![]() ![]() With the imminent threat of war in l939, a number of scientists began to consider the possibility that a new and very powerful “ atomic bomb ” could be built from uranium. Through fission, neutrons of low energy can trigger off a very large energy release. This decrease of mass, described in the correct units and multiplied by c, accounts numerically for why the fission fragments are so energetic. One can calculate that the total mass of the fission products remaining at the end of the reaction is slightly less than the mass of the uranium atom plus neutron at the start. The Einstein equation says that massm can be converted to energy E, and the conversion factor is a huge number, c, which is the velocity of light squared. To understand the high energy released in uranium fission, scientists made some theoretical calculations based on Albert Einstein ’s famous equation E = mc 2. The size of the pulses showed that the fragments had a very large amount of energy. Evidently, the large pulses were due to uranium fission fragments. A Geiger tube without the uranium lining did not generate large pulses. When the neutron source was taken away, the large pulses stopped. ![]() When a neutron source was brought near it, large voltage pulses were observed, much larger than from ordinary radioactivity. For this experiment, the inside of a modified Geiger tube was lined with a thin layer of uranium. Clearly, a nucleus had been photographed in the act of fission.Īnother experimental procedure used a Geiger counter, which is a small, cylindrical tube that produces electrical pulses when a radioactive particle passes through it. When it was irradiated by neutrons, photographs showed a pair of tracks going in opposite directions from a common starting point in the uranium. In one experiment, a thin sheet of uranium was placed inside a cloud chamber. For example, a cloud chamber is a device in which vapor trails of moving nuclear particles can be seen and photographed. Scientists in the United States and elsewhere quickly confirmed the idea of uranium fission, using other experimental procedures. It takes only a small amount of energy to start the vibration which leads to a major breakup. Meitner named this process “fission ” because it is similar to the process of cell division in biology. If the vibration is violent enough, the drop can break into two pieces. When an extra neutron enters, the drop begins to vibrate. One can visualize the uranium nucleus to be like a liquid drop containing protons and neutrons. Lise Meitner (1878 –1968), a long –time colleague of Hahn who had left Germany due to Nazi persecution, suggested a helpful model for such a reaction. It was very puzzling to them how a single neutron could transform element 92 into element 56. Hahn and Strassmann published a scientific paper showing that small amounts of barium (element 56) were produced when uranium (element 92) was bombarded with neutrons. The evidence for this supposed process came from chemical analysis. The uranium nucleus apparently underwent a major disruption. When uranium was bombarded with neutrons, however, an entirely different reaction seemed to occur. Other elements became radioactive in the same way. If they bombarded copper, for example, a radioactive form of copper was produced. They had been doing a series of experiments in which they used neutrons to bombard various elements. The fission reaction was discovered in l938 by two German scientists, Otto Hahn (1879 –1968) and Fritz Strassmann (1902 –1980). Reactors are utilized for production of electricity at nuclear power plants, for propulsion of ships and submarines, and for the creation of radioactive isotopes used in medicine and industry. After the war, controlled energy release from fission was applied to the development of nuclear reactors. The fission reaction was the basis of the atomic bomb, which was developed by the United States during World War II. Fission fragments were shown to fly apart with a large release of energy. This reaction was discovered when a target of uranium was bombarded by neutrons. Nuclear fission when the nucleus of an atom splits, usually into two pieces. The controversy about nuclear power plants Nuclear reactors for electric power production
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