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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 is a concatenation of Cal. U.-tron, in tribute to the University of California, Lawrence's institution and the contractor of the Los Alamos laboratory. 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.
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.
Due to the wartime copper shortage, the electromagnets were made using thousands of tons of silver borrowed from the U.S. Treasury. 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.
Magnetic separation was later abandoned in favor of the more complicated, but more effective, gaseous diffusion method.
Calutrons and the Manhattan project
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Uranium isotope separation
The world did not lack methods for separating isotopes when it discovered the possible utility of a kilogram of uranium-235 (235U). Known techniques, pursued simultaneously in Germany and the United States, included ultra-centrifugation, diffusion across thermal or osmotic pressure barriers, and deflection in electric and magnetic fields. The last method appealed to Lawrence, who had made his reputation on the precise control of beams of charged particles. The principle is simple: When passing between the poles of a magnet, a monoenergetic beam of ions of naturally occurring uranium splits into several streams according to their momentum, one per isotope, each characterized by a particular radius of curvature; collecting cups at the ends of the semicircular trajectories catch the homogeneous streams.
Most physicists in 1941 doubted that electromagnetic separation would succeed in practice, because they expected that the mutual repulsion of the like-charged ions (the space charge effect) would prevent the formation of narrow beams. Notwithstanding, Lawrence, who had seen a line of positively charged ions pour from his cyclotron, had the 37-inch (94 cm) cyclotron modified to demonstrate the feasibility of electromagnetic separation of uranium isotopes using the principle of the mass spectrograph. "It will not be a calamity," he wrote Compton, if uranium turned out to have no military applications; but if "fantastically positive and we fail to get them first, the results for our country may well be a tragic disaster." By December 1941 the uranium ion beam was passing 5 microamperes to the collector; a small amount, but enough to assure Lawrence that space charge would not be a formidable problem.
The fact that beams of uranium ions could be concentrated well enough to yield small quantities of isotopes suitable for laboratory research by no means assured that electromagnetic separation could be worked on the industrial scale necessary to make a kilogram of 235U. The process has little to work on, only the slight difference in mass: 1.25% between uranium 235 and 238. Because the lighter ions respond slightly more to the magnetic field than the heavier, their trajectories bend in a tighter arc. At the end of their semicircular travel, the ions of 235U are more plentiful on the inside than on the outside of the beam. The maximum separation even in the ideal case is small, only one-tenth inch for an arc with a diameter of 37 inches (94 cm). Actual beams are far from ideal.
The alpha calutron
Many technical problems had to be solved before even a prototype could be tested in the field of the nearly completed 184-inch (4.67 m) magnet. The beams, though of low power, could melt the collectors during long hours of operation; water cooling was installed for the collectors and tank liner. Electric arcs were contrived to ionize the uranium tetrachloride feed. Ways were devised to extract the enriched uranium that collected at the receiver, and the still valuable feed material that condensed along with chloride "gunk" all over the inside of the tank. Scrapers were made to clean the exit slits of the feed sources regularly to prevent the accumulated "crud" from reducing beam strength. Lawrence's optimistic conclusion: by the fall of 1942, ten calutrons, each with a 100-milliampere source and all operating within the 184-inch field, would produce four grams of enriched uranium a day. The S-1 Uranium Committee that oversaw the uranium project for Office of Scientific Research and Development recommended expending $12 million to create a plant with 25 times that capacity before the fall of 1943. Lawrence did not doubt that other means, particularly reactor production of fissile plutonium, might ultimately be the most efficient way to a bomb. In mid-1942 no reactor worked, and the calutron did.
The calutron design settled on in 1942, called "alpha", provided for enrichment of natural uranium to about 15% 235U. Much effort went into designing powerful ion sources and aptly shaped, eventually parabolic collecting slots. The many modifications and security codes proliferated whimsical names: sources Plato, Cyclops, Bicyclops, and Goofy mated with receivers Gloria, Irene, Mona, or Zulu. Ions from Plato and his friends traversed an arc 48 inches (1.22 m) in radius to reach collector slits placed 0.6 inch (15 mm) apart. The guiding magnetic field was shimmed not by the old black art, but in obedience to calculations. Accurately machined and installed, the shims greatly increased the usable beam that reached the collectors.
Scaling up at Oak Ridge
Among results obtained with the 184-inch magnet was a design superior to it for large-scale calutrons, the so-called "XA". The prototype of the magnets to be installed at Oak Ridge, XA was a rectangular, three-coil magnet giving a horizontal field in which the calutron tanks could stand side-by-side. It had room for four alpha tanks, each with a double source. 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. Laboratory expenditures exceeded half a million dollars a month.
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 calutron
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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- This article incorporates public domain material from the United States Government document "Lawrence and His Laboratory: The Calutron".
- This article incorporates public domain material from the United States Government document "The Manhattan Project: An Interactive History".