Text: Using a source of neutrons consisting of radon sealed in a glass tube with some beryllium powder, Fermi discovered that strong radioactivity was induced in such common elements as phosphorus, iron, silicon, and iodine when exposed to this source. The list of elements which showed some radioactivity after exposure to neutrons was considerably longer. These experiments marked the beginning of an entirely new method of producing so-called artificial radioactivity. The irradiation of stable isotopes with neutrons to produce radioactive isotopes was ultimately developed into the most prolific means of manufacturing radioisotopes in use today. In early investigations no mention was made of a moderator to slow down the relatively fast neutrons from the source. However, within a short time, it was realized that neutrons do not require kinetic energy to overcome Coulomb forces in penetrating the nucleus. Hence the chance of a reaction should increase as the transit time through the force-field of the nucleus became longer. In the original experiments in which neutrons were identified, the transfer of kinetic energy from neutrons to protons played an important role. Collision with protons is also the most effective means available for slowing neutrons.

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