The Making of the Atomic Bomb: 25th Anniversary Edition

by Richard Rhodes

Frisch wanted to work with full critical masses to determine by experiment what Los Alamos had so far been able to determine only theoretically: how much uranium Little Boy would need. Hence his daring proposal: The idea was that the compound of uranium-235, which by then had arrived on the site, enough to make an explosive device, should indeed be assembled to make one, but leaving a big hole so that the central portion was missing; that would allow enough neutrons to escape so that no chain reaction could develop.2263 But the missing portion was to be made, ready to be dropped through the hole so that for a split second there was the condition for an atomic explosion, although only barely so. Brilliant young Richard Feynman laughed when he heard Frisch’s plan and named it: he said it would be like tickling the tail of a sleeping dragon.2264 The Dragon experiment it became. At a remote laboratory site in Omega Canyon that Fermi also used, Frisch’s group built a ten-foot iron frame, the “guillotine,” that supported upright aluminum guides. The experimenters surrounded the guides at table level with blocks of uranium hydride. To the top of the guillotine they raised a hydride core slug about two by six inches in size. It would fall under the influence of gravity, accelerating at 32 feet per second/per second. When it passed between the blocks it would momentarily form a critical mass. Mixed with hydride, the U235 would react much more slowly than pure metal would react late...