The Woman Who Explained Nuclear Fission – On Lise Meitner’s 50th Death Anniversary

With Donna Stricklandin 2018 only for the third time a woman was awarded the Nobel Prize in Physics, after Marie Curie in 1903 and Maria Goeppert-Mayer in 1963 (only the economists have less Nobel Prize laureates, but the Nobel Prize for Economics has only been awarded since 1969). The most well-known physicist, who remained without a Nobel Prize, and next to Marie Curie probably the most famous physicist of all times, is Lise Meitner, whose death these days marks its50thand her most significant discoverysoon its 80thanniversary.

In 1938, Lise Meitner together with her research colleague Otto Hahn and his assistant Fritz Straßmann conducted experiments with uranium nuclei. In the midst of these experiments, the originally Austrian citizen (thus protected from persecution) had to flee from Berlin as a Jew, because with the German annexation of Austria she had suddenly become German citizen and thus subject to the Nazi persecution of the Jews. With Otto Hahn’s help, she fled via Holland and Denmark to Stockholm to continue working at the Nobel Institute there. The communication with her research colleague was sustained by letters. In December 1938, she received a letter from Hahn, in which he told her that barium atoms had been produced by the bombardment of the uranium atom (92 protons) with slowed neutrons. Barium atoms, however, have an atomic number of 56! Hahn asked Meitner to think about this result. Meanwhile, Hahn did not inform the physicists in his institute about this; Lise Meitner was the only one to be updated about all the experiments and results from Berlin (here, Otto Hahn showed astonishing courage: in 1938 Germany people with far less offensesended up in a concentration camp than a German director of a scientific institutesharing an important discovery with his emigrated Jewish colleague).

Together with her nephew, the physicist Otto Frisch, Lise Meitner brooded over this result during a walk in the winter snow in Kungälv in Sweden (where both spent their Christmas holidays). Could it really be that the uranium nuclei had been split? Back at home (or as the anecdote goes on a tree trunk during their walk), she performed a calculation based on the „liquid drop model“ of Bethe and Weizsäcker. She came to the conclusion that due to the high number of repulsive protons the Uranium must be quite unstable – like a large drop of water whose surface tension is barely able to hold it together. To bomb it with a neutron could make it burst. However, there was another problem: After splitting the nucleus, due to the electrical repulsive forcesthe fragments of the nucleiare greatly accelerated. They thusobtain a lot of energy. With calculated 200 million electron volts, this is significantly larger than any energy that arises in previously known atomic (chemical) processes. Where did this enormous amount of energy come from? At this point, Einstein’s famous formula came into play: Meitner calculated that the two nuclei resulting from the split plus free neutrons that arose from it are in sum slightly lighter in weight than the original nucleus of uranium. The difference of the mass corresponded according to the formula E = mc2 exactly to the energy of 200 million electron volts! The energy of the fragments had to come directly from the mass of the uranium atomic nucleus. For the first time, a process had become known in which the equivalence of energy and mass formulated by Einstein was directly revealed. This also made it clear that the atomic nucleus of uranium can be split.And soon thereafter something else became clear (first suggested in a discussion of Frisch with the Danish physicist Christian Møller): The free neutrons can split more nuclei, which can trigger a chain reaction of nuclear fissions.

In February 1939, Lise Meitner published her resultstogether with her nephew in a paper entitledDisintegration of Uranium by Neutrons: a New Type of Nuclear Reaction. This article is the first to speak of „nuclear fission“. This news spread like wildfire in the scientific world. Already during their walk in the snow Meitner and Frisch had realized that the discovery of nuclear fission could potentially have significant social and military consequences. Shortly thereafter, Otto Frisch and his British colleague Rudolf Peierls wrote a memorandum describing the technical construction of a bomb based on nuclear fission.

This made even non-physicists sit up and take notice, as meanwhileAdolf Hitler had attacked Poland and started the Second World War. As a leading nation in research and technology, National Socialist Germany was predestined to be the first to use nuclear energy militarily and produce nuclear bombs. A bomb with such enormous explosive power in the hands of Hitler would have disastrous consequences for the world, so not only the two Jews Meitner and Frisch thought. Nevertheless, the convinced pacifist (unlike her nephew) refused to accept research assignments for the construction of an Allied nuclear bomb, although Meitner was repeatedly called upon by the USfortheir “Manhattan Project”. This did not prevent Meitner from being referred to in the American press as „Jewish mother of the atom bomb“ and „woman of the year“ a year after the atomic bombs on Hiroshima and Nagasakito her own great displeasure

Otto Hahn, but not Lise Meitner, was awarded the Nobel Prize for Chemistry in 1944 for the discovery of nuclear fission. She was proposed a total of 48 times for the prize, 29 times for physics and 19 times for chemistry during her life. Lise Meitner herself was able to deal well with the lacking award, which Ernst Peter Fischer once said, can be called a „stupidity of the Swedish Academy“. Even when her inaugural lecture in 1922,entitled The Importance of Radioactivity for Cosmic Processes, was renamed to „cosmetic processes“ by an ignorant journalist (what else could one expect from a woman, was the assumption), she took it with humor. She knew worse, had she as a doctorate had to enter the university always through the back door, since before 1909 women were not allowed to attend lectures in Prussia (in 1926, she was the first woman in Germany, who received a professorship in physics).

To this day, she serves us as a shining example „, says Dieter Meschede, President of the DPG, the German Association of Physicists, on the occasion of the 50th anniversary of her death:“She was one of the most important physicists of her time and is a role model for the word in the best sense thanks to her outstanding scientific and social commitment.“Lise Meitner died on 27 October 1968 at the age of 89, a few months after her research associate Otto Hahn, which whom she had held alife-long friendship.

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