Diana Preston - Before the Fallout

With the British effort focused on research on separating U-235, Chadwick decided resources could not be spared. However, worries that his decision was mistaken gnawed at him. In December 1940 he learned that there might be an alternative way of producing element 94. Franz von Halban and Lew Kowarski, now working for the Maud Committee at Cambridge University, were continuing their investigations, initiated in Paris with Frederic Joliot-Curie, into producing chain reactions by bombarding natural uranium with slow neutrons using heavy water as a moderator. They concluded that, given enough uranium and heavy water, a chain reaction would indeed be possible. Although their primary interest was harnessing the chain reaction to produce nuclear power, they saw the potential military applications of their process: that neutrons could convert the heavy and easily obtainable isotope U-238 into the new element 94. Like Rotblat they believed that it was fissionable and could be used to fuel a bomb.

A few months later, in March 1941, research in the United States brought further confirmation. At Berkeley, also using Lawrence’s new sixty-inch cyclotron, the young chemist Glenn Seaborg and the Italian physicist Emilio Segre, who had emigrated from Italy in 1938, isolated and analyzed a tiny amount of the new element for the first time and confirmed that, like U-235, it would fission. Seaborg would name it “plutonium” for the planet Pluto, itself discovered only in 1930.

In July 1941 the Maud Committee submitted its final report to the British government, concluding that “an atomic bomb was feasible.” Written largely by Chadwick, it was in two parts. The first explained with compelling clarity how initial skepticism had turned into conviction that a “very powerful weapon of war” could definitely be made using U-235. Given “the destructive effect, both material and moral… every effort should be made to produce bombs of this kind.” Some twenty-five pounds of U-235 would be needed, and the project would take two years. The second part discussed the possible peaceful applications of nuclear energy: the generation of power by “uranium boilers” as envisaged by von Halban and Kowarski, the use of nuclear energy for ship propulsion, and the production of radioisotopes for medical purposes. The report made no reference to plutonium.

On 30 August 1941 Winston Churchill assented to the proposal to build the atomic bomb with typically mordant wit: “Although personally I am quite content with the existing explosives, I feel we must not stand in the way of improvement.”

GENERAL ERICH SCHUMANN, the head of German weapons re­search and a descendant of the composer Robert Schumann, was also skeptical about the prospect of a revolutionary new weapon. Although a professor of physics, he knew little of atomic science. The letter sent in April 1939 to the army by Professor Paul Harteck had left him unmoved, despite its tempting suggestion that nuclear explosives would confer an “unsurpassable advantage” on the country that possessed them.

While Harteck waited impatiently for a reply, he succeeded in coaxing a private company to give him five thousand dollars to initiate some research into fission since, as he later recalled, “in those days in Germany we got no support for pure science. We were very, very poor.” Harteck’s motive for alerting the German army to the potential of fission was, he claimed, financial. He was not a Nazi, and his sister, who had married into a prominent Jewish family in Vienna, had fled to the United States with her husband and son. What mattered most to Harteck was that “the War Office had the money and so we went to them. If we had gone somewhere else, we would have got nothing.”

By August 1939, having still received no reply, Harteck wrote again. Unknown to him, Schumann had referred the problem to Kurt Diebner, one of his juniors in Army Ordnance. Diebner was an expert in both atomic physics and explosives, and he took Harteck’s letter, with all its implied threat and promise, seriously. His first move was to summon to Berlin an able young physicist, Erich Bagge, then working as Werner Heisenberg’s assistant at the University of Leipzig and whose work on heavy water had come to the army’s attention. A nervous Bagge arrived, expecting to be dispatched to the front. Instead, Diebner instructed him to draw up an agenda for a meeting at the war office to discuss how best to exploit nuclear fission before the war ended. Bagge noticed that the list of invitees consisted almost entirely of experimentalists. He urged that they must have “a theoretical physicist with a big name” and that it “should be Heisenberg.” Diebner refused. The German program would, he insisted, be experimental only. He seems to have been partly motivated by pique that in former years Heisenberg had faulted his scientific work. Heisenberg certainly had little time for Diebner, later describing him as a “decent physicist” but “not absolutely first rate… one of the many people who had come from a low class level into rather high responsibility through the [Nazi] Party.”

On 16 September an initial meeting took place at the war office. Those present included Carl-Friedrich von Weizsacker, Otto Hahn, Hans Geiger, Walther Bothe, and Paul Harteck. Officials in the war office instructed them that their task was to determine whether it was feasible that Germany—or its enemies—could harness fission to produce power or bombs. It was not an easy question, and the group debated for several hours. At the end of this time, Geiger, who had until then remained silent, rose to his feet. Once a pupil of Rutherford and now seemingly a convinced Nazi, he stated that if there was “the slightest chance” of releasing nuclear energy through fission, “it must be done.” Bothe echoed this zeal, declaring, “Gentlemen, it must be done.”

Otto Hahn was much less certain. According to von Weizsacker, Hahn took much convincing to have anything to do with the project. Von Weizsacker pleaded, “Please join… not to help us, but to help yourself, because you will protect your Institute by doing so. You will be doing something which is officially judged to be important for the war effort, and therefore your Institute will continue. Your people will not be dispersed to other projects or to the front.” Hahn replied, “Well, I think you are right, I shall,” but then became “quite emotional,” privately saying, “But if my work leads to a nuclear bomb for Hitler, I will commit suicide.”

Having agreed with mixed feelings and motivations to study the potential applications of nuclear fission, Kurt Diebner’s scientists turned to practicalities—what studies should be undertaken and by whom. Bagge returned to his argument that his mentor, Werner Heisenberg, had to be involved. Not only did the project need his intellect, but there was a serious risk that he might otherwise be called up and perhaps killed in the fighting. This time Diebner assented, and on 20 September Heisenberg was finally ordered to Berlin.

In the fortnight since the war began, Heisenberg had been waiting anxiously with his wife and family at Urfeld in the Bavarian Alps. He had learned of Germany’s invasion of Poland from the proprietor of the local hotel, who assured him cheerily that it would “all be over and done with in three weeks’ time.” Heisenberg had expected immediate orders to join the Mountain Rifle Brigade, with which he had been training, but days passed and he heard nothing. He wrote to his former professor Arnold Sommerfeld that his call-up “strangely enough has not yet come through…. I have no idea what will happen to me.” The summons to Berlin must have been both a relief and a puzzle.