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Egon Bretscher (1901–1973) was a Swiss physicist.
Born near Zurich, Switzerland and educated at the ETH there, Bretscher gained a PhD degree in organic chemistry at Edinburgh in 1926. He returned to Zurich as privat docent to Peter Debye, later moving in 1936 to work in Rutherford’s laboratory at the Cavendish in Cambridge as a Rockefeller Scholar. Here he switched to research in nuclear physics, proposing (with Norman Feather) in 1940 that the 239 isotope of element 94 could be produced from the common isotope of uranium-238 by neutron capture and that, like U-235, this should be able to sustain a nuclear chain reaction. A similar conclusion was independently arrived at by Edwin McMillan and Philip Abelson at Berkeley Radiation Laboratory. In addition, he devised theoretical chemical procedures for purifying this unknown element away from the parent uranium; this element was named Plutonium by Nicholas Kemmer. In 1944 he became a part of the British Mission to the Manhattan Project in Los Alamos, New Mexico led by James Chadwick, where he made the first measurements on the energy released in fusion processes. His contributions up to 1945 are discussed by Margaret Gowing in her "Britain and Atomic Energy, 1935-1945", published in 1964. During his time in Los Alamos, he took many Kodachrome slides which appear to constitute a unique coloured record of that research site. His pictures, which are now held by the Churchill Archives Centre, include photographs of Enrico Fermi, Edward Teller and the Trinity site in New Mexico after the first atomic bomb was detonated, showing the surface light brown sand turned to a green-blue glass.
In 1947 he was invited by John Cockcroft to head the Chemistry Division at the newly established Atomic Energy Research Establishment at Harwell, Oxfordshire, England and in 1948 succeeded Otto Frisch as head of the Nuclear Physics Division there. Amongst his colleagues were Bruno Pontecorvo (in the Nuclear Physics Division) and Klaus Fuchs (head of the Theoretical Physics Division). He was awarded a CBE on retirement from Harwell and died in Switzerland in 1973.
He used to joke that his main contribution to physics occurred in the summer of 1930, when he was climbing in the Bergel region near Engadin with another student, Felix Bloch, in the Swiss Alps. Bloch slipped over an icy edge but was saved, as he fell, by the rope joining him to Bretscher. The latter's swift action in driving his ice axe into the ice prevented their combined demise. After raising the alarm, Bretscher returned with a guide and spent the night with Bloch discussing physics. It took guides a further three days to bring Bloch down. Bloch later won the Nobel Prize for physics for his discovery of nuclear magnetic resonance.