© Emilio Segre Visual Archives/American Institute of Physics/Science Photo Library
Lise Meitner and Otto Hahn, in their laboratory at Dahlem, Germany.
Lise Meitner and Otto Hahn, in their laboratory at Dahlem, Germany.
Although the experiments of John Cockcroft and Ernest Walton (see here) did involve ‘splitting the atom’, the atoms involved were very light. What most people think of today when atom splitting is mentioned is the kind of fission that occurs when the nucleus of a heavy element such as uranium splits into two or more fragments. In the 1930s, several teams made this happen in their laboratories without realizing what was going on. It was Otto Hahn, based in Berlin, who, together with his colleagues, not only triggered nuclear fission but explained what had happened in their experiment.
Hahn and his colleague Lise Meitner decided to follow up the discovery, made by Enrico Fermi and his team in Rome in 1934, that what seemed to be ‘new’ elements could be made by bombarding uranium with neutrons. Fermi thought that he had made elements with 93 and 94 protons in their nucleus, which he named ausonium and hesperium. Uranium has 92 protons in each nucleus, and comes in several different varieties (isotopes) including those with 143 and 146 neutrons per nucleus, dubbed uranium235 and uranium238. The German chemist Ida Noddack immediately suggested an alternative, that ‘it is conceivable that the nucleus breaks up into several large fragments,’ but nobody followed this up.
Hahn and Meitner, working with Fritz Strassman, set out to investigate Fermi’s discoveries. Meitner had been working with Hahn since before the First World War, but before they could complete this new project, Meitner, an Austrian Jew, had to leave Berlin because of the risk of Nazi persecution. In the summer of 1938 she moved to Sweden. But she corresponded with Hahn about the progress of the experiments he continued to carry out with Strassman, and contributed a theoretical understanding of what was going on.
Before and after Meitner’s departure, the team carried out experiments along the same lines as those of Fermi, which seemed to show the production of new ‘transuranium’ elements. But towards the end of 1938 Hahn discovered something unexpected. Chemical analysis showed that one of the ‘new’ elements being produced by bombarding uranium with neutrons was an isotope of barium – an atom with 60 per cent of the mass of a uranium atom. The analysis that revealed the trace of barium involved only a few thousand atoms, but Hahn was a superb chemist and when Meitner received a letter from him with news of the discovery, in December 1938, she had no doubt that he was right. Hahn himself was baffled. He wrote ‘we are more and more coming to the awful conclusion that our Ra [radium] isotopes behave not like Ra, but like Ba [barium] … Perhaps you can suggest some fantastic explanation.’ She could. Uranium nuclei had been split apart by the bombardment.
The explanation was that the nucleus of an atom is like a drop of liquid, held together by the strong nuclear force (see here), but with the positive charge of all the protons trying to blow it apart. In the case of uranium, with so many protons the electric repulsion is nearly enough to overcome the strong force, and if the drop is hit by a fast-moving neutron it can break into two or more droplets, which are then repelled from one another by the electric force. When uranium, with 92 protons, splits to produce barium, with 56 protons, the other ‘droplet’ is krypton, with 36 protons. Several neutrons are spat out in the process. The energy carried by these fragments turned out to be about 200 million electron Volts (200 MeV) in each fission.
Meitner was by now collaborating with her nephew Otto Frisch. Frisch was a nuclear physicist based in Denmark, but had been visiting her when Hahn’s letter arrived. She calculated the masses of the two nuclei formed by the division of a uranium nucleus and found that their combined mass is less than the mass of a uranium nucleus by one-fifth the mass of a proton. In line with Einstein’s famous equation E = mc2, one-fifth of a proton mass is equivalent to 200 MeV. So everything fitted and uranium fission provided a potential source of energy for peaceful and military use.
As Hahn commented, the ‘active breakdown products, previously considered to be transuraniums, were in fact not transuraniums but fragments produced by splitting.’42 It was Frisch who suggested the name nuclear fission (in German, Kernspaltung) which appeared in a paper he published with Meitner in 1939. Hahn alone received the Nobel Prize for chemistry for this work. It was the prize for 1944, but held over until 1945. The citation read ‘for his discovery of the fission of heavy nuclei’. This highlights the curious fact that one of the most important discoveries in physics was indeed made by chemical analysis of tiny traces of a substance.