Nuclear fission is a nuclear reaction in which the nucleus of an atom splits into two or more smaller nuclei, along with the release of a significant amount of energy. This process typically occurs in heavy elements such as uranium-235 or plutonium-239. The fission process can be initiated by the absorption of a neutron by the nucleus of the fissile atom. When the nucleus absorbs the neutron, it becomes unstable and splits into two smaller nuclei, known as fission fragments.
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Nuclear fission splits an atomic nucleus into two major fragments, usually accompanied by emitted neutrons and gamma rays. It can follow neutron absorption or occur spontaneously in some heavy nuclides. When the products have greater total nuclear binding energy, the decrease in rest energy supplies fragment kinetic energy and radiation.
The fragments are often neutron-rich and undergo subsequent beta decay, producing an evolving inventory of fission products. Some emitted neutrons can cause further nuclear fission, enabling a nuclear chain reaction; others are absorbed without fission or escape.
The discovery combined chemical and physical evidence. Otto Hahn and Fritz Strassmann identified barium among products of neutron-irradiated uranium, rather than merely an adjacent heavy element. Lise Meitner and Otto Robert Frisch interpreted this as division of the atomic nucleus and estimated the large released energy from mass differences. Chemical separation identified the unexpected product: the same procedure could separate a radium isotope from barium, whereas the new radioactive activity followed barium. This chemical comparison did not by itself measure every fragment's motion.[ref]
Disintegration of Uranium by Neutrons: a New Type of Nuclear Reaction by Lise Meitner and Otto Robert Frisch. Opening barium discussion and mass-defect interpretation, Nature 143, pp. 239–240
