|MadSci Network: Physics|
Hi Bruce, That is an interesting question. A historian might provide you with a better answer, but here goes. Sir James Chadwick discovered the neutron in 1932. In the following years Fermi studied the effects of neutrons on elements. The neutrons emitted by radioactive decay tend to have a lot of energy, which means they are moving at high velocity. In 1934 Fermi discovered that neutrons could be slowed down or moderated by passing them through light elements (low atomic number) such as carbon. Fermi found that the slow or thermal neutrons produced reactions more frequently than fast neutrons. Slow neutrons were essential to producing a chain reaction. The chain reaction occurs because each atom of uranium that splits produces two or more neutrons. Each of the neutrons produced by the first reaction can then start another reaction. Physicist later found ways to use fast neutrons in reactions, which enabled the development of the fast breeder reactor. Enrico Fermi received the Nobel Prize in physics in 1938 for his work on neutron reactions. The World Book encyclopedia states that Fermi believed the neutrons were absorbed by the uranium atoms to produce elements heavier than uranium. In 1938 Lise Meitner and Otto Frisch showed that the uranium atom had been split, and named the process nuclear fission. The discovery that the reaction was fission (splitting) accompanied with the production of excess neutrons led to the realization that fission could be accelerated, and controlled, with a chain reaction by using slow neutrons. Had Enrico Fermi recognized that the reactions he produced were fission reactions, the development of the chain reaction might have occurred several years sooner. Fermi was correct that the truansuranic elements (atomic number greater than 92) can be produced by the capture of slow neutrons. Bombarding uranium-238 with slow neutrons produces plutonium-239. The capture of the neutron by the uranium-238 nucleus forms neptunium-239, which decays by beta particle emission to form plutonium-239. Both the fissioning of uranium-235 and the capture of a neutron to produce Plutonuim-239 occur in modern fission reactors which use uranium enriched in the 235 isotope for fuel. For more information you might try using the keywords Physics and history with your web browser. There a many good web sites devoted to the history of science and physics. Bob Novak Sr. Process Engineer Carpenter Technology
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