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A calutron is a mass spectrometer originally designed and used for separating the isotopes of uranium. It was developed by Ernest O. Lawrence during the Manhattan Project and was based on his earlier invention, the cyclotron. Its name was derived from California University Cyclotron, in tribute to Lawrence's institution, the University of California in Berkeley, California, where it was invented. Calutrons were used in the industrial-scale Y-12 uranium enrichment plant at the Clinton Engineer Works in Oak Ridge, Tennessee. The uranium-235 produced there was used in the Little Boy atomic bomb that was detonated over Hiroshima on 6 August 1945. The calutron is a type of sector mass spectrometer, an instrument in which a sample is ionized and then accelerated by electric fields and deflected by magnetic fields. The ions ultimately collide with a plate and produce a measurable electric current. Since the ions of the different isotopes have the same electric charge but different masses, the heavier isotopes are bent less by the magnetic field, causing the beam of particles to separate out into several beams by mass, striking the plate at different locations. The mass of the ions can be calculated according to the strength of the field and the charge of the ions. During World War II, calutrons were developed to use this principle to obtain substantial quantities of high-purity uranium-235, by taking advantage of the small mass difference between uranium isotopes. Electromagnetic separation for uranium enrichment was abandoned in the post-war period in favor of the more complicated, but more efficient, gaseous diffusion method. Although most of the calutrons of the Manhattan Project were dismantled at the end of the war, some remained in use to produce isotopically enriched samples of naturally occurring elements for military, scientific and medical purposes. == Origins == News of the discovery of nuclear fission by German chemists Otto Hahn and Fritz Strassmann in 1938, and its theoretical explanation by Lise Meitner and Otto Frisch, was brought to the United States by Niels Bohr. Based on his liquid drop model of the nucleus, he theorized that it was the uranium-235 isotope and not the more abundant uranium-238 that was primarily responsible for fission with thermal neutrons. To verify this Alfred O. C. Nier at the University of Minnesota used a mass spectrometer to create a microscopic amount of enriched uranium-235 in April 1940. John R. Dunning, Aristid von Grosse and Eugene T. Booth were then able to confirm that Bohr was correct. This made it almost certain that a nuclear chain reaction could be initiated, and therefore that the development of an atomic bomb was a theoretical possibility. There were fears that a German atomic bomb project would develop one first, especially among scientists who were refugees from Nazi Germany and other fascist countries. At the University of Birmingham in Britain, the Australian physicist Mark Oliphant assigned two refugee physicists—Otto Frisch and Rudolf Peierls—the task of investigating the feasibility of an atomic bomb, ironically because their status as enemy aliens precluded their working on secret projects like radar. Their March 1940 Frisch–Peierls memorandum indicated that the critical mass of uranium-235 was within an order of magnitude of 10 kg, which was small enough to be carried by a bomber of the day. The British Maud Committee then unanimously recommended pursuing the development of an atomic bomb. Britain had offered to give the United States access to its scientific research, so the Tizard Mission's John Cockcroft briefed American scientists on British developments. He discovered that the American project was smaller than the British, and not as far advanced. A disappointed Oliphant flew to the United States to speak to the American scientists. These included Ernest O. Lawrence at the University of California's Radiation Laboratory. The two men had met before the war, and were friends. Lawrence was sufficiently impressed to commence his own research into uranium. Uranium-235 makes up only about 0.72% of natural uranium, so the separation factor of any uranium enrichment process needs to be higher than 1250 to produce 90% uranium-235 from natural uranium. The Maud Committee had recommended that this be done by a process of gaseous diffusion, but Oliphant had pioneered another technique in 1934: electromagnetic separation. This was the process that Nier had used. The principle of electromagnetic separation was that charged ions are deflected by a magnetic field, and lighter ones are deflected more than heavy ones. The reason the Maud Committee, and later its American counterpart, the S-1 Uranium Committee, had passed over the electromagnetic method was that while the mass spectrometer was capable of separating isotopes, it produced very low yields. The reason for this was the so-called space-charge limitation. Positive ions have positive charge, so they tend to repel each other, which results in the beam scattering. However, Lawrence had considerable experience with the precise control of charged-particle beams from his work with his invention, the cyclotron, and he suspected that the air molecules in the vacuum chamber would neutralize the ions, resulting in a focused beam. Oliphant inspired Lawrence to convert his old cyclotron into a giant mass spectrometer for isotope separation. The 37-inch cyclotron at Berkeley was dismantled on 24 November 1941, and its magnet used to create the first calutron. Its name came from California University and cyclotron. The work was initially funded by the Radiation Laboratory from its own resources, with a $5,000 grant from the Research Corporation. In December Lawrence received a $400,000 grant from the S-1 Uranium Committee. The calutron consisted of an ion source, in the form of a box with a slit in it and hot filaments inside. Uranium tetrachloride was ionized by the filament, and then passed through a slot into a vacuum chamber. The magnet is then used to deflect the ion beam by 180°. The enriched and depleted beams went into collectors. When the calutron was first operated on 2 December 1941, just days before the Japanese attack on Pearl Harbor brought the United States into World War II, a uranium beam intensity of 5 microamperes (μA) was received by the collector. Lawrence's hunch about the effect of the air molecules in the vacuum chamber was confirmed. A nine-hour run on 14 January 1942 with a 50 μA beam produced 18 micrograms (μg) of uranium enriched to 25% uranium-235, about ten times as much as Nier had produced. By February, improvements in the technique allowed it to generate a 1,400 μA beam. That month, 75 μg samples enriched to 30% were shipped to the British and the Metallurgical Laboratory in Chicago. Other researchers also investigated electromagnetic isotope separation. At Princeton University, a group led by Henry D. Smyth and Robert R. Wilson developed a device known as an isotron. Using a klystron, they were able to separate isotopes using high-voltage electricity rather than magnetism. Work continued until February 1943, when, in view of the greater success of the calutron, work was discontinued and the team was transferred to other duties. At Cornell University a group under Lloyd P. Smith that included William E. Parkins, and A. Theodore Forrester devised a radial magnetic separator. They were surprised that their beams were more precise than expected, and, like Lawrence, deduced that it was a result of stabilization of the beam by air in the vacuum chamber. In February 1942, their team was consolidated with Lawrence's in Berkeley. They posted their results to the ''Physical Review'' from a railroad station en route to California. Their paper was classified secret, but was published after the war. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「calutron」の詳細全文を読む スポンサード リンク
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