The Making of the Atomic Bomb: 25th Anniversary Edition

by Richard Rhodes

In December at this Pajarito Canyon field station Segrè made a significant discovery. The spontaneous fission rate for natural uranium was much the same at the field station as at Berkeley, but at the field station the rate was seemingly higher for U235. Segrè deduced that cosmic-ray neutrons, which were usually too slow to fission U238 but effective to fission U235, caused the difference. Cosmic rays batter neutrons from the upper reaches of the atmosphere and the field station was 7,300 feet nearer that region than was sea-level Berkeley. Shield out such stray neutrons and the U235 bomb core could be purified less rigorously than they had assumed. Predetonation would be less likely: the gun that assembled the U235 to critical mass would need less muzzle velocity and could be significantly shorter and lighter. Thus was Little Boy engendered, Thin Man’s modest brother, a gun assembly six feet long instead of seventeen that would weigh less than 10,000 pounds, an easy load for a B-29: in a log cabin in a grove beyond fields of bright asters, up a trail visited by rattlesnakes.