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Summary Article: Hahn, Otto (1879-1968)
From The Hutchinson Dictionary of Scientific Biography

Place: Germany

Subject: biography, physics

German radiochemist who discovered nuclear fission. For this achievement he was awarded the 1944 Nobel Prize for Chemistry.

Hahn was born on 8 March 1879 in Frankfurt am Main and studied at the University of Marburg, gaining his doctorate at Marburg in 1901. From 1904 to 1905 he worked in London under William Ramsay, who introduced Hahn to radiochemistry. In 1905 he moved to McGill University at Montréal, Canada, to further these studies with Ernest Rutherford. The next year Hahn returned to Germany to work under Emil Fischer at the University of Berlin. There he was joined in 1907 by Lise Meitner, beginning a collaboration that was to last more than 30 years. In 1912 Hahn was made a member of the Kaiser-Wilhelm Institute for Chemistry in Berlin-Dahlem and in 1928 was appointed its director. He held this position until 1944, when the Max Planck Institute in Göttingen took over the functions of the Kaiser-Wilhelm Institute and Hahn became president.

Perturbed by the military uses to which nuclear fission had been put, Hahn was concerned in his later years to warn the world of the great dangers of nuclear weapons. In 1966 he was co-winner of the Enrico Fermi Award with Meitner and Fritz Strassmann (1902-1980). Hahn died in Göttingen on 28 July 1968.

Hahn's first important piece of research was his discovery of some of the intermediate radioactive isotopes formed in the radioactive breakdown of thorium. These were radiothorium (discovered in 1904) and mesothorium (1907). In 1918 Hahn and Meitner discovered the longest-lived isotope of a new element, which they called protactinium, and in 1921 they discovered nuclear isomers - radioisotopes with nuclei containing the same subatomic particles but differing in energy content and half-life.

In the 1930s, Enrico Fermi and his collaborators showed that nearly every element in the periodic table may undergo a nuclear transformation when bombarded by neutrons. In many cases, radioactive isotopes of the elements are formed and slow neutrons were found to be particularly effective in producing many of these transformations. In 1934 Fermi found that uranium is among the elements in which neutron bombardment induces transformations. The identity of the products was uncertain, but it was suspected that artificial elements higher than uranium had been found.

Hahn, Meitner, and Strassmann investigated Fermi's work, believing the formation of radium to have occurred, and treated the bombarded uranium with barium. Radium is chemically very similar to barium and would accompany barium in any chemical reaction, but no radium could be found in the barium-treated fractions. In 1938 Meitner, being Jewish, fled from Germany to escape the Nazis, and Hahn and Strassmann carried on the search. Hahn began to wonder whether radioactive barium was being formed from the uranium when it was bombarded with neutrons, and that this was being carried down by the barium he had added. The atomic number of barium is so much lower than that of uranium, however, that the only way it could possibly have been formed was if the uranium atoms broke in half. The idea of this happening seemed so ridiculous that Hahn hesitated to publish it.

Meitner was now in exile in Copenhagen and with Otto Frisch quickly confirmed Hahn's explanation, showing the reaction to be an entirely new type of nuclear process and suggesting the name nuclear fission for it. Meitner and Frisch communicated the significance of this discovery to Niels Bohr, who was preparing to visit the USA. Arriving in January 1939, Bohr discussed these results with Albert Einstein and others. The presence of barium meant that uranium had been split into two nearly equal fragments, a tremendous jump in transmutation over all previous reactions. Calculations showed that such a reaction should yield 10-100 times the energy of less violent nuclear reactions. This was quickly confirmed by experiment, and later the same year it was discovered that neutrons emitted in fission could cause a chain reaction to occur.

Fearing that Nazi Germany would put Hahn's discovery to use and build an atomic bomb, Einstein communicated with the US president and urged that a programme be set up to achieve a nuclear chain reaction. Fermi subsequently built the first atomic reactor in 1942 and the first atomic bomb was exploded in 1945, bringing World War II to a close. Hahn had in fact not worked on the production of nuclear energy for Germany, and the German effort to build a reactor and a bomb had not got anywhere.

Otto Hahn made a discovery of enormous importance for the human race for, as well as providing a new and immense source of power, it has given us enormous destructive ability, as he was well aware. He is commemorated in science in the naming of element 105 as hahnium.

© RM, 2018. All rights reserved.

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