The Making of the Atomic Bomb: 25th Anniversary Edition

by Richard Rhodes

In 1935, using a more powerful instrument, physicist Arthur Jeffrey Dempster of the University of Chicago detected a second, lighter isotope. “It was found,” Dempster announced in a lecture, “that a few seconds’ exposure was sufficient for the main component at 238 reported by Dr. Aston, but on long exposures a faint companion of mass number 235 was also present.”1091 Three years later a gifted Harvard postdoctoral fellow named Alfred Otto Carl Nier, the son of working-class German emigrants to Minnesota, measured the ratio of U235 to U238 in natural uranium as 1:139, which meant that U235 was present to the extent of about 0.7 percent.1092 By contrast, thorium in its natural form is essentially all one isotope, Th232. And that natural difference in the composition of the two elements was the clue that set Bohr off.