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Old September 13th, 2010, 02:41 PM   #1
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from basic discoveries to atom bombs (1939)

HOW WAS NUCLEAR FISSION DISCOVERED





Practical applications of science, desirable and undesirable, are rooted in our attempts to understand and control nature. Nuclear fission was discovered accidentally, during investigations whose purpose was to create trans-uranium elements. But it was at once recognized as a reaction by which a large amount of energy can be released.



The 1939 discovery of nuclear fission, leading to the development of atomic bombs and nuclear reactors, was mostly the result of an international effort. It was preceded by the discovery of neutrons (James Chadwick; England, 1932) and artificial radioactivity (Irene and Frederic Joliot-Curie, France, 1933). In 1934, when uranium was the last known element in the periodic chart, a team of Italian scientists headed by Enrico Fermi started bombarding uranium with neutrons. They wanted to artificially produce elements “beyond uranium.”



German chemists Otto Hahn, Lise Meitner, and Fritz Strassmann also bombarded uranium with neutrons for several years, investigating properties of reaction products. At first they suspected these products were Ra, Ac and Th. But in a January 1939 paper, published without Meitner’s co-authorship (she was forced to leave Germany due to Hitler's racial policy), they wrote that these products might be Ba, La, and Ce. But this was not a definite statement; the authors hesitated because they had no idea how such elements could be produced.



Credit for understanding the mechanism reactions belongs to Meitner, who was in Sweden when the paper was published. She was the first to realize that an atomic nucleus could split into two large fragments. Meitner coined the name “nuclear fission” and subsequently published a paper in which the amount of energy released in fission was calculated on the basis of Einstein's E=m*c2 formula. Two scientists, Otto Frisch in Denmark and Joliot-Curie in France, provided experimental evidence for the magnitude of released energy. This was at once confirmed in early nuclear fission experiment in the United States.



Another important discovery, made in 1939 by Joliot-Curie and his collaborators (Halban, from Austria, and Kowarski, from Russia), was emission of several energetic neutrons during each fission event. This was a clear indication of the possibility of an explosive chain reaction. Basic principles of future applications of nuclear energy (both military, as in Hiroshima, and peaceful, as in a nuclear reactor producing electricity) were already known when WWII started. But technical details leading to practical applications were far from clear. Fearing that Nazi Germany was planning to develop nuclear weapons, Leo Szilard, one of many refugees from Europe, persuaded Albert Einstein to write a letter to president Roosevelt. On August 2, 1939, the famous scientist wrote:



“ . . . In the course of the last four months it has been made probable through the work of Joliot in France as well as Fermi and Szilard in America--that it may be possible to set up a nuclear chain reaction in a large mass of uranium, by which vast amounts of power and large quantities of new radium-like elements would be generated. Now it appears almost certain that this could be achieved in the immediate future.



This new phenomenon would also lead to the construction of bombs, and it is conceivable--though much less certain--that extremely powerful bombs of this type may thus be constructed. A single bomb of this type, carried by boat and exploded in a port, might very well destroy the whole port together with some of the surrounding territory. . . . “ The large-scale US nuclear bomb project (1942-1945) can be traced to that letter.



Early American nuclear science efforts, prior to discovery of fission, focused on accelerators, such as the Columbia University cyclotron built by John Dunning (in 1937). The first American nuclear fission experiments were conducted using this accelerator in January and February of 1939. Fermi, by then one of many European refugees, joined Dunning’s team. After confirming high energy of fission fragments, and the possibility of a chain reaction, the team showed that only a rare uranium isotope, 235U, and not the common 238U, was responsible for fission fragments. This important American discovery opened the path toward both military and peaceful applications of nuclear energy.



Reacting to Einstein’s letter, President Roosevelt asked the director of the National Bureau of Standards to organize a secret “Advisory Committee on Uranium.” The first meeting of that body took place on October 21, 1939. A modest sum of six thousand dollars was at once budgeted to support basic chain reaction research. The United State was still a neutral country at that time. The Manhattan Project, whose purpose was to develop atomic bombs, was created in August 1942. Enrichment of uranium with 235U became the high priority task . The first atomic bomb was exploded in Alamogordo, New Mexico (July 1945). A month later two atomic bombs were dropped on Hiroshima and Nagasaki, killing and seriously injuring several hundred thousand people.
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