Szilard and the chain reaction

On 12 September 1933 the London newspaper The Times carried a report of a speech given the previous day in Leicester, at the annual meeting of the British Association for the Advancement of Science, by Lord Ernest Rutherford, the most famous nuclear physicist of his day. Rutherford described recent experiments in which atomic nuclei had been split, but warned against illusions. According to the paper, he said that anyone who looked for a source of power in the transformation of the atoms was “talking moonshine”. The article was read by a thirty-five-year-old Hungarian physicist, Leó Szilárd, for some months a refugee from Germany, who was living in a hotel in the Bloomsbury district of London. As he recounted years later, Rutherford’s certainty irritated him. He was turning it over in his mind as he walked the streets of London, and as he waited for the traffic lights to change at a crossing on Southampton Row, it occurred to him that if one could find an element whose nucleus, on absorbing one neutron, emitted two, it would be possible to set off a nuclear chain reaction – a self-sustaining process releasing enormous energy. More than five years before the discovery of uranium fission, this was the first clear statement of the principle on which the nuclear reactor and the atomic bomb are based.

Chronologies usually record under this date that Szilárd “conceived the idea of the nuclear chain reaction”, and that wording matches what we know. It is worth remembering, however, where the date comes from. Szilárd kept no diary at the time and did not record the day of his flash of insight. In later recollections he said only that it happened after he read the newspaper report of Rutherford’s address. Since The Times reported it on 12 September, historians – among them Richard Rhodes in his classic history of the atomic bomb – link the idea to that day. The thought may have matured over several days. The exact wording of Rutherford’s remarks is also uncertain: different newspapers gave slightly different versions, and some softened them with the proviso that he was speaking of “the present state of knowledge”.

A refugee from Berlin

Szilárd was born in Budapest in 1898 into the Jewish family of an engineer. After the First World War, in which he served in the Austro-Hungarian army, and the fall of the Hungarian Soviet Republic, he left in 1919 a country where antisemitism was on the rise, and from 1920 studied in Berlin. He wrote his doctorate in physics under Max von Laue, and his work was praised by Albert Einstein, with whom he later designed and patented a refrigerator with no moving parts. He became a Privatdozent at the University of Berlin, filed patents for a linear accelerator and a cyclotron, and in 1930 acquired German citizenship.

He followed politics more closely than most of his colleagues. When Hitler became Chancellor on 30 January 1933, Szilárd kept his suitcases packed in his room at the guest house of the Kaiser Wilhelm Institutes in Dahlem. By his own account, on 31 March 1933 he boarded an almost empty train to Vienna. The next day, during the boycott of Jewish shops, the trains on that route were packed, and travellers were stopped and questioned at the border. “You don’t have to be cleverer than other people, you just have to be one day ahead of them,” he commented years later. A few days afterwards, on 7 April 1933, Germany adopted the Law for the Restoration of the Professional Civil Service, which allowed officials of Jewish descent and political opponents to be removed from state posts, including university posts. Within a few months German universities lost hundreds of scholars, among them many of the most eminent physicists.

By the beginning of May Szilárd was in London. For his first months there he was occupied not with physics but with rescuing others. He helped to found the Academic Assistance Council, an organisation chaired by the economist William Beveridge that sought work for scholars driven out of Germany. By the outbreak of war it had helped more than 2,500 scientists. Szilárd himself was then considering giving up physics for biology.

The year of the neutron

Nuclear physics was going through a breakthrough at the time. In February 1932, in Cambridge, James Chadwick discovered the neutron, a particle with no electric charge. In April of the same year Rutherford’s pupils John Cockcroft and Ernest Walton split the atomic nucleus for the first time using artificially accelerated particles: they bombarded lithium with protons and obtained alpha particles. Each single transformation released far more energy than the projectile carried, but only a tiny fraction of the protons hit nuclei. The energy balance of the experiment as a whole was therefore heavily negative. That was exactly Rutherford’s point when he said in Leicester that this was a very inefficient way of producing energy.

Szilárd noticed what Rutherford had not taken into account. A proton, being positively charged, is repelled by the nucleus, so it has to be accelerated to high speed and even then rarely hits its target. A neutron has no charge, penetrates the nucleus freely and can trigger a transformation even when moving slowly. If a transformation caused by one neutron released two new ones, each of them could trigger another, and the number of transformations would grow like an avalanche. Szilárd carried over into nuclear physics the concept of the chain reaction familiar to chemists. He also understood that such a reaction could be sustained only in a sufficiently large quantity of material, because in a small lump the neutrons escape through the surface before they hit further nuclei. Thus was born the concept of critical mass.

Szilárd had a particular relationship with this idea. In 1932 he had read H. G. Wells’s 1914 novel The World Set Free, in which the writer described a war fought with “atomic bombs” and the destruction of great cities. The book, he recalled, made an enormous impression on him. From the outset, therefore, he knew not only that a chain reaction could be a source of energy but also that it could be used to build a weapon.

Patent and secrecy

Szilárd did not know which element might sustain the reaction. He suspected beryllium, and later indium. He wanted to examine systematically all the elements then known, but could not find the money to do so. In 1934 Rutherford refused him a place in his laboratory in Cambridge. Szilárd therefore worked at St Bartholomew’s Hospital in London, where together with the physicist Thomas Chalmers he discovered a method of separating artificial radioactive isotopes, known as the Szilard–Chalmers effect. From 1935 he worked in Oxford.

On 12 March 1934 he filed his first application with the British Patent Office, and over the following months, including on 28 June, he supplemented it with a description of a chain reaction induced by neutrons and of the condition of critical mass. He knew that publicity could be dangerous. In 1936 he assigned the patent to the British Admiralty, which kept it secret. It was not published until 1949. Most physicists, including Niels Bohr and Enrico Fermi, then regarded the release of nuclear energy as a distant pipe dream, and treated Szilárd’s urging to be cautious in publishing results as an eccentricity.

From idea to bomb

The missing piece appeared at the end of 1938. In December in Berlin the chemists Otto Hahn and Fritz Strassmann found that uranium bombarded with neutrons yielded barium, and Lise Meitner, who had earlier been forced to flee Germany, and her nephew Otto Frisch explained that the uranium nucleus splits into two parts. Szilárd, who had been living in New York since the beginning of 1938, immediately realised that uranium might be the element he had been looking for for five years. At the beginning of March 1939, at Columbia University, he and Walter Zinn showed that the fission of uranium releases new neutrons. “That night, there was very little doubt in my mind that the world was headed for grief,” he recalled.

Szilárd tried to persuade physicists in the West to stop publishing the results of their research on uranium. He failed, because Frédéric Joliot-Curie’s team in Paris published similar measurements. In the spring of 1939 Germany began its own research into the use of nuclear energy. In July Szilárd, together with Eugene Wigner and later with Edward Teller, visited Einstein and prepared a letter that Einstein signed on 2 August 1939. The letter warned President Franklin Delano Roosevelt that a chain reaction in uranium could be used to build “extremely powerful” bombs and that Germany might be working on them. It was delivered in October 1939, after the war had broken out. It began with a modest advisory committee and ended with the Manhattan Project. On 2 December 1942 in Chicago, Szilárd was present at the start-up of the first nuclear reactor, Chicago Pile-1, in which Fermi’s team achieved the first controlled chain reaction. Its design drew heavily on Szilárd’s ideas; among other things, he had made sure the graphite was free of boron, which absorbs neutrons.

In 1945, when the bomb was nearly finished and Germany had already surrendered, Szilárd collected scientists’ signatures for a petition to President Harry S. Truman urging him not to use it against Japan before Japan had been told the terms of surrender and had rejected them. The petition did not reach the President before the atomic bombings of Hiroshima and Nagasaki.

The idea conceived on Southampton Row is one example of how the policies of Nazi Germany turned against it. The laws of 1933 drove dozens of physicists of Jewish descent out of the country. Many of them – Szilárd, Einstein, Wigner, Teller, Hans Bethe and Otto Frisch among them – later contributed to building the bomb in Britain and the United States, while the German nuclear programme had come nowhere near its goal by the end of the war. A thought formulated by one of the exiles on a London street in the autumn of 1933 became, six years later, one of the reasons why America began work on the weapon that ended the war in the Pacific and changed the world that came after it.