Uranium fission at Columbia University

On the evening of 25 January 1939, in the basement of Pupin Hall, the physics building of Columbia University in New York, a group of physicists were watching the screen of an oscilloscope connected to an ionisation chamber. Inside the chamber was a thin layer of uranium oxide, which was being irradiated with neutrons. Against a background of small fluctuations, single enormous pulses appeared every so often, dozens of times larger than those produced by the alpha particles emitted by the uranium itself. They meant that fragments with an energy of around 200 million electronvolts were flying through the chamber. Only the splitting of a heavy uranium nucleus into two roughly equal parts could release so much energy. John R. Dunning, who led the experiment, wrote in his notebook that night: “Believe we have observed new phenomenon of far-reaching consequences.”

It is often said that on 25 January at Columbia University “the uranium atom was split for the first time”. This was not the case. Uranium had been split in laboratories for years without anyone realising it, and the phenomenon itself had been discovered a month earlier in Berlin and confirmed physically in Copenhagen. The Columbia experiment was the first measurement on the American continent to demonstrate directly the fission of the uranium nucleus and the enormous energy released in the process. It was from that day that the research began in the United States which six years later led to the atomic bomb.

The discovery in Berlin

The story began in Rome in 1934. Enrico Fermi and his team were bombarding one element after another with neutrons – particles without electric charge, discovered two years earlier, which easily penetrate atomic nuclei. When they reached uranium, they obtained a mixture of new radioactive substances and concluded that they had produced elements heavier than uranium, unknown in nature. For this, and for discovering the effect of slow neutrons, Fermi received the Nobel Prize in 1938. As early as 1934 the German chemist Ida Noddack had pointed out that the uranium nucleus might have broken into several large pieces, but no one took her remark seriously.

Fermi’s experiments were repeated in Berlin-Dahlem, at the Kaiser Wilhelm Institute for Chemistry, by the chemist Otto Hahn, the Austrian physicist Lise Meitner and the younger chemist Fritz Strassmann. Meitner, who was of Jewish descent, had for several years been protected by her Austrian citizenship. After the Anschluss of Austria in March 1938 she became a citizen of the Reich, subject to the racial laws, and in July she secretly fled via the Netherlands to Sweden. Hahn and Strassmann carried on working, corresponding with her by letter.

In December 1938 Hahn and Strassmann established beyond all doubt that one of the substances they had taken for radium was in fact barium, an element with roughly half the mass of uranium. On 19 December Hahn wrote to Meitner about it, and on 22 December he sent a paper to the journal Die Naturwissenschaften, which printed it on 6 January 1939. As a chemist, however, he could not explain how this was possible.

The explanation was found by Meitner. She was spending Christmas in Kungälv in Sweden with her nephew, the physicist Otto Frisch, who worked in Copenhagen under Niels Bohr. On a walk through the snowy woods they calculated, using the liquid-drop model of the nucleus, that the uranium nucleus was so unstable that after absorbing a neutron it could stretch and divide into two parts, like a drop of water. The fragments, repelled by their electric charges, would fly apart with an energy of around 200 million electronvolts – many times more than in any known chemical or nuclear reaction. Borrowing the biologists’ term for cell division, Frisch called the phenomenon “fission”. On 13 January 1939 in Copenhagen he measured the pulses produced by the fragments and so became the first to confirm nuclear fission by a physical method. Their short papers appeared in Nature on 11 and 18 February 1939.

The news crosses the Atlantic

On 7 January 1939 Bohr sailed for the United States to attend a conference on theoretical physics in Washington and to spend several months at Princeton. He had promised Frisch that he would tell no one about fission until the paper had been sent for publication. During the voyage, however, he discussed it with the Belgian physicist Léon Rosenfeld, who was travelling with him, without warning him that it was a secret. The ship docked in New York on 16 January. That same evening Rosenfeld told a meeting of physicists at Princeton University about the discovery, and the news spread like wildfire.

It was brought to Columbia by two of its own physicists then working at Princeton, Isidor Isaac Rabi and Willis Lamb. It reached Fermi, who had been working at Columbia for a few weeks. In December 1938 he had gone with his wife and children to Stockholm to collect his Nobel Prize and had not returned to Italy. His wife Laura was Jewish, and in the autumn of 1938 Benito Mussolini’s regime had introduced racial laws on the German model. The Fermi family arrived in New York by sea on 2 January 1939. For Fermi the news was awkward, because the “new elements” for which he had received the prize turned out to be largely fission products. He added a footnote acknowledging this to the text of his Nobel lecture. Soon afterwards Bohr himself came to Columbia and, not finding Fermi, explained the new phenomenon to a doctoral student he met at the cyclotron, Herbert L. Anderson.

The experiment in Pupin Hall

Columbia had everything needed to test the discovery: powerful neutron sources, including the cyclotron that Dunning, a specialist in neutron physics, had built in the basement of Pupin Hall in 1935–1936, and equipment for detecting particles. Fermi and Dunning agreed that the energy of the fragments had to be measured directly. Anderson built an ionisation chamber with a layer of uranium oxide and an amplifier, and the signal was fed to an oscilloscope. If uranium nuclei really were splitting in two, each fission should produce a pulse far larger than anything recorded before.

Fermi helped to plan the experiment but was not in the laboratory when it produced its result, having left for the conference in Washington. The measurement was made on 25 January by Dunning and Anderson; the team that later signed the paper also included Eugene T. Booth, G. Norris Glasoe and Francis G. Slack. The expected huge pulses appeared on the screen. The paper “The Fission of Uranium”, signed by Anderson, Booth, Dunning, Fermi, Glasoe and Slack, was sent to Physical Review on 16 February 1939. It appeared on 1 March.

Washington and a flood of experiments

The next day, 26 January, the fifth Washington Conference on Theoretical Physics, organised by George Washington University and the Carnegie Institution, opened in Washington. There Bohr, in agreement with Fermi, announced the discovery of fission. According to participants, several physicists left the hall while the session was still in progress to repeat the experiment in their own laboratories. Within the next few days it was confirmed at the Carnegie Institution in Washington, where the result was shown to Bohr and Fermi on 28 January, at Johns Hopkins University, at Berkeley and elsewhere. By the end of January American newspapers were already writing about “splitting the uranium atom”. In Paris Frédéric Joliot-Curie confirmed fission independently.

The most important question, however, concerned not fission itself but whether new neutrons were released when the uranium nucleus split. If so, they could split further nuclei and set off a nuclear chain reaction, something Leó Szilárd had been thinking about since 1933. Szilárd, who had conceived the idea on a London street and had been living in New York since 1938, understood from the first moment what the Berlin discovery meant. At the beginning of March 1939 at Columbia he and Walter Zinn – and, in parallel, Fermi with Anderson – showed that fission produces on average more than two new neutrons. At the same time a similar result was announced in Paris by Joliot-Curie’s team, which included Hans von Halban and Lew Kowarski. In Warsaw the emission of neutrons in fission was observed, in the radiological laboratory headed by Ludwik Wertenstein (PL), by the young physicist Józef Rotblat, who, as he later recalled, realised even then that a runaway reaction in uranium could cause an explosion.

Szilárd tried to persuade his colleagues to hold back from publishing such results, so as not to hand them to the Germans. Fermi was sceptical, and Joliot-Curie refused. Bohr and John Wheeler soon explained theoretically that in natural uranium slow neutrons split mainly the rare isotope uranium-235, which makes up less than one per cent of it. Their paper appeared on 1 September 1939, the day Germany attacked Poland.

From Pupin Hall to Hiroshima

The experiment of 25 January was not in itself a breakthrough that changed the course of history, but one of many confirmations of the discovery made in Berlin. It does, however, show how quickly knowledge of fission crossed borders and how great a part was played in this by refugees from the fascist states: Meitner and Frisch, driven out of the Reich; Fermi, who left Italy after the racial laws; Szilárd, who fled Berlin in 1933. It was in their circle that the fear arose that Germany – where fission had been discovered and where physicists such as Werner Heisenberg were working – would build the bomb first.

The fear was not groundless. As early as April 1939 the first group of German physicists studying the use of nuclear energy was formed in Göttingen, known as the Uranverein (Uranium Club), and the physicist Paul Harteck informed the Army Ordnance Office of the possibility of building a new weapon. After the outbreak of war the army took over the programme. In the summer of 1939 Szilárd persuaded Albert Einstein to warn President Franklin Delano Roosevelt that Germany might build bombs of “extremely powerful” force. In Britain in 1940 Frisch and Rudolf Peierls, also refugees, calculated that a bomb would need only a few kilograms of pure uranium-235, not tons, as had been believed. Their memorandum and the MAUD Committee report of 1941 convinced the British and Americans that a bomb was feasible. In 1942 the American Manhattan Project got under way, and many members of the Columbia team worked on it. At Columbia Dunning led research into uranium enrichment by gaseous diffusion, in which Booth also took part, while Anderson was one of Fermi’s closest collaborators in building Chicago Pile-1, in which the first controlled chain reaction was achieved on 2 December 1942.

The German nuclear programme, scattered among several institutes and never treated as a priority by the Reich authorities, had not even built a working reactor by the end of the war. On 16 July 1945 the first American atomic bomb exploded in the New Mexico desert, and on 6 and 9 August 1945 the Americans dropped bombs on Hiroshima and Nagasaki. Rotblat, who worked on the bomb at Los Alamos, was the only scientist to leave the project on moral grounds, at the end of 1944, when it became clear that Germany would not build one. His wife, who had not managed to get out of Warsaw in August 1939, was killed in the German death camp at Bełżec. The road that began with pulses on an oscilloscope screen in a New York basement ended the war in the Pacific and opened the age of nuclear weapons.