Nuclear Fission
The first splitting of an atom by John Douglas Cockroft and Ernest Thomas Sinton Walton in 1932 was a momentous occasion in physics. The two researchers found that the two helium atoms produced when a proton collides with a lithium nucleus have more than fifty times the kinetic energy of the protons used to initiate the reaction. The question these results posed for physicists was whether similar reactions could be used as a practical source of energy. Further research indicated that other reactions could be designed in which the energy produced was many times greater than the energy used to bring about the reaction initially.
The hopes for finding practical applications of such reactions were dashed, however, when studies showed that only one to ten particles out of a billion actually come into contact with a target nucleus. The total energy needed to initiate such reactions, therefore, is much too great for them to have commercial value.
Just as this reality was dawning on physicists, however, remarkable news on the same topic appeared from another front. In the early 1930s, Enrico Fermi, Emilio Segrè, and their colleagues at the University of Palermo were studying the effect of bombarding uranium with neutrons. They knew that the result of neutron bombardment of an element was often the production of a new element with an atomic number one greater than the original target. The Fermi-Segré team hoped, therefore, to produce the first synthetic transuranium element in their research.
Initial results of their studies prompted Fermi and Segré to conclude that they had, indeed, obtained element number 93 as a result of bombarding uranium with neutrons. However, as time passed and their work was repeated, evidence for this result became less convincing. One team attempting to replicate the Fermi-Segré findings consisted of the German scientists, Fritz Strassman and Otto Hahn. In their research, Hahn and Strassman found evidence for the formation of elements with atomic numbers lower than that of uranium, in particular, of actinium and radium. Such results were unlike anything that had ever been reported with neutron reactions and were, therefore, totally unexpected.
The problem became even more complex in 1938 when Irène Joliot-Curie and P. Savitch showed that the products of this reaction were more similar to lanthanum and barium than they were to actinium and radium. These results were startling because lanthanum and barium are much farther down in the periodic table than are actinium and radium.
On January 6, 1939, Hahn and Strassman published a report of their findings, along with the results obtained by Joliot-Curie and Savitch. They acknowledged the possibility that the bombardment of uranium by neutrons could have resulted in the formation of lanthanum and barium. But that would have meant that the uranium nucleus had been broken apart into two large pieces. They wrote that they really could not accept that such a change had occurred because "As 'nuclear chemists' with close ties to physics, we can not decide to make a step so contrary to all existing experience of nuclear physics."
Ten days later, another pair of researchers decided that they could take that step. Two Austrian physicists, Lise Meitner and Otto Frisch, presented a paper in which they explained the Hahn-Strassman result as the breaking apart of the uranium nucleus into "two nuclei of roughly equal size," a process they named nuclear fission.
Within a short time, the world community of physicists recognized the significance of this discovery. Calculations showed that the amount of energy released during fission is many times greater than that produced in any chemical or nuclear reaction known theretofore. In theory, at least, here was an important new source of energy for human use.
All of which is not to say that all scientists thought that nuclear fission was a feasible source of energy. Many argued that fission was purely an interesting laboratory phenomenon with no practical applications. The great Ernest Rutherford, for example, referred to the idea of using fission as a practical source of energy as "moonshine."
Nonetheless, a number of scientists were convinced of fission's potential and began to search for methods of putting it to use. One of the critical practical problems to be solved was how to keep a fission reaction going. Each time a uranium nucleus fissions, an average of about two additional neutrons is produced. This means that the particle needed to initiate a fission reaction is also produced in the reaction. In theory, once a fission reaction starts in a block of uranium, it should continue on its own until all or most uranium nuclei have fissioned. Leo Szilard first conceived of such a notion in 1934, although he proposed using beryllium rather than uranium for the chain reaction, a process that proved unfeasible.
Having heard of Hahn and Meitner's paper, however, Szilard realized that uranium rather than beryllium could be used in a chain reaction. In the decade that followed, Szilard was a major force in encouraging the United States government to go forward in the development of a new type of weapon based on the use of nuclear fission. Working with Enrico Fermi during the Manhattan Project of World War II, he designed a technique for attaining the first controlled nuclear fission reaction on December 2, 1942. Information gained from the Fermi-Szilard "pile" at the University of Chicago was crucial to the development of the first fission (atomic) bombs produced in the years that followed.
Following the war, scientists turned their attention to developing methods for using controlled nuclear fission as a dependable source of power for peaceful uses. The result of that research has been the nuclear reactor that forms the core of a nuclear power plant and that provides power in other settings.
The development of nuclear fission is a complex story with many sub-plots. Countless theoretical and technical problems had to be solved before the discovery by Hahn and Strassman could become a reality. For example, in 1939, the American biophysicist, Richard Roberts, made an important discovery about the nature of fission reactions. He learned that, during fission, some neutrons are not released at the instant a uranium nucleus splits apart, but are delayed slightly before emission. That discovery meant that nuclear fission could be controlled sufficiently to allow its use for peaceful purposes. If there were no delayed neutrons, all fission reactions would proceed so rapidly that they could be used only to produce sudden, violent explosions. But the phenomenon of delayed neutron emission allows the use of control rods in a nuclear reactor to slow down or stop a fission reaction that might otherwise go out of control.
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