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Lise Meitner

Lise Meitner, forever linked in people's minds with the monumental discovery of nuclear fission, made many significant contributions to science throughout a long and productive career. Upon receiving a doctorate in physics in 1906, Meitner went to the University of Berlin where she began her collaborations with Otto Hahn. The first significant result of this collaboration was an important technique for purifying radioactive material that took advantage of the recoil energy of atoms produced in alpha decay. Later, at the Kaiser Wilhelm Institute in Austria, she was the first to explain how conversion electrons were produced when gamma ray energy was used to eject orbital electrons. She also provided the first description of the origin of auger electrons, i.e., outer-shell orbital electrons ejected from the atom when they absorbed the energy released by other electrons falling to lower energies. When Nazi Germany annexed Austria in 1938, Meitner, a Jew, fled to Sweden. In her absence, Hahn and Fritz Strassmann continued experiments they had begun earlier with Meitner and demonstrated that barium was produced when a uranium nucleus was struck by neutrons. This was absolutely startling because barium is so much smaller than uranium! Hahn wrote to Meitner, "it [uranium] can't really break up into barium . . . try to think of some other possible explanation." While visiting her nephew Otto Frisch for the Christmas holidays in Denmark, she and Frisch proved that a splitting of the uranium atom was energetically feasible. They employed Niels Bohr's model of the nucleus to envision the neutron inducing oscillations in the uranium nucleus. Occasionally the oscillating nucleus would stretch out into the shape of a dumbbell. Sometimes, the repulsive forces between the protons in the two bulbous ends would cause the narrow waist joining them to pinch off and leave two nuclei where before there had been one. Meitner and Frisch described the process in a landmark letter to the journal Nature with a term borrowed from biology: fission.

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