| abstract
| - The discovery of nuclear fission by Lise Meitner and Otto Hahn in December 1938 generated intense interest among physicists. The news was brought to the United States by Danish physicist Niels Bohr, who opened the Fifth Washington Conference on Theoretical Physics with Enrico Fermi on January 26, 1939. The results were quickly corroborated by experimental physicists, most notably Fermi and John R. Dunning at Columbia University. The Hungarian physicist Leó Szilárd, who was residing in the United States at the time, realized that the neutron-driven fission of heavy atoms could be used to create a nuclear chain reaction that could yield vast amounts of energy for electric power generation or atomic bombs. Such a reaction using neutrons was an idea he had first formulated in 1933, upon reading Ernest Rutherford's disparaging remarks about generating power from his team's 1932 experiment using protons to split lithium. However, Szilárd had not been able to achieve a neutron-driven chain reaction with neutron-rich light atoms. In theory, if in a neutron-driven chain reaction the number of secondary neutrons produced was greater than one, then each such reaction could trigger multiple additional reactions, producing an exponentially increasing number of reactions. Szilárd teamed up with Fermi to build a nuclear reactor from natural uranium at Columbia University, where they were fortunate in having a sympathetic head of the physics department in George B. Pegram. At the time there was disagreement about whether it was uranium-235, which made up less than 1% of natural uranium, or, as Fermi maintained, the more abundant uranium-238 isotope that was primarily responsible for fission. Fermi and Szilárd conducted a series of experiments, and concluded that a chain reaction in natural uranium could be possible if they could find a suitable neutron moderator. They found that the hydrogen atoms in water was good at slowing neutrons, but tended to capture them. Szilárd then suggested using carbon as a moderator. They now needed large quantities of carbon and uranium to create a reactor. Szilárd was convinced that they would succeed if only they could get the materials. Szilárd was concerned that German scientists might also attempt this experiment. The German nuclear physicist Siegfried Flügge published two influential articles on the exploitation of nuclear energy in 1939. After discussing this prospect with fellow Hungarian physicist Eugene Wigner, they decided that they should warn the Belgians, as the Belgian Congo was the best source of uranium ore. Wigner suggested that Albert Einstein might be a suitable person to do this, as he knew the Belgian Royal Family. The connection between Einstein and Szilárd pre-dated the letter. They had known each other in Berlin in the early 1920s. The so-called Einstein-Szilárd refrigerator, an absorption refrigerator with no moving parts that runs at constant pressure and requires a heat source to operate, was jointly invented in 1926 by Einstein and Szilárd and patented in the US on November 11, 1930. The invention of freon as a fridge coolant limited the commercial success of this kind of refrigeration process.
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