Spontaneous fission is an extremely rare phenomenon. For a nucleus like uranium 238, for instance, spontaneous fission only takes place for one decay in two million.
However, fission may be triggered by the capture of incoming neutrons by very heavy nuclei. The process become predominant for an handful of such heavy nuclei called fissile. These nuclei are made fragile by too many nucleons. After the capture of a neutron, they split into two sets of nucleons, liberating in the process a great deal of energy.
The only fissile nucleus which exists in nature is an isotope of uranium (uranium 235) which makes up 0.7% of uranium minerals. This material is extremely rare on Earth. Other fissile nuclei exist, but these need to be artificially generated in reactors. The main ones are plutonium 239, which is produced from uranium 238, and uranium 233, produced from thorium 232.
When a low-energy neutron is captured by a fissile nucleus, it increases the internal energy of the nucleus. This supplement of energy can be enough to spark off nuclear fission. The nucleus deforms and then splits into two pieces which are generally of unequal size.
Uranium-235, plutonium-239 and uranium-233 are fissile with slow neutrons whose kinetic energies are considered small., the neutron kinetic energy adds up to the energy released by the capture. When the neutron energy is no more small, the internal energy of the nucleus is significantly increased. This allows ‘fast’ neutrons to spark off nuclear fission of more nuclei than uranium-235, plutonium-239 and uranium-233.
More tha sixty years after the dicovery fission may seem a common phenomenon, but the discoveries of its multiple features required several steps. It was Otto Hahn and Lise Meitner who in 1938 put in evidence the splitting of the uranium nucleus into two smaller fragments, the fission products, but it was the Joliot's tean which one year later discovered the production of neutrons and the chain reaction process. With an extraordinary intuition on its future applications, Frederic Joliot filed three patents on the eve of the Second World War.
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