page_title: Discovery of nuclear fission
section_title: Research
hierarchical_section_title: Discovery of nuclear fission / Discovery / Research
caption_attribution_description: Exhibition, organised by the Permanent Mission of Federal Republic of Germany, to mark the 75th anniversary of the discovery of nuclear fission, at the Vienna International Centre. Replica of Dr. Otto Hahn’s laboratory table, loaned by the Deutsches Museum Munich. Vienna, Austria, 25 November 2013. Photo Credit: Dean Calma / IAEA
context_page_description: Nuclear fission was discovered in December 1938 by physicists Lise Meitner and Otto Robert Frisch and chemists Otto Hahn and Fritz Strassmann. Fission is a nuclear reaction or radioactive decay process in which the nucleus of an atom splits into two or more smaller, lighter nuclei. The fission process often produces gamma rays and releases a very large amount of energy, even by the energetic standards of radioactive decay. Scientists already knew about alpha decay and beta decay, but fission assumed great importance because the discovery that a nuclear chain reaction was possible led to the development of nuclear power and nuclear weapons.
  Hahn and Strassmann at the Kaiser Wilhelm Institute for Chemistry in Berlin bombarded uranium with slow neutrons, and discovered that barium had been produced. They reported their findings by mail to Meitner in Sweden, who a few months earlier had fled Nazi Germany. Meitner and her nephew Otto Robert Frisch theorised and then proved that the uranium nucleus had been split and published their findings in Nature. Meitner calculated that the energy released by each disintegration was approximately 200 megaelectronvolts, and Frisch observed this.
context_section_description: The Berlin group started by irradiating uranium salt with neutrons from a radon-beryllium source similar to the one that Fermi had used. They dissolved it and added potassium perrhenate, platinum chloride and sodium hydroxide. What remained was then acidified with hydrogen sulphide, resulting in platinum sulphide and rhenium sulphide precipitation. Fermi had noted four radioactive isotopes with the longest-lived having 13- and 90-minute half-lives, and these were detected in the precipitate. The Berlin group then tested for protactinium by adding protactinium-234 to the solution. When this was precipitated, it was found to be separated from the 13- and 90-minute half-life isotopes, demonstrating that von Grosse was incorrect, and they were not isotopes of protactinium. Moreover, the chemical reactions involved ruled out all elements from mercury and above on the periodic table. They were able to precipitate the 90-minute activity with osmium sulphide and the 13-minute one with rhenium sulphide, which ruled out them being isotopes of the same element. All this provided strong evidence that they were indeed transuranium elements, with chemical properties similar to osmium and rhenium.
  Fermi had also reported that fast and slow neutrons had produced different activities. This indicated that more than one reaction was taking place. When the Berlin group could not replicate the Rome group's findings, they commenced their own research into the effects of fast and slow neutrons. To minimise radioactive contamination if there were an accident, different phases were carried out in different rooms, all in Meitner's section on the ground floor of the Kaiser Wilhelm Institute. Neutron irradiation was carried out in one laboratory, chemical separation in another, and measurements were conducted in a third. The equipment they used was simple and mostly hand made.
  By March 1936, they had identified ten different half-lives, with varying degrees of certainty. To account for them, Meitner had to hypothesise a new (n, 2n) class of reaction and the alpha decay of uranium, neither of which had ever been reported before, and for which physical evidence was lacking. So while Hahn and Strassmann refined their chemical procedures, Meitner devised new experiments to shine more light on the reaction processes. In May 1937, they issued parallel reports, one in Zeitschrift für Physik with Meitner as the principal author, and one in Chemische Berichte with Hahn as the principal author. Hahn concluded his by stating emphatically: Vor allem steht ihre chemische Verschiedenheit von allen bisher bekannten Elementen außerhalb jeder Diskussion ("Above all, their chemical distinction from all previously known elements needs no further discussion.")
  Meitner was increasingly uncertain. They had now constructed three (n, γ) reactions:
  ²³⁸
  ₉₂U + n → ²³⁹
  ₉₂U (10 seconds) → ²³⁹
  ₉₃ekaRe (2.2 minutes) → ²³⁹
  ₉₄ekaOs (59 minutes) → ²³⁹
  ₉₅ekaIr (66 hours)  → ²³⁹
  ₉₆ekaPt (2.5 hours) → ²³⁹
  ₉₇ekAu (?)
  ²³⁸
  ₉₂U + n → ²³⁹
  ₉₂U (40 seconds) → ²³⁹
  ₉₃ekaRe (16 minutes) → ²³⁹
  ₉₄ekaOs (5.7 hours) → ²³⁹
  ₉₅ekaIr (?)
  ²³⁸
  ₉₂U + n → ²³⁹
  ₉₂U (23 minutes) → ²³⁹
  ₉₃ekaRe
  Meitner was certain that these had to be (n, γ) reactions, as slow neutrons lacked the energy to chip off protons or alpha particles. She considered the possibility that the reactions were from different isotopes of uranium; three were known: uranium-238, uranium-235 and uranium-234. However, when she calculated the neutron cross section it was too large to be anything other than the most abundant isotope, uranium-238. She concluded that it must be a case of nuclear isomerism. This had been discovered by Hahn in protactinium in 1922, and provided a physical explanation by von Weizsäcker, who had been her assistant in 1936, but had since taken a position at the Kaiser Wilhelm Institute for Physics. Different isomers of protactinium had different half-lives, and this could be the case for uranium too, but if so it was somehow being inherited by the daughter and granddaughter products, which seemed to be stretching the argu
caption: Exhibition to mark the 75th anniversary of the discovery of nuclear fission, at the Vienna International Centre in 2013. The table (on loan from the Deutsches Museum Munich) is now described as a replica and images of Meitner and Strassmann are  prominently displayed.
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