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Frisch, Otto Robert

uranium reaction chain confirmed

(1904–79) Austrian–British physicist: early investigator of nuclear fission of uranium.

Frisch studied physics in his home city, Vienna, and then took a job at the national physical laboratory in Berlin. In 1930 he went to Hamburg to work with , but he was sacked in 1933 as a result of the Nazi anti-Jewish laws and worked for a year in London and then in Copenhagen (with ) until the Second World War. At Christmas 1938 he visited his aunt, , then a refugee in Stockholm and a former co-worker with in Berlin. She had a letter from Hahn reporting that uranium nuclei bombarded with neutrons gave barium. Frisch and Meitner walked in the snow and talked about it ‘and gradually the idea took shape that this was no chipping or cracking of the nucleus but rather a process to be explained by Bohr’s idea that the nucleus was like a liquid drop; such a drop might elongate and divide itself’. This division would give lighter elements such as barium, and more neutrons, so a chain reaction should occur. Frisch worked out that this should be easiest for heavy nuclei such as uranium; and Meitner calculated that it would release much energy (about 200 MeV). Back in Copenhagen, Frisch confirmed the energy of the fragments in experiments in an ionization chamber. He named the effect ‘nuclear fission’. Working in Birmingham from 1939, he and confirmed Bohr’s view that a chain reaction should occur more readily with the rare isotope uranium-235, rather than the common uranium-238. They also calculated that the chain reaction would proceed with huge explosive force even with a few kilograms of uranium. If an atomic bomb based on this was made in Germany it would clearly decide the war, and they wrote to the British scientific adviser on this; their letter probably spurred government to action, and soon Frisch was working at Los Alamos on the A-bomb project, which reached success in 1945.

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