Quantum generations, p.38
Quantum Generations, page 38
There can be no doubt that American physics became greatly strengthened as a result of the influx of the European emigrants. Especially in many theoretical fields, such as quantum electrodynamics, nuclear theory, relativity, and solid state theory, the emigrants were an invaluable asset. However, they could flourish in the American environment only because there already was a strong basis, both institutionally and intellectually, and both in experiment and theory. Contrary to what is often believed, the United States did not become the world’s leading nation in physics research simply as a result of the brain gain. In 1936 Newsweek could proudly, and correctly, declare, “The United States leads the world in physics.” The leadership was further enhanced by the wave of European emigrants, but it was created mostly by American physicists and the country’s impressive achievements in higher education and scientific institutions. The emigrants were welcomed to American universities in part because of general humanitarian sentiments and in part because American physicists and science administrators realized that they would make a most valuable contribution to the research system. The motives were not political in the sense that America wanted to deprive Germany of its best brains, but when the war came, this was realized to be an extra bonus. In June 1941, at a time when the United States was still formally a neutral country, this strategic result of the intellectual migration was pointed out by a professor Gortner in a letter to the president of the Emergency Committee, Stephen Duggan. Gortner argued as follows: “I firmly believe that we could reduce the technological achievement of Central Europe to the basis of a technological achievement of Spain or Portugal if we could move out 1000 of their strategic men who are leaders in the field of natural sciences, and in the long run the battle for democracy would be won more cheaply by doing just this and the results would be much more permanent than can ever be accomplished by the billions of dollars which we are pouring into our own defense program” (Fischer 1988, 84). An interesting thought, but this was not the way things came to happen.
The migrating physicists were persons, not merely statistical figures. Consider as an example the fate of Fritz London, a Polish-born German physicist of Jewish descent born in 1900. London, who had started his academic career as a student of philosophy, did important work in quantum mechanics and worked in Zurich with Schrödinger, whom he followed to Berlin. During his Zurich period in 1927 he wrote, with Heitler, the pioneering paper in quantum chemistry, for the first time explaining the covalent bond in terms of quantum mechanics. During his years in Berlin, he was occupied mostly with problems of chemical physics. By 1933, London was known as an original and eminent physicist, although not quite of Nobel prize caliber (he was once nominated for the prize, but in chemistry). With the advent of the Nazi laws of 1933, he was forced to take a leave of absence from the University of Berlin, which in reality meant a dismissal. Like many of his colleagues, he received help through the informal physics network and in August 1933, was offered an ICI fellowship at Oxford University. When in England, he changed his focus to low-temperature physics and joined the group around Simon and Mendelssohn. Together with his younger brother Heinz, another refugee member of the Oxford group, London developed the first successful (macroscopic) theory of superconductivity. Although scientifically fruitful, London’s stay in England was not happy and after three years, he was informed that the ICI fellowship had been terminated without possibility of extension. He then managed to obtain a research position at the Institut Henri Poincaré in Paris, for the first year of which he received a grant from the Comité Français d’Accueil aux Savants Etrangers (the French Committee for the Help of Foreign Scholars), the French counterpart of the AAC. In Paris he continued his studies of superconductivity and super-fluidity. London liked Paris and declined an offer to come to the Hebrew University in Jerusalem, but in 1938 he accepted the post of visiting lecturer for 1938 39 at Duke University in North Carolina. Back in Europe, he received a new offer from Duke, now for a permanent position as professor of theoretical chemistry. He left for America on September 1, 1939, the day the German army invaded Poland. Many of the refugee physicists assimilated quickly into the American environment, but not all could do it as easily as Bethe, Fermi, or Weisskopf. London was deeply immersed in the European intellectual culture and strongly felt the difference between his world and that of the American south. To Frédéric Joliot, he wrote: “I am too European to be able to become enthusiastic about life here, which even for those childlike adults here is too calm. . . . It appears to me that people here are free of passions except bridge and football” (Gavroglu 1995, 169).
Chapter 18
FROM URANIUM PUZZLE TO HIROSHIMA
THE ROAD TO FISSION
AS SOON AS THE neutron was discovered, physicists realized that the new particle, owing to its lack of electrical charge, might be used as an effective projectile in nuclear reactions. The earliest reported nuclear transmutations of 1932 34 made use of fast neutrons impinging on light target nuclei such as aluminium. The results were (n, α), (n, p) and (n, γ) processes, that is, the expulsion of either alpha particles, protons, or gamma radiation. At that time, Fermi and his group in Rome began a systematic study of neutron reactions with all the elements of the periodic system, from hydrogen onward. For a neutron source, they used a sealed glass tube containing beryllium powder and radon. In the course of this work, the Italian scientists discovered purely accidentally that neutrons that had passed through paraffin, wood, or water were much more effective in producing radioactive isotopes. They concluded that the neutrons had been slowed down by collisions with hydrogen nuclei. Further experiments confirmed that slow neutrons were more easily captured than fast neutrons. When the Italians bombarded uranium with slow neutrons, they were able to identify several beta-emitting products, one of them with a half-life of 13 minutes. Fermi, Franco Rasetti, and Oscar D’Agostino found that the activity could not be due to isotopes between uranium and lead, and that this negative evidence “suggests the possibility that the atomic number of the element may be greater than 92” (Wohlfarth 1979, 58) The announcement made headlines in the press, and in Italy it was celebrated as a great triumph of fascist culture. Although disturbed about the publicity, Fermi believed that he had manufactured the first transuranium elements. As late as December 1938, in his Nobel lecture in Stockholm, he spoke confidently about “ausonium” and “hesperium,” the names used in Rome for elements 93 and 94.
The 1934 Rome announcement caused Otto Hahn and Lise Meitner at the Kaiser Wilhelm Institute of Chemistry in Berlin to engage in similar work. The institute, which had been founded in 1912, was funded at the time mainly by the chemical industry, directly or indirectly by the I. G. Farben Company, Germany’s giant chemical corporation. Meitner and Hahn at first believed that they too had found transuranic elements and reported in 1935 that “it seems very probable that the 13 and 90 minute activities are elements beyond number 92” (Graetzer and Anderson 1971, 24). On the other hand, Fermi’s results were criticized by Ida Noddack (née Tacke), a German chemist who, together with her later husband Walter Noddack, had discovered the element rhenium in 1925. Ida Noddack found Fermi’s conclusions completely unwarranted and denied that transuranic elements had been produced. After all, she argued, almost nothing was known about neutron-induced nuclear reactions, so why assume that the product belonged to the end of the periodic table? “It is conceivable,” she wrote, “that in the bombardment of heavy nuclei with neutrons, these nuclei break up into several large fragments that are actually isotopes of known elements, but are not neighbors of the irradiated elements” (Wohlfarth 1979, 63). Noddack’s anticipation of nuclear fission made no impact at all on the course of events. Although published in a chemistry journal (the Zeitschrift für angewandte Chemie), it was known to both Fermi and Hahn and Meitner, but they did not take the suggestion seriously. Not only was Noddack’s paper highly critical and her suggestion speculative, but the author’s scientific reputation was also somewhat undermined because of her controversial claim to have discovered element 43 (which she called masurium and is now known as technetium, first produced in 1937 by E. Segré and Carlo Perrier). Noddack was not “rehabilitated” as a precursor of the fission hypothesis until the 1990s.
From 1935, the centers of uranium research moved from Rome to Berlin and Paris, with the two groups entering what can be better described as a rivalry than a cooperation. Although the Paris and Berlin groups were by far the most important, they were not the only ones interested in neutron-irradiated uranium. For example, in Berkeley, Philip Abelson tried to identify the supposedly transuranic products by means of the tested and precise x-ray spectroscopic method. However, in looking for atomic numbers larger than 92, Abelson failed to interpret his x-ray lines correctly. When the fission hypothesis became known, Abelson quickly found evidence for tellurium and thus confirmed the hypothesis. In Berlin, Hahn and Meitner made numerous experiments, suggested elaborate decay schemes, and thought of a variety of hypotheses in order to clarify what happened when uranium was bombarded with neutrons. After two years of strenous work, their main conclusion was disappointing, namely, that irradiated uranium produced complex products of an unknown nature, probably including some transuranic isotopes. Yet not all their work was in vain. One of their hypotheses was that the uranium products were isomers of uranium, that is, isotopes with different half-lives but with the same number of protons and neutrons. At that time, nuclear isomerism was not generally accepted, the only known (and controversial) case being the “uranium Z” that Hahn had reported as a protactinium isomer in 1921.
The work of Hahn and Meitner proved the existence of isomers but did not solve the uranium puzzle. In Paris, Irène Joliot-Curie worked on the same problem but adopted a somewhat different approach. In 1937, together with Pavel Savitch, a Yugoslavian physicist working in Paris, she reported a substance with a half-life of 3.5 hours in irradiated uranium. At first they thought it was thorium, but after more work they concluded in October 1938 that it followed lanthanum in chemical separations and was therefore possibly actinium although “on the whole the properties of R 3.5 hr are those of lanthanum.” Then, in the third round, they suggested that the 3.5-hour substance could not be an actinium isotope, but was probably a new transuranic element. Had they suggested that the close chemical similarity with lanthanum was evidence of a lanthanum isotope with half-life 3.5 hours, they might have discovered fission. But they did not. The results of Curie and Savitch puzzled the Berlin team, which from 1935 had been extended with the inclusion of Friedrich Strassmann, an analytical chemist. While pondering how to understand the Paris experiments, Meitner decided in July 1938 to leave Germany; it was now left to Hahn and Strassmann to find a solution. However, they communicated by mail with Meitner, who unofficially still belonged to the Berlin group.
It was the attempt to explain the Curie-Savitch results that led Hahn and Strassmann to fission. Among the activities resulting from the bombardment of uranium with neutrons, they found one that was precipitated with barium and therefore concluded that it was probably a new radium isotope. It seemed to them that the lanthanum-like isotope might be actinium, created from the radium by beta decay. But could radium be produced from uranium by the emittance of two alpha particles? Bohr, Meitner, and other theorists said no, and Hahn and Strassmann returned to the laboratory. In early December 1938 they began to realize that what they had thought of as radium behaved very much like barium, much more than would be expected from the chemical similarity of the two elements. If so, the Curie-Savitch substance might be lanthanum, produced by beta-active barium. By December 18, 1938, they had experimental evidence that what behaved like barium was, in all likelihood, barium. But it seemed incredible that uranium could turn into a much lighter element, and Hahn did not easily draw the conclusion. “Perhaps you can propose some kind of fantastic explanation,” he wrote to Meitner on December 19. “We ourselves know that [uranium] cannot really burst apart into barium” (Weart 1983, 112). Even in Hahn’s and Strassmann’s paper of January 6, 1939, the two authors avoided a definite statement that barium had been produced by neutron-irradiated uranium. “As chemists,” they wrote, “we should replace the symbols Ra, Ac and Th . . . in [our] scheme . . . by Ba, La, and Ce. . . . [But] as nuclear chemists, closely associated with physics, we cannot decide to take this step in contradiction to all previous experience in nuclear physics” (Wohlfarth 1979, 58). But they now glimpsed a possible explanation. They had found among the supposed transuranium elements one that resembled rhenium. If “radium” was barium, then the “transuranic rhenium” might be a lower homologue of rhenium, that is, element 43 or masurium (Ma). As Hahn and Strassmann remarked: “The sum of the mass numbers Ba + Ma, thus e.g. 138 + 101, gives 239!”
The insight that the uranium nucleus may split when capturing a slow neutron was first reached by Meitner and her nephew Otto Frisch, both refugees from the Third Reich. Frisch worked with Bohr in Copenhagen and his aunt was in Stockholm, where she had a position at Manne Siegbahn’s institute. When the two met in late December 1938 to spend the Christmas holidays at Kungälv near Gothenburg, they had not yet received a copy of Hahn and Strassmann’s paper. But they knew about its results and tried to figure out what had happened in the uranium nucleus in the Berlin laboratory. Frisch recalled: “We walked up and down in the snow, I on skis and she on foot . . . and gradually the idea took shape that this was no chipping or cracking of the nucleus but rather a process to be explained by Bohr’s idea that the nucleus was like a liquid drop; such a drop might elongate and divide itself” (Frisch and Wheeler 1967, 276). The liquid drop model of the nucleus went back to work performed by Gamow in 1929 and during the following decade, it was developed by Bohr, von Weizsäcker, and others. Bohr’s 1936 version, known as the compound nucleus, was particularly important and well suited to illuminate the mechanism of neutron reactions. The theory of the compound nucleus was well known to Frisch, who realized that it might provide an explanation of the Hahn-Strassmann anomaly. The splitting process was termed “fission,” a name suggested to Frisch by an American biologist working at Bohr’s institute. Meitner and Frisch reported their fission hypothesis in a letter to Nature on January 16, 1939. The hypothesis was that the uranium nucleus “after neutron capture, divides itself into two nuclei of roughly equal size.” Moreover, the fission would be a violent process: “These two nuclei will repel each other and should gain a total kinetic energy of about 200 MeV, as calculated from nuclear radius and charge. This amount of energy may actually be expected to be available from the difference in packing fraction between uranium and the elements in the middle of the periodic system” (Graetzer and Anderson 1971, 52). Meitner and Frisch also used the occasion to suggest that thorium underwent fission in a manner similar to uranium. They had privately suggested to Hahn and Strassmann to look for radioactive inert gases (krypton and xenon) as fission products, and when Strassmann found the gases, the fission hypothesis was substantiated. It is notable that the discovery of fission, one of the most important discoveries in twentieth-century physics, was made by two chemists working at a chemical laboratory, and not by nuclear physicists. Indeed, the discovery took the physics community by surprise. Not even the Berlin physicists were aware that something highly interesting was going on at the Kaiser Wilhelm Institute of Chemistry.
MORE THAN MOONSHINE
News about the splitting of the uranium nucleus spread rapidly in the international physics community. The route started in Copenhagen, where Frisch had discussed the matter with Bohr, who was preparing to leave for the United States. Bohr was greatly surprised, but immediately accepted the fission hypothesis. He was, Frisch wrote to Meitner on January 3, 1939, “only astonished that he had not thought earlier of this possibility, which follows so directly from the present conceptions of nuclear structure,” that is, the model of the compound nucleus (Stuewer 1994, 78). Bohr and his collaborator, Léon Rosenfeld, arrived in New York on January 16, 1939, and Rosenfeld went straight to Princeton, where he discussed the conclusions obtained in Germany, Sweden, and Denmark. The announcement, made before Meitner and Frisch’s paper had appeared, caused a sensation. Fermi, John Wheeler, and other American physicists immediately started working on the fission process. In late January 1939, Bohr attended the Fifth Washington Conference on Theoretical Physics, where he and Fermi discussed the new type of process and Bohr explained it qualitatively from the point of view of the liquid drop model. “The whole matter was quite unexpected news to all present,” three American physicists reported in the February 15 issue of Physical Review. Fission was still a hypothesis, and the first phase of work, in both Europe and America, was concerned with verifying the suggestion of Meitner and Frisch. Using different methods, this was done within one or two months, first by Frisch in Copenhagen, who used an oscillograph to record the electrical pulses produced by the fission fragments in an ionization chamber. Shortly afterward, the Berkeley physicists Dale Corson and R. Thornton produced the first visual proof of fission by means of a cloud chamber photograph.
