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Hahn was born in Frankfurt-am-Main, Germany, on March 8, 1879. He studied chemistry at the universities of Munich and Marburg, earning his Ph.D. from Marlburg in 1901. After graduation, he worked successively at the Chemical Institute in Marburg (1902-1903), with William Ramsay at University College, London (1904-1905), and with Rutherford at McGill University in Montreal (1905-1906). In 1906 he joined the Chemical Institute at the University of Berlin. During World War I, he served in the German army.
Hahn's earliest research concerned radioactive elements. He analyzed some of the members of the thorium radioactive family, identifying some of the isotopes that make up that family. In 1918, he discovered the element protactinium with his long-time collaborator, Lise Meitner. Three years later, the two also identified the existence of "nuclear isotopes," atoms that have identical numbers of protons and neutrons in their nuclei, but that decay by different mechanisms.
The achievement for which Hahn is best known, however, is his discovery of nuclear fission. The discovery of the neutron by James Chadwick in 1932 provided scientists with a new particle to use in nuclear reactions. The neutron has the advantage that it carries no electrical charge and is not, therefore, repelled by the atomic nucleus. Researchers quickly began to study nuclear reactions initiated by neutron bombardment.
One of the most successful of these researchers was Enrico Fermi, who was able to convert a number of stable isotopes to radioactive isotopes by means of neutron bombardment. One experiment in which Fermi was especially interested was the bombardment of uranium by neutrons. Fermi suspected that this reaction might result in the formation of an element heavier than uranium, a transuranium element which did not exist in nature.
The results of this experiment turned out to be very complex, however. Although evidence for the existence of element 93 was obtained, it was obvious that other changes had occurred also. It fell to Hahn and his colleagues to explain fully Fermi's results.
Between 1934 and 1938, Hahn and Meitner attempted to unravel Fermi 's results. After Meitner fled Germany in 1938, Fritz Strassman took her place in the research. The most confusing point about Fermi's results was the existence of a radioactive isotope that appeared to be an isotope of either radium or barium. All previous experience suggested that it should be a radium isotope. All nuclear reactions studied to that point in time involved the conversion of one isotope into another isotope of the same element, or one close to it in the periodic table. Radium is only four places from uranium in the table, while barium is 36 places distant.
The isotope in question simply should not have behaved like barium, but it did. By 1938, Hahn and Strassman were ready to admit that the bombardment of uranium by neutrons produced, as one of its products, an isotope of barium. But they were reluctant to announce this conclusion. The splitting of a nucleus, such as the conclusion implied, was chemical heresy.
While Hahn and Strassman hesitated about publishing their " heresy," their former colleague, Meitner, and her nephew Otto Frisch published their explanation for the process of nuclear fission. Within a month of the January 1939 Meitner-Frisch paper, Hahn and Strassman followed with their own report of experiments that showed beyond doubt that fission had occurred. For his research on nuclear fission, Hahn was awarded the Nobel Prize of chemistry in 1944.
Hahn remained in Germany during World War II. Since the German government was never convinced of the potential value of nuclear energy for weapons development, there was no effort similar to the Manhattan Project in the United States. Hahn first heard about the use of nuclear weapons against Japan while he was being held by Allied troops after the conquest of Germany. He was so distressed by the news that he briefly considered committing suicide. He never changed his mind about nuclear weapons and refused to participate in West Germany's plans to build an atomic bomb in the late 1950s.
Hahn served as president of the Max Planck Society from 1946 until his retirement in 1960. He died in Göttingen on July 28, 1968.
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