A Calutron was a mass spectrometer used for separating the isotopes of uranium 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 were implemented for industrial scale uranium enrichment at the Oak Ridge, Tennessee Y-12 plant established during the war and provided much of the uranium used for the "Little Boy" nuclear weapon, which was dropped onto Hiroshima in 1945.

In a mass spectrometer, a vaporised quantity of a sample is bombarded with high energy electrons which causes them to become positively charged ions. They are then accelerated and subsequently deflected by magnetic fields. They then collide with a plate, producing a measurable electric current. The mass of the ions can be calculated according to the strength of the field and the charge of the ions.

To maximize the separation and the use of the required large electromagnet, multiple Calutrons were arranged around the magnet in a massive oval, which resembled (and were called) race tracks. Two types of Calutrons were created, known as Alpha and Beta, as the technology was improved. Magnetic separation was later abandoned in favor of the more complicated, but more effective, gaseous diffusion method. Due to the copper shortage during WWII, the electromagnets were made from thousands of tons of silver borrowed from the U.S. Treasury.

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. In principle the technique 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. But Lawrence, who had seen a line of positively charged ions pour from his cyclotron, guessed that negative particles formed in the air kept the beam from dispersing under its own electrical influence. He 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 to be sure, but enough to assure Lawrence that space charge would not be a formidable problem.

The fact that beams of uranium ions could be defined 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 percent-between uraniums 235 and 238. Because the lighter ions respond slightly more readily 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 relatively more plentiful on the inside than on the outside of the beam. But 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 small, could melt the collectors during long hours of operation; the staff therefore installed water cooling for the collectors and tank liner. They contrived electric arcs to ionize the uranium tetrachloride feed. They devised ways 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. They made scrapers to clean the exit slits of the feed sources regularly lest the accumulated "crud" cut down 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. But in mid-1942 no reactor worked, and the calutron did.


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; from the XA and a scale model of the production magnet alone would come procedures for alpha operation at Oak Ridge. Tests of the first, full-scale system installed there, the XAX, were scheduled for July.


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 drafted 14,700 tons (13,300 tonne) of pure silver from a government vault for the purpose. 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 beta calutrons


The gaseous diffusion procedure for separation of uranium isotopes, which had consumed more money even 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 slightly enriched uranium and processed through beta calutrons. To augment the calutron feed, the Manhattan Engineering District constructed still another plant at Oak Ridge, this one working by thermal diffusion, a method developed by Philip Abelson.

Weapons grade uranium


Modern calutrons


Calutron patents

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).

See also


External links

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