Hahn , Otto

(1879–1968) German chemist

Hahn's father, a successful merchant in the German city of Frankfurt, was keen for his son to train as an architect and it was against much family opposition that Hahn was finally allowed to study chemistry at the University of Marburg in 1897. After obtaining his doctorate in 1901 he studied abroad, first with William Ramsay in London and then at McGill University, Canada, with Ernest Rutherford. Hahn returned to Germany in 1907, where he took up an appointment at the University of Berlin, being made professor of chemistry in 1910. Two years later he joined the Kaiser Wilhelm Institute of Chemistry where he served as director from 1928 to 1945.

Hahn had trained as an organic chemist and had really gone to London to learn English in order to prepare himself for an industrial career. Ramsay had however asked him to separate radium from some radioactive material he had recently acquired from Ceylon. In so doing Hahn found a new material, a highly active form of thorium which he named ‘radiothorium’. So impressed was Ramsay with this work that he wrote to Emil Fischer in Berlin suggesting that he employ Hahn after he had acquired more experience of radioactivity with Rutherford at McGill.

Hahn was thus diverted into an academic career, most of which was spent in research on radioactivity and much of it in collaboration with Lise Meitner. With her he discovered a new element, protactinium, in 1917. He went on to define, in 1921, the phenomenon of nuclear isomerism. This arises when nuclei with different radioactive properties turn out to be identical in atomic number and mass.

Hahn's most important work however, was done in the 1930s when, with Meitner and the German chemist Fritz Strassmann(1902––sp;–sp;), he made one of the most important discoveries of the century, namely nuclear fission. One of the strange features about Hahn's work was that he was repeating experiments already done and formulating hypotheses already rejected as nonsense or due to some contamination of the materials used. Chemists at this time felt that they understood the process of nuclear transformation. After all it was some twenty years since Rutherford had first detected the transformation of nitrogen into oxygen, and a newer form of the same ‘alchemy’ had been described by Irène and Frédéric Joliot-Curie in 1934. Two basic rules were involved in this understanding. First, that nuclear transmutations always involved the emission of either an alpha particle (helium nucleus) or a beta particle (electron); and secondly, that the change could take place only between elements separated by no more than two places in the periodic table. If more substantial transformations appeared to occur, as in the transformation of uranium into lead, this was explained as the result of a series of such intermediate steps, each one taking place with the emission of the appropriate particle.

Thus when in 1938 Hahn bombarded uranium with slow neutrons and detected some strange new half-lives, he assumed that the uranium had changed into radium, a close neighbor, with some undetected alpha particles. But when he tried to remove the radium all he could find was barium. This Hahn simply could not understand, for barium was far too low in the periodic table to be produced by the transmutation of uranium; and if the transformation was taking place it should be accompanied by the emission of a prodigious number of alpha particles, which Hahn could not have failed to detect. The thought that the heavy uranium nucleus could split into two lighter ones was too outrageous for him to consider seriously. He could not dismiss it entirely for he asserted at the time that “we must really state that we are not dealing with radium but with barium.” But to suppose the barium arose from what he then called nuclear ‘bursting’ he felt would be “in contradiction to all previous experience in nuclear physics.” He did realize that something of importance was going on and quickly sent off for publication a joint paper with Strassmann even though, as he recalled twenty years later, “After the manuscript had been mailed, the whole thing once more seemed so improbable to me that I wished I could get the documents back out of the mail.” Appropriately enough it was his old collaborator Meitner, in exile from the Nazis in Sweden, and her nephew Otto Frisch, who made the necessary calculations and announced fission to the world early in 1939. Hahn received the Nobel Prize for chemistry in 1944.