Calutron
A calutron is a mass spectrometer originally designed and used for separating the isotopes of uranium. It was developed by Ernest 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, 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 enriched uranium produced 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 deflected less by the magnetic field, causing the beam of particles to separate 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. Leo Szilard and Walter Zinn soon confirmed that more than one neutron was released per fission, which 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 Lawrence at the University of California's Radiation Laboratory in Berkeley. 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 125 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 is 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 Section of the Office of Scientific Research and Development, 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 causes the beam to scatter. Drawing on his experience with the precise control of charged-particle beams from his work with his invention, the cyclotron, Lawrence suspected that the air molecules in the vacuum chamber would neutralize the ions, and create a focused beam. Oliphant inspired Lawrence to convert his old cyclotron into a giant mass spectrometer for isotope separation.
File:Calutron emitter.jpg|thumb|alt=Four men in suits bend over a piece of machinery.|Frank Oppenheimer and Robert Thornton examine the 4-source emitter for the improved Alpha calutron.
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 was 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 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 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.
Research
While the process had been demonstrated to work, considerable effort 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 large cyclotron under construction at Berkeley, one with a 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, whence the temperature could be regulated, the position of the electrodes adjusted, and even components replaced 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 minimized 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 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 ion beam. Work would continue on the shims through 1943. The main calutron patents were Methods of and apparatus for separating materials, Magnetic shims, and Calutron system.
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 usage 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" that condensed inside the vacuum tank. A particular problem was blockage of the slits by "crud", which caused the ion beams to lose focus, or stop entirely.
The chemists had to find a way of producing quantities of uranium tetrachloride from uranium oxide. Initially, they produced it by using hydrogen to reduce uranium trioxide to uranium dioxide, which was then reacted with carbon tetrachloride 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 and phosgene. 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. The presence of phosgene, a lethal gas responsible for 85,000 deaths as a chemical weapon during World War I, required that the chemists wear gas masks when handling it.
Of the $19.6 million spent on research and development of the electromagnetic process, $18 million was spent at the Radiation Laboratory in Berkeley, and further work conducted at Brown University, Johns Hopkins University and Purdue University, and by the Tennessee Eastman corporation. During 1943, the emphasis shifted from research to development, engineering, and the training of workers to operate the production facilities at the Clinton Engineer Works in Oak Ridge, Tennessee. By the middle of 1944, there were nearly 1,200 people working at the Radiation Laboratory.