Bohr also considered U235’s behavior under fast-neutron bombardment. “For fast neutrons,” he wrote near the end of the paper, “ . . . because of the scarcity of the isotope concerned, the fission yields will be much smaller than those obtained from neutron impacts on the abundant isotope.”1098 The statement implies but does not ask a pregnant question: what would the yields be for fast neutrons if U235 could be separated from U238?
