 In 1939, the Pupin Physics Laboratories at Columbia where Zinn worked were the center of intensive research into the properties of uranium and nuclear fission, which had recently been discovered by Lise Meitner, Otto Hahn and Fritz Strassmann.At Columbia, Zinn, Enrico Fermi, Herbert L. Anderson , John R. Dunning and Leo Szilard investigated whether uranium-238 fissioned with slow neutrons, as Fermi believed, or only the uranium-235 isotope, as Niels Bohr contended.Since pure uranium-235 was not available, Fermi and Szilard chose to work with natural uranium.