Hiroshima_ The World's Bomb - Andrew J. Rotter [52]
The Alsos mission revealed perhaps the single most important reason why the Germans failed to build an atomic bomb. For all the manifest brilliance of Heisenberg and his fellow scientists, and notwithstanding the limits of German resources and heightened pace of Allied attacks on German facilities after 1942, the Germans lost the first nuclear arms race because they did not fully grasp the science and technology required to build an atomic bomb. It was Heisenberg, the most eminent of the atomic scientists, who made two fundamental miscalculations. First, misunderstanding the fission process, he dramatically overestimated the amount of enriched U-235 needed to sustain a chain reaction, believing it to be a ton or several tons, rather than the 56 kilograms actually needed. As Jeremy Bernstein has demonstrated, even after the Farm Hall Germans got word of the Hiroshima bombing, Heisenberg failed to understand the physics of U-235. Attempts to refine enough uranium to produce its reactable form in the amount Heisenberg thought necessary proved time-consuming and frustrating. Second, the equally frustrating pursuit of many gallons of heavy water was the result of Heisenberg’s belief that it was the only possible moderator of a nuclear chain reaction. The Germans had tried experiments using graphite as a moderator; these had proved unavailing. But this was because the Germans had used industrial graphite contaminated with boron, a substance that, as Bernstein puts it, ‘soaks up neutrons like a sponge’. The Allies would understand the problem and demand pure graphite from their manufacturers. It was graphite that worked perfectly as a moderator in the atomic pile superintended by Enrico Fermi in a University of Chicago squash court in 1942.32
German scientists got a good deal of the bomb’s physics right: they experimented, for example, with creating a transuranic element that might be easier to use for a chain reaction than U-235 (plutonium, element 93, would be the basis for the Nagasaki bomb), seemed at times to grasp the proper scale of the bomb (Heisenberg may have told Albert Speer and others, in June 1942, that a bomb the size of a pineapple would be sufficient to destroy a city), and appeared to understand the difference between running a reactor and constructing a bomb (principally the speed of the chain reaction). But the miscalculation of the chain reaction’s critical mass and mistakes made in choosing a moderator for the reaction fatally undermined the Germans’ bomb project. These errors offer the simplest, and in this case the best, explanation of the German failure.
There is one thing more to be said. The mistakes of the German nuclear physicists during the war were in part the result of the enforced insularity of German physicists, pariahs to most of the rest of the world by their association with a murderous aggressor state. The Germans had left, and had been expelled from, the republic of science. They were expelled, wrote the American physicist Philip Morrison after the war, because, unlike those who planned and built the American bomb, ‘they worked for the cause of Himmler and Auschwitz, for the burners of books and the takers of hostages’. Brilliant as they were, Heisenberg, Diebner, Hahn, and the others were trapped inside a hardened, darkened bubble, unable to see or hear what was going on in the scientific community outside Germany, beguiled by the echoes of their own voices bounced back at them by the bubble’s inner surface. The community’s self-reflexiveness was made worse by the rivalries within it. Heisenberg,