Calutron - Calutrons and The Manhattan Project - Uranium Isotope Separation

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. But 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 percent-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. 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.