Otto Hahn
1879-1968
German Chemist
Often regarded as the leading nuclear and radiochemical experimentalist of the twentieth century, Otto Hahn won the 1944 Nobel Prize for his discovery of nuclear fission. Widely respected for both his scientific research and personal integrity, he also played a leading role in reestablishing scientific research in Germany following the destruction of World War II.
Hahn was one of four children born to a professional glazier, and was initially attracted to organic chemistry in college, taking his doctorate at Marburg University in 1901 under Theodor Zincke. After a year of infantry service, he returned to work as Zincke's assistant in 1903. Dissuading Hahn from his intention to work in industry, in 1904 Zincke obtained a position for him as a research assistant in London with William Ramsay (1852-1916), where heisolated radiothorium, a radioactive isotope of thorium, by chemical analysis of a radioactive mineral blend. In 1905 Hahn left to spend a year with Ernest Rutherford (1871-1937) at McGill University in Montreal, where he repeated his success under Ramsay by discovering radioactinium.
Otto Hahn. (Library of Congress. Reproduced with permission.)In 1906 Hahn returned to Germany to work in the Berlin Chemical Institute headed by Emil Fischer (1852-1919), where in 1907 he was promoted to Privatdozent (non-stipendiary lecturer) and in 1910 to a professorship. Two other significant developments also occurred in 1907: Hahn identified mesothorium, an intermediate radioactive isotope between thorium and radiothorium, and he began a 30-year collaboration with the brilliant female physicist Lise Meitner (1878-1968). In 1912 Hahn and Meitner moved to positions at the newly established Kaiser Wilhelm Institute for Chemistry, for which Hahn later served as president in 1928.
At the Institute, Hahn was initially engaged in supporting the German military effort in World War I by research under Fritz Haber (1868-1934) on poisonous gases, a role he later greatly regretted. During the 1920s he studied emissions of beta particles (electrons ejected from a nuclear proton that changes into a neutron) by extremely weak radioactive substances, particularly radioisotopes of potassium and rubidium.His method of determining the production of strontium by the rate of radioactive decay and half-life of rubidium was subsequently utilized as a new method for dating geological strata and artifacts. In 1921 he also discovered "uranium-Z," the first nuclear isomer, though its nature remained unexplained for more than a decade until the discovery of the neutron and of artificially induced radioactivity by neutron bombardment in the 1930s.
Another important area of research was Hahn's development of the "emanation method" to study the character of and changes in the surfaces of finely divided solution precipitates. A radioactive "tracer" element was added to the precipitate, and the rate of diffusion was then measured by tracking the path and rate of emanation of a rare gas due to radioactive decay. The method proved particularly useful for working with minute quantities of matter insufficient for measurement by other techniques, and also provided information on temperature changes and crystal lattice structures. Hahn showed a direct correlation between the surface/volume ratio of the precipitate and the emanation rate.
During the 1930s, with the discovery of the neutron by James Chadwick (1891-1974), the development of neutron bombardment techniques by Enrico Fermi (1901-1954), and the creation of artificial radioactivity using Fermi's techniques by Frédéric Joilot-Curie (1900-1958) and Irène Joilot-Curie (1897-1956), Hahn's research interests shifted to the study of decay patterns and products of nuclear isotopes, particularly the transuranium elements, using electrolysis and precipitation techniques. His refusal to cooperate with the new Nazi regime made his position at the Institute increasingly difficult, especially after Lise Meitner, an Austrian Jew, was forced to flee to Sweden in 1938. Later that year, Hahn and his colleague Fritz Strassmann sent a letter to Meitner with the baffling report that neutron bombardment of a uranium sample had not produced radium as expected, but barium instead. Meitner supplied the correct interpretation of the result as the first observed example of nuclear fission.
During World War II, Hahn concentrated on research of fission products, compiling a table of over 100 nuclear isotopes by 1945. Captured by Allied troops and interred with other leading German scientists in England, Hahn learned of the belated award of the 1944 Nobel Prize for his work on fission, but heard with disbelief and despair the news of the dropping of atomic bombs on Hiroshima and Nagasaki. As Germany's most prestigious physical scientist who had not been involved in atomic weapons research during the war, Hahn was asked to take direction of the newly re-founded Kaiser Wilhelm Institute, renamed the Max Planck Institute in 1948. Despite his advanced years, Hahn worked energetically to reestablish scientific research in Germany, and became an outspoken opponent of nuclear weapons and a cautionary critic of nuclear power.
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