Before the Fallout

by Diana Preston

  In the early months of 1941 the German Luftwaffe attacked Liverpool with high explosive bombs, parachute land mines, oil bombs, and incendiary bombs. In March a parachute landmine hit the courtyard of Chadwick's physics department and blew out all the windows. Scientists hurried to the engineering department to find hammers and nails for makeshift repairs to their labs. Luckily, Chadwick's cyclotron, deep in the basement, was unharmed. Frisch and the fellow occupants of his boardinghouse spent many nights huddling under the staircase. After one particularly frightening raid, they emerged to find that their landlady had fled. Frisch packed a case and scrambled through inner-city streets littered with debris to seek sanctuary with friends in the suburbs. The Chadwicks, who had sent their daughters to Canada for safety, were sleeping on the ground floor of their house for greater protection. Chadwick was discreetly going out with a Geiger counter and checking bomb craters to reassure himself that the Germans were not mixing radioactive material with the explosive in a kind of "dirty bomb."

  Despite the dangers and difficulties of living in a city under attack, Frisch settled down in Liverpool. As aliens he and Joseph Rotblat were formally subject to restrictions on their movements, but Chadwick persuaded his friend the chief constable of Merseyside to exempt them from what Rotblat called the more "ridiculous" strictures. Frisch was thus allowed to own a bicycle and found himself being fined ten shillings for riding without turning on his lights. He enjoyed working for Chadwick, who encouraged members of the team to discuss their work, "putting no great trust in the bogus security which relies on compartmentalising knowledge, on letting every scientist know only what he needs to know." Rotblat was lecturing openly on chain reactions.

  Frisch's task was to test the thermal diffusion method for separating isotopes, pioneered by the German scientist Klaus Clusius, and which he and Peierls had recommended in their memorandum. Frisch told Chadwick that to do this he needed uranium hexafluoride, the only gaseous compound of uranium stable enough to be put into a tube. According to Frisch, Chadwick sat for about thirty seconds, "turning his head side to side like a bird," then said simply, "How much hex do you want?" Frisch set to work with a student assistant, John Holt—the pair were soon nicknamed "Frisch and Chips"—but they discovered that the process would not work with uranium hexafluoride. As Peierls put it, "The effect happens to be practically zero."

  Working with a fellow refugee, the German-born Franz Simon, Peierls thought up another diffusion method for separating isotopes. This involved forcing atoms of uranium hexafluoride gas through fine holes in a porous barrier or membrane made from nickel. Peierls hoped that the lighter U-235. would pass through more quickly than the heavier U-238 and that, by repeating the exercise again and again, a U-235-rich gas would result. The process was difficult because the gas was highly corrosive and broke down on contact with moist air, but it seemed to work. Their research suggested that an industrial separation plant covering forty acres could yield one kilogram of 99 percent pure U-235. a day. The huge complex would take eighteen months to construct.

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  Chadwick was feeling the pressure. With his overview of all the experimental work, it was becoming ever clearer to him that "a nuclear bomb was not only possible—it was inevitable." Yet he felt that he had "nobody to talk to." Although he had a high regard for his chief helpers, Frisch and Rotblat, he was conscious that they "were not citizens of this country" and that the other scientists were "quite young boys." Isolated and anxious, Chadwick found "the only remedy" was to take sleeping pills—a habit that remained with him for life.

  Chadwick also bore the burden of deciding how to prioritize the research. Back in June 1940—on the day after the Germans marched into Paris—a letter, published in the U.S. journal Physical Review by the American scientists Edwin McMillan and Philip Abelson, had reported their results from working with the largest cyclotron yet built, by Ernest Lawrence at Berkeley. It was a giant device with a 60-inch vacuum chamber, compared with the 4. 5-inch chamber of Lawrence's first model. This machine provided a source of high-energy particles that, when they hit a beryllium or similar target, produced a copious stream of neutrons. Using these neutrons, McMillan and Abelson had bombarded uranium and created a hitherto unknown radioactive element. This element, with atomic number 93—named neptunium for the planet next in line to Uranus—decayed into another unnamed element occupying slot 94 in the periodic table. Joseph Rotblat recognized at once that, since the mysterious element shared characteristics with uranium, it would be likely to fission under neutron bombardment. If so, it could be an alternative to U-235. as atomic bomb fuel. He asked Chad­wick to allow him to use the Liverpool cyclotron to produce and explore the new element.

  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.

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  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."

  *A substantial sum, then equivalent to £290,000 or $ 1.4 million.

  * Suffolk would die the following year, 1941, while defusing a bomb.

  ELEVEN

  "HITLER'S SUCCESS COULD

  DEPEND ON IT"

  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 go
t 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."

  Carl-Friedrich von Weizsacker

  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.

  Heisenberg reported to the war office, where, as he later wrote, he was told that he had been conscripted into the new nuclear physics research group "to work on the technical exploitation of atomic energy." The group became known, with surprising casualness about security, as the Uranverein (the Uranium Club). According to von Weizsacker, Heisenberg joined the club without hesitation in order to protect German science. His argument was "Well, we must do it. Hitler will lose this war. It is like the end game in chess, with one castle less than the others. . . . Consequently, much of Germany will be destroyed, or its value will have disappeared. The value of science will still be there and it is necessary that science should live through the war, and we must do something for that."

  Heisenberg's own subsequent recollections described sessions of deep soul-searching with von Weizsacker, during which both men agreed that the prospect of successfully building an atomic bomb was very remote. The technical problems were formidable, probably insuperable, at least over the likely lifespan of the war. The greatest challenge was obtaining enough fissionable material to create an explosive device. Niels Bohr and John Wheeler had shown that it could not be done with natural uranium; only a sufficient quantity of the rare and highly fissionable isotope U-235. would do. But to separate this from U-238, the less-fissionable isotope of which natural uranium was chiefly composed, would, in Heisenberg's words, require "a gigantic technical feat" that would take until "the distant future." However, according to Heisenberg's postwar account, the two men agreed that it might well be possible to use natural uranium to trigger a chain reaction capable of yielding controllable amounts of energy that could be used for "power stations, ships and the like." They also agreed that when the war ended such technology would be important for the rebuilding of Germany. They could, they convinced themselves, work on it "with a clear conscience."

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  The two broad thrusts of the Uranium Club's research were how to separate enough U-23c. and how to build a chain-reacting nuclear pile—a "reactor." Meanwhile, Army Ordnance swiftly requisitioned the Kaiser Wilhelm Institute for Physics, which became the heart of the army project, and gave the institute's Dutch director, Peter Debye, an ultimatum: renounce his Dutch nationality and take German citizenship or resign his directorship. Debye departed for the United States to teach at Cornell University.

  Heisenberg's role was to drive the theoretical side of the project. Still only thirty-seven years old and brimming with drive and energy, the man whom James Chadwick would identify later in the war as "the most dangerous possible German in the field because of his brain power" got quickly to work. His first priority was to develop a theoretical basis for a workable reactor. By December 1939, just weeks after his appointment, he submitted to the army a secret twenty-four-page report, which suggested that the production of power through nuclear fission in a reactor was technically possible using natural uranium. But "enriched" uranium, where the percentage of the rare isotope U-235. had been increased by means of isotopic separation, would be better. He offered an alluring scenario: Enriched uranium could be used to run a smaller reactor at a higher temperature than achievable with natural uranium and to generate enough power to drive German warships and submarines.

  Heisenberg also suggested, in a statement in his report somewhat at odds with his later justification of his motives, that enriching natural uranium could create an explosive surpassing "the explosive power of the strongest existing explosive materials by several orders of magnitude." Isotopic separation was, he said, the "surest method" for achieving a nuclear reactor but the "only method for producing explosives."

  Heisenberg's report and a follow-up paper in February 1940 would provide the template for the Nazi fission research program until the end of the war. However, he made a critical misjudgment over the choice of a suitable material to use as a moderator to slow neutrons down and thus to enhance the
ir chances of hitting their target uranium nuclei, causing fission and thereby triggering the release of more neutrons to sustain a chain reaction. Heisenberg had initially focused on two substances as a moderator: heavy water or carbon, which, as he later wrote, "I had suspected, for theoretical reasons . . . could be used as a moderator in place of heavy water." However, in his second report to the army, he declared it doubtful whether the uranium machine (i.e., a reactor) could be built with carbon.

  Heisenberg had been misled by imprecise data from experiments he had had conducted. The error was compounded by von Weizsacker, whose calculations in Berlin supported Heisenberg's views. So did measurements made in early 1941 by Walther Bothe, by then Germany's leading experimental physicist despite some difficult times. In 1933 he had been ejected from his professorship at Heidelberg University for failing to show due enthusiam for the Nazi Party. However, he had managed to obtain a post at the Kaiser Wilhelm Institute for Medical Research in Heidelberg.

  At first Bothe believed that carbon was a promising material for a moderator: It did not absorb neutrons and was freely available. However, just as von Weizsacker had done, Bothe chose, as his form of carbon, industrial graphite. Both men failed to realize that even the best industrial graphite contains too many impurities to function well as a moderator. In particular, it contains boron, which absorbs, or mops up, neutrons. Had they experimented with completely pure graphite, they would have discovered, as had Enrico Fermi in his experiments at Columbia University, that it was an excellent moderator. Thanks, however, to Leo Szilard's persistence, Fermi's results had not been published, so Bothe remained unaware of his mistake.