Tinn. Background Enrico Fermi and his colleagues studied the results of bombarding uranium with neutrons in 1934. That year, Ida Noddack first mentioned the concept of nuclear fission. In December 1938, four years after the Fermi publication, Lise Meitner and Otto Robert Frisch correctly interpreted Otto Hahn and Fritz Strassmann's radiochemical experimental results as evidence of nuclear fission. News of the discovery spread quickly among physicists and it was realized that if chain reactions could be controlled, fission might be a new source of great power. What was needed was a substance which could moderate the energy of the secondary neutrons emitted by fission, so they could be captured by other fissile nuclei. Heavy water and graphite were the prime candidates for moderating neutron energy. When Nazi Germany investigated the production of an atomic bomb, a range of options was identified. Although historical records provide limited detail on the German decision to pursue the heavy water approach, it became clear after the war that they had explored that option. Although ultimately unsuccessful, the approach chosen has been demonstrated as technically viable. Plutonium-239 (239Pu) makes effective weapons material, although requiring an implosion-type mechanism as a simpler Thin Man gun-type bomb is not feasible. Heavy water has been demonstrated as an effective moderator for 239Pu production, and may be separated from ordinary water by electrolysis. The German program had already been handicapped by the Nazi purging of German Jewish physicists and the conscription of others and ended in the autumn of 1942. Approaches to developing a weapon In nuclear-weapon development, the main problem is securing sufficient weapons-grade material; it is particularly difficult to acquire fissile isotopes of uranium-235 (235U) or 239Pu. Weapons-grade uranium requires mining, extracting and enriching natural ore. Plutonium can be "bred" in reactors fueled by unenriched uranium, which requires chemical separation of the 239Pu produced. Plutonium production Although the most common isotope of uranium, uranium-238 (238U), can be used as secondary fissile material in hydrogen (fusion) bombs, it cannot be used as the primary fissile material for an atomic (fission-only) bomb. 238U can be used to produce 239Pu through the fission of 235U, which produces neutrons (some of which will be absorbed by 238U, creating 239U). The 239U will decay after a few days, turning into weapons-usable 239Pu. The Germans found that a chain reaction could not be sustained if graphite was used as a moderator, and abandoned it.