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A calutron is a mass spectrometer used for separating the isotopes of uranium. It was developed by Ernest O. Lawrence during the Manhattan Project and was similar to the cyclotron invented by Lawrence. Its name was derived from Califrnia University Cyclotron, in tribute to Lawrence's institution, the University of California, where it was invented. They implemented industrial scale uranium enrichment at the Oak Ridge Y-12 plant established during the war and provided much of the uranium used for the Little Boy nuclear weapon, which was dropped on Hiroshima in 1945.
In a mass spectrometer, a vaporized sample is bombarded with high-energy electrons, which cause the sample components to become positively charged ions. They are then accelerated by electric fields and subsequently deflected by magnetic fields, ultimately colliding with a plate and producing 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. An ordinary mass spectrometer is designed to analyse the composition of very small samples; the calutron uses the same principle, but is designed to separate substantial quantities of known isotopes.
Due to the wartime copper shortage, the electromagnets were made using thousands of tons of silver borrowed from the U.S. Treasury. Initially a type of calutron known as Alpha was used; it enriched uranium to about 15% 235U. A later design, called Beta, further enriched the output of Alpha, optimising the initial design for the smaller quantities of already enriched feedstock. To take full advantage of the required large electromagnet, multiple calutrons were arranged around it in a large oval, called a race track because of its shape.
Electromagnetic separation was later abandoned in favor of the more complicated, but more effective, gaseous diffusion method.
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. The news was brought to the United States by Neils 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, ironically because their status as enemy aliens precluded their working on secret projects like radar. They made a breakthrough investigating the critical mass of uranium-235. Their March 1940 Frisch–Peierls memorandum indicated that it was within an order of magnitude of 10 kilograms (22 lb), which was small enough to be carried by a bomber of the day. Oliphant was quick to recognize the significance of this. 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 one part in 140 of natural uranium, so it needs to enriched 1,260-fold to produce 90% uranium-235. 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 is simple: 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 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 37-inch (94 cm) 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 0.04 by 2 inches (1.0 by 50.8 mm) slot into a vacuum chamber. The magnet is then used to deflect the ion beam by 180 degrees. 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 intro World War II, a 5 microampere (μA) beam was received by the collector. While still very small, it was ten times as much as Nier's. 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. 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.
While the process had been demonstrated to work, considerable work was still required before a prototype could be tested in the field. Lawrence assembled a team of physicists to tackle the problems, including David Bohm, Edward Condon, Donald Cooksey, A. Theodore Forrester, Irving Langmuir, Kenneth Ross MacKenzie, Frank Oppenheimer, J. Robert Oppenheimer, William E. Parkins, Bernard Peters and Joseph Slepian. In November 1943 they were joined by a British Mission headed by Oliphant that included fellow Australian physicists Harrie Massey and Eric Burhop,and British physicists such as Joan Curran and Thomas Allibone.
Lawrence had a much larger cyclotron under construction at Berkeley, one with a 184-inch (4,700 mm) magnet. This was converted into a calutron that was switched on for the first time on 26 May 1942. Like the 37-inch version, it looked like a giant C when viewed from above. The operator sat in the open end, from whence he could regulate the temperature, adjust the position of the electrodes, and even replace components through an airlock while it was running. The new, more powerful calutron was not used to produce enriched uranium, but for experiments with multiple ion sources. This meant having more collectors, but it multiplied the throughput. The problem was that the beams interfered with each other, producing a series of oscillations called hash. An arrangement was devised that minimised the interference, resulting in reasonably good beams being produced in September 1942. Robert Oppenheimer and Stan Frankel invented the magnetic shim, a device used to adjust the homogeneity of a magnetic field. These were sheets of iron about 3 feet (0.91 m) in width that were bolted to the top and bottom of the vacuum tank. The effect of the shims was to slightly increase the magnetic field in such a way as to help focus the ionic beam. Work would continue on the shims through 1943.
Burhop and Bohm later studied the characteristics of electric discharges in magnetic fields, today known as Bohm diffusion. Their papers on the properties of plasmas under magnetic containment would find utilization in the post-war world in research into controlled nuclear fusion. Other technical problems were more mundane but no less important. Although the beams had low intensity, they could, over many hours of operation, still melt the collectors. A water cooling system was therefore added to the collectors and the tank liner. Procedures were developed for cleaning the "gunk" and "crud" from the inside of the vacuum tank. A particular problem was blockage of the slits, which caused the ionic beams to lose focus, or stop entirely.
The chemists had to find a way of producing quantities of uranium tetrachloride (UCl
4) from uranium oxide. (Nier had used uranium bromide (UBr
4).) Initially, they produced it by using hydrogen to reduce uranium trioxide (UO
3) to uranium dioxide (UO
2), which was then reacted with carbon tetrachloride (CCl
4) to produce uranium tetrachloride. Charles A. Kraus proposed a better method for large-scale production that involved reacting the uranium oxide with carbon tetrachloride at high temperature and pressure. This produced uranium pentachloride (UCl
5) and phosgene (COCl
2). While nowhere near as nasty as the uranium hexafluoride used by the gaseous diffusion process, uranium tetrachloride is hygroscopic, so work with it had to be undertaken in gloveboxes that were kept dry with phosphorus pentoxide (P
10). The presence of phosgene, a lethal gas, required the chemists wore gas masks when handling it. The danger was brought home when Sam Ruben died from an accident involving phosgene, although unrelated to the Manhattan Project. One worker at Oak Ridge, died from exposure to phosgene.
In addition to the work carried out at Berkeley, research into the electromagnetic process was also conducted at Brown University, Johns Hopkins University and Purdue University, and by the Tennessee Eastman corporation. However, of the $19.6 million spent of research and development of the electromagnetic process, $18 million (92 percent) was spent at the Radiation Laboratory in Berkeley. During 1943, the emphasis shift from research to development, engineering, and the training of workers to operate the production facilities at the Clinton Engineer Works in Oak Ridge. By the middle of 1944, there were nearly 1,200 people working at the Radiation Laboratory.
The alpha racetracks
Much of the great progress on the electromagnetic process can be attributed to Lawrence's leadership style. His audacity, optimism and enthusiasm were contagious, and inspired his staff to put in long hours, University of California administrators to slice through red tape despite not knowing what the project was about, and government officials to view the development of atomic bombs in time to affect the outcome of the war as a genuine possibility. Vannevar Bush, the director of the Office of Scientific Research and Development (OSRD), which was overseeing the project, visited Berkeley in February 1942, and found the atmosphere there "stimulating" and "refreshing". This infected him as well. On 9 March 1942, he reported to the President, Franklin D. Roosevelt, that it might be possible to produce enough material for a bomb by early 1942, based on new estimates from Robert Oppenheimer that the critical mass of a sphere of uranium-235 was between 2.0 and 2.5 kilograms.
The experiments with the 184-inch magnet led to the construction of a prototype calutron called the XA. This contained a rectangular, three-coil magnet with a horizontal field in which the calutron tanks could stand side-by-side, with four vacuum tanks, each with a double source. At the 25 June 1942 meeting of the S-1 Executive Committee, which had superseded the S-1 Uranium Committee on 19 June, there was a proposal to build the electromagnetic plant at Oak Ridge, where the other Manhattan Project facilities would be located, for reasons of economy and security. Lawrence objected. He wanted the electromagnetic separation plant located much nearer to Berkeley. The Shasta Dam area in California remained under consideration for the electromagnetic plant until September 1942, by which time Lawrence had dropped his objection. The 25 June meeting also designated Stone & Webster as the primary contractor for the design and engineering of the electromagnetic plant.
The Army assumed responsibility for the Manhattan Project on 17 September 1942, with Brigadier General Leslie R. Groves, Jr., as director. Between October 1942 and November 1943, Groves would pay monthly visits to the Radiation Laboratory in Berkeley. The Army formally took over the electromagnetic project contracts with the University of California from the OSRD on 1 May 1943.
By the spring of 1943, convinced that the Germans might be ahead, General Leslie Groves decided to skip the scheduled pilot plant: procedures for alpha operation at Oak Ridge came from the XA and a scale model of the production magnet alone. Tests of the first full-scale system installed there, the XAX, were scheduled for July.
The spring and early summer of 1943 brought hundreds of trainees to Berkeley from Tennessee Eastman Company, the operator for the Oak Ridge plant. The Laboratory labored to ensure that the test XA magnet system and alpha units were working by April in spite of delays in delivery of steel. Between April and July the training sessions ran continuously. In June a migration that by 1944 would reach 200 started for Oak Ridge.
The first wave of Berkeley workers at Oak Ridge had to see that the XAX magnet worked. Then runs could begin on the first production system, or "racetrack"; a 24-fold magnification of the XA that could hold 96 calutron alpha tanks. To minimize magnetic losses and steel consumption, the assembly was curved into an oval 122 feet (37 m) long, 77 feet (23 m) wide and 15 feet (4.6 m) high. Want of copper for the large coils to produce the magnetic fields prompted a solution possible only in wartime: Groves borrowed 14,700 short tons (13,300 tonnes, 429 million troy ounces) of pure silver from a government vault for the purpose; all was later returned, the last few tons in 1970. Late in the summer of 1943 the XAX was ready for testing. After a week of difficulty, it cleared the hurdle for full-scale racetrack runs.
The first two of five projected racetracks started up in November and failed from contaminated cooling oil; the second was limping in January, but produced 200 grams of uranium enriched to 12% 235U by the end of February 1944, one fifth of the total goal of one kilogram of enriched uranium per month. By April four racetracks were functioning, including the repaired number 1. They required constant attention. Many people from the Laboratory helped modify the units to reach production goals.
The calutrons were initially operated by scientists from Berkeley to remove bugs and achieve a reasonable operating rate. Then Tennessee Eastman operators who had only a high-school education took over. Kenneth Nichols compared unit production data, and pointed out to Ernest Lawrence that the young "hillbilly" girl operators were outproducing his Ph.Ds. They agreed to a production race and Lawrence lost, a morale boost for the Tennessee Eastman workers and supervisors. The girls were trained like soldiers not to reason why, while "the scientists could not refrain from time-consuming investigation of the cause of even minor fluctuations of the dials". Responsibility for operation passed entirely to Tennessee Eastman after the spring of 1944, and the Laboratory staff at Oak Ridge turned their attention to redesigning the calutron system for higher efficiency.
The beta racetracks
Many at the Laboratory, especially Edward Lofgren and Martin Kamen, thought that a second stage would be necessary to reach the required enrichment. Groves approved the idea. In the spring of 1943, during training at Berkeley for alpha operations, design began on the second or beta stage. Because beta would have only the enriched product of alpha as feed, it would process proportionately less material; its beam therefore did not need to be as broad, nor its dimensions as large, as alpha's. Beta design emphasized recovery, not only of the further enriched output but also of the already enriched feed. The first units were tried at Oak Ridge in late February 1944, but the sources had to be redesigned, and even by June difficulties persisted in recovering the precious beta feed strewn throughout the calutron. Process efficiencies stayed low: only 4 or 5 percent of the 235U in the feed ended up in the output. A better source of enriched uranium feed would have to be found to create the 10 kilograms or so of 90 percent 235U that Robert Oppenheimer thought necessary for a bomb.
The gaseous diffusion procedure for separation of uranium isotopes, which had consumed more money than the calutron, had not met its design goals by late 1944. Groves decided that it could not be counted on to produce high enrichment, and that whatever it did produce would have to be supplemented with other less enriched uranium and processed through beta calutrons. To augment the calutron feed, the Manhattan Engineering District constructed a further plant at Oak Ridge, this one working by thermal diffusion, a method developed by Philip Abelson.
In the critical production period in the first months of 1945, the calutrons, particularly the six betas of 36 tanks each, produced weapons-grade 235U using feed from the modified alpha calutrons, the small output from the gaseous diffusion plant, and whatever the new thermal process had to offer. Virtually all the 235U sent by courier on the train to Chicago and on to Los Alamos, New Mexico had passed through the beta calutrons. From these shipments Oppenheimer's physicists assembled the bomb that was to destroy Hiroshima.
After the 1990 Gulf War, UNSCOM determined that Iraq had been pursuing a calutron program to enrich uranium. Iraq chose to develop the program over more modern, economic, and efficient methods of enrichment because it would require fewer imports. At the time the program was discovered, Iraq was a number of years away from developing material for weapons, but the program was destroyed in the Gulf War.
The main Calutron patents are Methods of and apparatus for separating materials (Ernest O. Lawrence), Magnetic shims (Robert Oppenheimer and Stanley Frankel), and Calutron system (Ernest O. Lawrence).
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