Hiroshima_ The World's Bomb - Andrew J. Rotter [55]
In Great Britain, there was also growing interest in an atomic bomb. Unlike the United States, Britain was from the beginning of the Second World War on the front line, and its refugee scientists had in their new government an ally in their urgency to beat the Germans in the nuclear weapons race. Otto Frisch, the nephew of Lise Meitner who had, with his aunt, worked through the implications of the Hahn-Strassmann fission research in late 1938, was in Birmingham when war broke out, and, rather than return to Copenhagen where he was now based, he decided to remain in Birmingham to work with his fellow refugee Rudolf Peierls. Initially skeptical that a chain reaction could be harnessed for a bomb—it would be ‘prohibitively expensive’ and probably ineffective, they thought—the scientists changed their minds as they contemplated using not a compound of uranium 235 and 238 but pure 235 at the core of the bomb. Early in 1940 Frisch and Peierls produced a three-page memorandum that laid out, more logically and bluntly than any single document previously written, how to go about building what they called a ‘Super-Bomb’. Using U-235 exclusively would mean not having to slow down neutrons, allowing fission to take, as it were, its natural course, and rapidly releasing an enormous amount of energy. They suggested fabricating a bifurcated uranium sphere, its halves to be thrown together at great speed to produce an explosion. In three paragraphs, Frisch and Peierls indicated that thermal diffusion, filtering a gaseous uranium compound through a long series of ‘separating units’, should produce enough U-235 for an atomic bomb.
The memo ended with a remarkably prescient warning concerning the dispersal of radiation from the bomb:
Most of it will probably be blown into the air and carried away by the wind. This cloud of radioactive material will kill everybody within a strip estimated to be several miles long. If it rained the danger would become even worse because active material would be carried down to the ground and stick to it, and persons entering the contaminated area would be subjected to dangerous radiations even after days. If 1% of the active material sticks to the debris in the vicinity of the explosion and if the debris is spread over an area of, say, a square mile, any person entering this area would be in serious danger, even several days after the explosion.
Frisch and Peierls added that radiation exposure would not be felt immediately by those subject to it. Like poison gas, it was an insidious killer.39
The Frisch-Peierls memorandum, as Margaret Gowing has pointed out, asked (and answered) the right questions. The Japanese and Germans never properly asked them. The Americans had not yet asked them, though the Hungarians in the United States, backed by Einstein, had started to do so. The memorandum made its way through the physics community in Britain during early 1940, as the blitzkrieg paused, ominously; ‘interest about the uranium bomb which had been waning now waxed rapidly,’ according to Gowing. As in the United States, a committee was formed to consider the feasibility of building a bomb. It included, among others, G. P. Thomson (its chair), whose request for a ton of uranium oxide the year before had stirred the curiosity of Henry Tizard,