A memorandum that changed the world

In the summer of 1939 Otto Frisch, who was in Copenhagen, received an invitation from Mark Oliphant, a professor of physics at Birmingham, to come to Birmingham for the summer to see if some arrangements might be made for Frisch to emigrate to Birmingham. War came that fall and Frisch could not return safely to Denmark. He was given a lectureship that enabled him to stay. The previous Christmas he had gone to visit his aunt Lise Meitner in Kungalv in Sweden. She had just received a letter from Otto Hahn, the radiochemist in Berlin with whom she had collaborated for many years before she was forced to flee Germany. Meitner and Hahn had been using neutron sources to bombard various elements including uranium. Hahn had continued the experiments with Fritz Strassmann, another chemist. They had found something they did not understand. Instead of finding elements in the residual bombarded uranium that had masses comparable to uranium, they found barium, which was somewhere in the middle of the Periodic Table. They were stuck for an explanation and Hahn appealed to Meitner. She and Frisch went for an excursion in the snowy woods—he on skis and she on foot. During that outing they realized that by using the liquid drop model of the heavy nuclei, what Hahn and Strassmann had done was to fission the uranium nucleus. One product was barium and the other was krypton with possible neutrons in addition. They worked out the energies and realized that about 200 MeV would be released, a very large amount of energy compared to any chemical reaction. Frisch returned to Copenhagen after this visit with his aunt his aunt remained in Sweden and told Bohr about what he and Meitner had concluded. Bohr’s reaction was like a great many other physicists when they heard about it—it was so obvious why had not they thought of it. As it happened Bohr was about to leave for the United States where he was going to spend time at the Institute for Advanced Study which was then located at Princeton University. He was accompanied by his assistant Lèon Rosenfeld. On the Atlantic crossing Bohr had a blackboard setup in his cabin, and he and Rosenfeld went over the theory. He had promised Frisch and Meitner not to say anything until their paper appeared, but he had forgotten to tell Rosenfeld who went at once to Princeton and told everybody. Even in those preinternet days the news spread all over the country and abroad and fission experiments proliferated. Among the first were those of Frisch, who was primarily an experimentalist. He looked for ionization pulses, which were produced by the charged fission fragments. Others found the additional neutrons—on average a little over two—produced