page_title: Discovery of nuclear fission
section_title: Transmutation
hierarchical_section_title: Discovery of nuclear fission / Transmutation
caption_attribution_description: English: Irène and Frédéric Joliot-Curie in their laboratory in 1935.
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: Patrick Blackett was able to accomplish nuclear transmutation of nitrogen into oxygen in 1925, using alpha particles directed at nitrogen. In modern notation for the atomic nuclei, the reaction was:
  ¹⁴
  ₇N + ⁴
  ₂He → ¹⁷
  ₈O + p
  This was the first observation of a nuclear reaction, that is, a reaction in which particles from one decay are used to transform another atomic nucleus. A fully artificial nuclear reaction and nuclear transmutation was achieved in April 1932 by Ernest Walton and John Cockcroft, who used artificially accelerated protons against lithium, to break this nucleus into two alpha particles. The feat was popularly known as "splitting the atom", but was not nuclear fission; as it was not the result of initiating an internal radioactive decay process.
  Just a few weeks before Cockcroft and Walton's feat, another scientist at the Cavendish Laboratory, James Chadwick, discovered the neutron, using an ingenious device made with sealing wax, through the reaction of beryllium with alpha particles:
  ¹¹
  ₅Be + ⁴
  ₂He → ¹⁴
  ₇N + n
  Irène Curie and Frédéric Joliot irradiated aluminium foil with alpha particles, they found that this results in a short-lived radioactive isotope of phosphorus with a half-life of around three minutes:
  ²⁷
  ₁₃Al + ⁴
  ₂He → ³⁰
  ₁₅P + n
  which then decays to a stable isotope of silicon
  ³⁰
  ₁₅P → ³⁰
  ₁₄Si + e⁺
  They noted that positron emission continued after the neutron emissions ceased. Not only had they discovered a new form of radioactive decay, they had transmuted an element into a hitherto unknown radioactive isotope of another, thereby inducing radioactivity where there had been none before. Radiochemistry was now no longer confined to certain heavy elements, but extended to the entire periodic table.
  Chadwick noted that being electrically neutral, neutrons would be able to penetrate the nucleus more easily than protons or alpha particles. Enrico Fermi and his colleagues in Rome—Edoardo Amaldi, Oscar D'Agostino, Franco Rasetti and Emilio Segrè—picked up on this idea. Rasetti visited Meitner's laboratory in 1931, and again in 1932 after Chadwick's discovery of the neutron. Meitner showed him how to prepare a polonium-beryllium neutron source. On returning to Rome, Rasetti built Geiger counters and a cloud chamber modelled after Meitner's. Fermi initially intended to use polonium as a source of alpha particles, as Chadwick and Curie had done. Radon was a stronger source of alpha particles than polonium, but it also emitted beta and gamma rays, which played havoc with the detection equipment in the laboratory. But Rasetti went on his Easter vacation without preparing the polonium-beryllium source, and Fermi realised that since he was interested in the products of the reaction, he could irradiate his sample in one laboratory and test it in another down the hall. The neutron source was easy to prepare by mixing with powdered beryllium in a sealed capsule. Moreover, radon was easily obtained; Giulio Cesare Trabacchi had more than a gram of radium and was happy to supply Fermi with radon. With a half-life of only 3.82 days it would only go to waste otherwise, and the radium continually produced more.
  Working in assembly-line fashion, they started by irradiating water, and then progressed up the periodic table through lithium, beryllium, boron and carbon, without inducing any radioactivity. When they got to aluminium and then fluorine, they had their first successes. Induced radioactivity was ultimately found through the neutron bombardment of 22 different elements. Meitner was one of the select group of physicists to whom Fermi mailed advance copies of his papers, and she was able to report that she had verified his findings with respect to aluminium, silicon, phosphorus, copper and zinc. When a new copy of La Ricerca Scientifica arrived at the Niels Bohr's Institute for Theoretical Physics at the University of Copenhagen, her nephew, Otto Frisch, as the only physicist there who could read Italian, found himself in demand from colleagues wanting a translation. The Rome group had no samples of the rare eart
caption: Irène Curie and Frédéric Joliot in their Paris laboratory in 1935.
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