John Dunning - American Physicist
(1907-1975)
March 1940
Dunning at Columbia bombards with neutrons a small sample of U-235 that had been separated by Alfred Nier at the University of Minnesota. Dunning shows that it was indeed the U-235 isotope that was responsible for the slow neutron fission of uranium, just as Bohr and Wheeler had hypothesied.

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Alfred Nier at his mass spectrograph which he used to separate a sample of U-235. Courtesy of Ardis Nier

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