such transuranic element by chemical analysis of one of the decay products.