contaminations of graphite after a first failed test and consequently ensured the next test would be run with one entirely devoid of them. As it turned out, both boron and cadmium were strong neutron poisons. In 1943, CP-1 was moved to Red Gate Woods, and reconfigured to become Chicago Pile-2 (CP-2). There, it was operated for research until 1954, when it was dismantled and buried. The stands at Stagg Field were demolished in August 1957; the site is now a National Historic Landmark and a Chicago Landmark. Origins The idea of a chemical chain reaction was first suggested in 1913 by the German chemist Max Bodenstein for a situation in which two molecules react to form not just the final reaction products, but also some unstable molecules that can further react with the original substances to cause more to react. The concept of a nuclear chain reaction was first hypothesized by the Hungarian scientist Leo Szilard on 12 September 1933. Szilard realized that if a nuclear reaction produced neutrons or dineutrons, which then caused further nuclear reactions, the process might be self-perpetuating. Szilard proposed using mixtures of lighter known isotopes which produced neutrons in copious amounts, and also entertained the possibility of using uranium as a fuel. He filed a patent for his idea of a simple nuclear reactor the following year. The discovery of nuclear fission by German chemists Otto Hahn and Fritz Strassmann in 1938, and its theoretical explanation (and naming) by their collaborators Lise Meitner and Otto Frisch, opened up the possibility of creating a nuclear chain reaction with uranium, but initial experiments were unsuccessful. In order for a chain reaction to occur, fissioning uranium atoms had to emit additional neutrons to keep the reaction going. At Columbia University in New York, Italian physicist, Enrico Fermi, with Americans John Dunning, Herbert L. Anderson, Eugene T. Booth, G. Norris Glasoe, and Francis G. Slack conducted the first nuclear fission experiment in the United States on 25 January 1939. Subsequent work confirmed that fast neutrons were indeed produced by fission. Szilard obtained permission from the head of the Physics Department at Columbia, George B. Pegram, to use a laboratory for three months, and persuaded Walter Zinn to become his collaborator. They conducted a simple experiment on the seventh floor of Pupin Hall at Columbia, using a radium-beryllium source to bombard uranium with neutrons. They discovered significant neutron multiplication in