by Philip Ball
Strassmann joined the KWIC in 1929 as a student intending to seek subsequent employment in industry, but he soon decided that he would rather stay and do fundamental research. His career after 1933 shows how difficult it could be for a young researcher not protected by wealth or status openly to oppose the National Socialists. He despised the regime and his refusal to join any Nazi organizations prompted his resignation from the Nazi-controlled Society of German Chemists. As a result he was blacklisted from jobs in academia and industry, and denied promotion or proper pay at the chemistry institute. Consequently, he was pitifully poor and malnourished, and considered himself lucky when Hahn and Meitner managed to find an assistantship for half-pay in 1935: ‘I value my personal freedom so highly that to preserve it I would break stones for a living’, he attested. His resistance to the Nazis never flagged; during the war, he and his wife hid a Jewish friend in their apartment. In retrospect Strassmann looks more heroic than most of his illustrious colleagues.
Splitting apart
After Lise Meitner’s flight from Germany in July 1938, Hahn and Strassmann continued the uranium studies. But without Meitner’s expertise they had difficulty interpreting what they saw. They found that uranium could be transformed by neutron bombardment into three radioactive substances that seemed chemically similar to barium, and which they therefore concluded must be isotopes of radium (which shares a column with barium in the periodic table). That, however, implied two alpha particles must be emitted at once from uranium, which had never before been seen. They wrote to Meitner, now in Stockholm, who replied that it did not seem credible. In fact, these forms of ‘radium’ resisted all attempts to separate them from barium itself—as if they were indeed nothing other than barium.*8
But that was even more absurd. Barium (element 56) had barely half the mass of uranium (element 92). There was a consensus that transmutations happened only a little at a time: a radioactive decay would turn one element into another very nearby in the periodic table, either by losing two protons (alpha decay) or gaining one (beta decay). You couldn’t get straight to barium from uranium—could you?
Yet Hahn and Strassmann were running out of other explanations for their weird findings. On 19 December Hahn wrote to Meitner in Stockholm:
Perhaps you can suggest some fantastic explanation. We understand that [uranium] really can’t break up into barium . . . So try to think of some other possibility . . . If you can think of anything that might be publishable, then the three of us would be together in this work after all.
At Christmas time Meitner, on holiday in a quiet Swedish village, discussed the peculiar results with her visiting nephew Otto Frisch, another exile from Germany who was now working in Copenhagen. They came to a conclusion that now seems inevitable but which then contravened all prevailing wisdom about nuclear transmutation. The uranium nuclei, they decided, had indeed been more or less split in half. Searching for a name for this process, Frisch recalled the division of living cells, and borrowed the biological term for it: uranium underwent nuclear fission. Frisch told Niels Bohr in Copenhagen, who was about to leave for a conference in the United States, and so Bohr brought the news to scientists across the Atlantic.
And here was the crux: in splitting apart, the uranium atoms also emitted neutrons, as demonstrated in early 1939 by Frédéric Joliot-Curie in Paris. Neutrons in produced neutrons out: here were the ingredients for Szilard’s chain reaction.
There was a catch. Like all elements, uranium has several isotopes differing in the number of neutrons their nuclei contain. By far the most abundant form is uranium-238, which constitutes over 99 per cent of natural uranium. But the form of uranium that underwent fission with the emission of more neutrons was the rarer of the two main isotopes, uranium-235, present in natural uranium at a level of just 0.7 per cent.
What’s more, the neutrons that induce fission of uranium-235 are slow neutrons—they move at speeds of a few kilometres per second, about the same as the speed of most gas molecules at ordinary temperatures, which is why they are also called thermal neutrons. But the neutrons emitted by uranium fission are fast, with speeds of many thousands of kilometres a second. So they need to be slowed down to develop a sustained chain reaction.
Enrico Fermi had discovered serendipitously in 1934 how to slow down fast neutrons: one needs a so-called moderator, a substance that will absorb some of the neutrons’ energy. Fermi found that he could enhance the radioactivity of uranium by bombarding it with fast neutrons if he placed paraffin wax between the neutron source and the target. The paraffin slows the neutrons so that they can be captured by uranium nuclei, causing them to decay.*9
Paraffin, a substance made of carbon and hydrogen atoms, is an effective moderator because neutrons can transfer some of their energy efficiently to hydrogen atoms. Hydrogen nuclei—lone protons—have essentially the same mass as a neutron. So when a neutron hits a hydrogen atom, the atom is light enough to recoil from the impact and absorb some of the energy. In contrast, a neutron strikes a heavy atom as if it were a solid wall, bouncing back while relinquishing hardly any of its energy. Other light atoms can also act as neutron moderators; one of the most effective is carbon, which may be used as a moderator in the form of graphite. Heavy hydrogen—the isotope called deuterium, with a nucleus containing a proton and a neutron—is more effective than normal hydrogen, for which reason heavy water (enriched in deuterium) was also soon identified as a potential moderator. These two substances—graphite and heavy water—became the two favourite candidates for a neutron moderator that could sustain a chain reaction of uranium decay in a nuclear reactor.
The release of energy in this chain reaction can be speeded up if the fissile component, uranium-235, is made more concentrated. The nuclear physicists calculated that to produce a runaway cascade—a nuclear explosion that releases the energy almost instantaneously—requires more or less pure uranium-235. No one knew how to separate it from the much more abundant isotope uranium-238, however—the two isotopes are chemically indistinguishable.
Although such details remained unclear, in April 1939 the chemist Paul Harteck and his assistant Wilhelm Groth at the University of Hamburg decided to inform the Reich Ministry of War of the awesome power that could be unleashed from the uranium nucleus. ‘We permit ourselves’, they wrote,
to direct your attention to the newest development in the field of nuclear physics, for in our estimation it holds a possibility for the creation of explosives whose effect would be many times greater than those presently in use . . . in case the means for creating energy in the manner sketched above becomes a reality, which is entirely within the realm of the possible, the country that first makes use of it would, in relation to other nations, possess a well-nigh irretrievable advantage.
One might imagine that, with war in Europe looking almost inevitable, the Nazis would have considered the discovery of Hahn and Strassmann too sensitive to disclose to the international scientific community. But Paul Rosbaud encouraged the researchers to publish an account of their work in Naturwissenschaften, before anyone saw fit to suppress it, and helped to rush it into print in January 1939 so that foreign scientists might become aware of it. (As we’ve seen, Bohr knew already.) For good measure Rosbaud told the British expert on particle accelerators, John Cockcroft, about it on a visit to Cambridge, apparently with Hahn’s blessing. At the end of April the Nazi government decided that nuclear research should thenceforth be kept secret. But it was too late. In August, Einstein, Szilard and Edward Teller, another Hungarian Jew in exile from Germany, wrote a letter to President Roosevelt warning of the feasibility of making an atom bomb.
Harteck, a specialist on isotope separation, later confessed that it was not so much through patriotic duty that he and Groth sent their memo to the military, but simply because they wanted funding for their research and hoped that the army would provide it. They were certainly not disclosing any well-kept secret: anyone could see the implications of fission for themselves in an article
published, again in Naturwissenschaften, in June by Hahn’s assistant Siegfried Flügge, titled ‘Can the energy content of atomic nuclei be utilized in technology?’ At any rate, the letter of Harteck and Groth reached the German Army Weapons Bureau, which decided to convene a group of specialists to decide what should be done about uranium. They became dubbed the Uranverein—the Uranium Club—and they met for the first time in early September, led by Kurt Diebner, a nuclear physicist who acted as the bureau’s specialist adviser on explosives. The Uranverein decided that research on this new potential source of energy and military supremacy should begin right away, and at a meeting of 26 September a suitable location was identified for this work. Where else should it be but the Kaiser Wilhelm Institute for Physics in Dahlem, headed by Peter Debye?
9
‘As a scientist or as a man’
On 16 September 1939, two weeks after Britain declared war on Germany, Peter Debye received a letter from Ernst Telschow, general secretary of the KWG, declaring that the Kaiser Wilhelm Institute for Physics was thenceforth to be deployed ‘for military technological ends and activities related to the wartime economy’. The clear implication was that it would use its formidable technical facilities to investigate nuclear science and the possibility of liberating energy from uranium.
Such sensitive research could not be left in the charge of a foreigner, even one as deeply immersed in German science as Debye. On the same day that Debye received written notification of the impending changes at the KWIP, Telschow visited him in person to deliver an ultimatum from Rudolf Mentzel of the Reich Science Council: he must either become a German citizen, giving up his Dutch passport, or he must resign.
Debye was not prepared to relinquish his citizenship. But neither would he resign. Instead, he simply refused the demand. As he explained in a letter two days later to KWG president Carl Bosch:
In response to Dr Telschow’s first question, I replied that I did not want to give up my Dutch nationality and that he could regard that answer as definitive. As for the second part of his statement, I pointed out to him that, as he knew, during the last two years I have been given the opportunity to take up a new post, and that each time I have declined the offer without claiming any compensation. Under pressure now, I do not intend to behave differently. I thus refuse to resign my position and must demand that any initiative in that direction will have to come from my superiors. I added that I will not let people say that I ran away from it.
‘There was no question of surrender’, he told Laue early the following year.
Very well, Telschow called subsequently to tell him: Mentzel was prepared to discuss the situation. They met in early October, when Mentzel made it clear that discussion did not mean negotiation. He advised Debye that if he would not comply then he would have to be removed from the directorship and, as Debye put it, should ‘stay at home and write a book’. Mentzel added that there would be no more funds for any research at the institute except that directed towards ‘knowledge and approaches for military objectives’.
Debye regarded the KWIP as an embodiment of his life’s work, and he was not going to give it up easily. That April he had accepted an invitation to deliver a series of prestigious lectures at Cornell University, and now he saw this as a way to stave off any final decision on his directorship. He arranged with the REM and the Army Weapons Bureau (which was assuming control of the KWIP) to take a six-month leave of absence from his post in order to go to America. After that, his position at the institute could be reconsidered. It was agreed that space would be made for him on his return to conduct experiments at very low temperatures, while in the meantime Kurt Diebner would become administrative director ‘at’ (explicitly not ‘of’) the institute.
On 7 October Debye wrote to Tisdale of the Rockefeller Foundation in New York to explain the situation:
Until now the institute has been dealing with purely scientific research only. I have been informed that the government itself wants from now on to decide the kind of questions to be treated in the institute and does not want that this shall be done under my directorship, because of my Dutch nationality. As I am not willing to change my nationality, I agree with the government that for the time being I cannot act as director. As a result of an interview between the leading director of the governmental department and myself, which took place the day before yesterday, we came to the following agreement. I do not resign, instead a leave of absence will be granted for the time of the occupation of the institute during which I will be free to direct my activities, as I think best. During this time my salary will be paid as usual . . . I am very sorry that for a lapse of time, of which the duration cannot be evaluated in this moment, my work in the Max Planck Institute has ended.
Debye clearly felt an obligation to keep the foundation informed of what was happening at the institute they had paid for, but he also hoped to enlist the help of his potential allies in New York. The Rockefeller indeed provided him with unflagging support: Tisdale and other staff stepped in on several occasions during the first few years of Debye’s residence in America to help with visa issues and other inconveniences, such as getting clearance for Debye’s scientific apparatus to be shipped to Cornell.
Debye never made any suggestion that he was leaving Germany because of moral scruples about the regime. He undoubtedly disapproved of the military takeover of the KWIP, but there was never an indication that he left because of the nature of the work that would be undertaken there. Indeed he probably shared the view of the German physicists that uranium research raised a host of interesting scientific challenges. Debye was invited to the first meeting of the Uranverein, and might well have joined the club if the Nazis had been less intransigent about the issues of nationality and autonomy. Since he was a more able experimentalist than either Diebner or Heisenberg (who later led much of the research), it is quite conceivable that the German uranium work would have progressed faster if he had been put in charge.
However, Debye’s departure from Germany later came to be seen as exemplifying his ‘resistance’ to the Nazis. It was presented as a flight from the fascist regime, in fact almost as a mission to warn the Allies of the developing nuclear threat in Berlin. A 1951 report in the New York Times put it like this:
Dr Peter J. W. Debye, a Dutch chemist and winner of the Nobel Prize in 1936, had been working at the Kaiser Wilhelm Institute at Berlin. Abruptly he was informed that his laboratory was needed ‘for other purposes’. He made a few discreet inquiries and learned that a large part of the institute was turned over to uranium research. He fled Germany and came to the United States . . . Upon his arrival he notified his fellow scientists about the new emphasis the Germans were placing on nuclear research. ‘His tidings’, Dr Pegram [George Pegram, who headed early work on the Manhattan Project] said, ‘started a race between our scientists and the German. From then on we worked day and night in a race to get ahead of the Germans.’
It is a very short step from here to the conclusion that Debye came to America to inform on the Nazis and thwart Hitler, rather than because he was seeking a way to postpone a final severance from his Berlin institute. That became the ‘Debye story’, which went essentially unchallenged until his death. Yet despite the claims of his accusers today, Debye didn’t exactly fabricate any part of it. He seems to have been content to let the myth evolve on its own.
Sailing to America
The German authorities were well aware what Debye’s departure might mean. Telschow made his concerns clear in a letter to Adolf Baeumker, chancellor of the Reich Academy of Aviation Research:
In my view there is also the possibility that Prof. Debye . . . may now decide to abandon Germany . . . The Kaiser Wilhelm Society will make efforts to find other possibilities of employment here in Germany after Prof. Debye has concluded his lectures in Ithaca, and it would be welcome if the Academy for Aviation Research could be deployed in a similar way.*1
In December Debye and his wife Mathilde went to Maastricht,
ostensibly to visit his sick mother but in fact to discuss plans for how to leave the country. With this in mind, Debye borrowed money in dollar currency from his mother. He was supposed to meet Telschow later that month to sign the contract of leave, but Telschow found when he arrived at the institute that Debye was once again ahead of the game, and had left already.
It was not exactly an escape, given that he had negotiated official permission, but the Debyes took care to ensure that the departure should not seem to have an air of finality. On 15 January 1940 Debye travelled from Munich across the Brenner Pass to Milan and then Genoa, where he picked up more money transferred by his mother and boarded the Conte di Savoia, setting sail for New York on the 23rd.
Debye’s son Peter, then working for a doctorate at the University of Berlin, had already gone to America in the summer of 1939 on a holiday with friends in Ohio, and the outbreak of war prevented his return. (It has been suggested by the Debye family that this was never intended anyway—that Peter’s departure was the initial move in a planned exodus.) Debye intended that Mathilde would join him at Cornell too. This required some ingenuity. Although she could not obtain an American visa directly (she had, on marrying Debye, become a Dutch national), she could travel to Switzerland where this might be arranged. It turned out to be far from easy: at first she was denied, and only after the intervention of the presidents of Cornell and the Massachusetts Institute of Technology did the US State Department finally grant Mathilde permission to travel. She left Lisbon in December 1940, sailing to Cuba just before Christmas and arriving in the United States in January 1941 to join her husband in Ithaca.