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By Root 1217 0

placed between projectile neutrons and target nucleii would slow the neutrons slightly and make them likely to hit more nuclear targets and produce greater radioactivity. In the course of his experiments Fermi split the uranium atom, though it was not clear to him that he had done so. Indeed, when the physical chemists Ida and Walter Noddack, at the University of Freiburg, suggested that he had, Fermi dismissed their suggestion. Instead, he thought, he had found a new element, one higher on the periodic table than uranium.

The mystery of exactly what Fermi had found was taken on by the German radium expert Otto Hahn and the Austrian physicist Lise Meitner, who had collaborated at the Kaiser Wilhelm Institute (KWI) outside Berlin since its opening in 1912. Hahn was the confident and generous German who had worked with Ernest Rutherford in Montreal before returning to Berlin in 1906 and during the First World War had worked on poison gas. Meitner, the daughter of assimilated (even baptized) Viennese Jews, was as shy as Hahn was gregarious, but a hardworking and imaginative scientist. Along with another colleague, Fritz Strassmann, Hahn and Meitner sought to repeat the Curie and Fermi experiments and comprehend their results. But in March 1938 the Anschluss brought Nazi racial laws to bear on Austrians, now citizens of greater Germany, and Meitner was forced to leave Berlin. That July, playing tourist, she bluffed her way across the border with the Netherlands, went next to Copenhagen as a guest of the Bohrs, then came to rest at a physics institute near Stockholm, as arranged by Niels Bohr.

Hahn and Strassmann carried on with the work at KWI. Skeptical especially of the Joilot and Curie findings—it seemed to them implausible that an alpha or a neutron could drive a particle out of a nucleus—they nevertheless pursued the experiment themselves. By bombarding uranium with neutrons, they produced a result that was, in Robert Jungk’s words, ‘chemically incontrovertible but physically inexplicable’: the process yielded small amounts of the element barium, which weighs slightly more than half as much as uranium. Hahn and Strassmann could hardly believe their data. (Neither, for that matter, had Joliot and Curie, Fermi, the Noddacks, or physicists at the Cavendish, who saw energy bursts off a bombarded uranium nucleus as the probable result of equipment malfunction, believed what their eyes told them.) Hahn sent a tentatively worded paper based on the experiment to the journal Naturwissenschaften. He sent a copy of the paper to Meitner in Stockholm. She brought it with her to an inn in the Swedish town of Kungalv, where she was spending the Christmas holiday with her nephew, the physicist Otto Frisch.

Together, Meitner and Frisch studied their colleagues’ paper and its tremulous conclusions; Frisch skied while Meitner walked briskly beside him. Imagining the nucleus as a drop of liquid, a model suggested by Bohr several years earlier, they decided that the intervention of a neutron projected into a uranium nucleus made the nucleus split into two roughly equal pieces. Each had about half the mass of uranium, hence the surprising production of barium in the Hahn-Strassmann experiment. Along with the large pieces came some neutrons liberated from the target nucleus, and these might then collide with other uranium nucleii, and so on, to create a chain reaction. (This feature of the process they did not immediately understand.) With this splitting came also the release of an enormous quantity of energy. Frisch likened the dividing of the nucleus to the way in which bacteria multiply and so dubbed it ‘nuclear fission’, terminology that appeared in the article Frisch wrote with his aunt for the journal Nature and published in February 1939. When Frisch told Bohr about the revelation a month before the article appeared, Bohr slapped his forehead and groaned, ‘How could we have overlooked that so long?’ He would disclose the news of fission to scientists in Washington two weeks later.37

A quarter century had passed since the publication