The distinction between U235 and U238 had already fired a debate. “Fermi and a number of others,” says John Dunning, “had considerable doubts about U-235 or even disagreed — they thought it was U-238 [that was responsible for slow-neutron fission].” The disagreement incensed Bohr, who told Leon Rosenfeld he was “outraged” that Fermi should question the logic of his argument that thorium and U238 stood on one side and U235 on the other. “It was both the strength and the weakness of Fermi,” writes Rosenfeld, “to be so intent on following his own lines of thought that he was impervious to any outside influence…. He fancied there could be a different interpretation of the evidence discussed by Bohr, and that only experiment could decide.” Dunning, on the other hand, “immediately accepted Bohr's argument.” The important outcome was that Dunning began to think of isotope separation, while Fermi continued to pursue the possibility of a chain reaction in natural uranium. With unusual and uncharacteristically Fermian conservatism, so did Szilard.
5
Compton's memory errs toward more optimism than Fermi's calculations warranted. After Compton's visit Gregory Breit, Briggs' theoretician on the Uranium Committee, asked Fermi to work his formulae on paper. Fermi was busy with his uranium-graphite experiment and produced, on October 6, a sketchy set of notes. He guessed at the cross sections and came up with 130,000 grams — 287 pounds. “One cannot,” he added, “in my opinion, exclude the possibility that [the critical mass] may be as low as 20,000 grams [44 pounds] or as high as one or more tons.”
6
The proximity fuse was a miniature radar unit shaped to replace the ballistic nose of anti-aircraft shells. It sensed its proximity to a target — an enemy plane — and exploded the shell it rode at a preset range, often turning a miss into a kill. Its development was another of Bush's responsibilities and it was one of science's most important contributions to the war. Merle Tuve, Richard Roberts and most of the physics team at the Department of Terrestrial Magnetism of the Carnegie Institution had turned from fission research in August 1940 to develop it.
7
Spontaneous fission, a relatively rare nuclear event, differs from fission caused by neutron bombardment; it occurs without outside stimulus as a natural consequence of the instability of heavy nuclei.
8
A betatron accelerates electrons to high speeds in a magnetic field; such beta ray-like electrons can then be directed onto a target to produce intense beams of high-energy X rays.
9
“The glory is departed.”
10
In modern “dry” bombs, by making tritium from lithium6 deuteride.
