2—
European Transuranics
In 1938 Fermi received the Nobel prize in physics "for his demonstrations of the existence of new radioactive elements produced by neutron irradiation, and for his related discovery of nuclear reactions brought about by slow neutrons." What was meant by "new elements" appears from a letter of nomination from G.P. Thomson, who specified besides the method of slow neutrons the "proof of the existence of radio-active elements of atomic number greater than 92 produced by the action of neutrons on uranium."[23] Fermi had advanced his claim cautiously; but like Lawrence, he had supporters who snatched at any novelty to justify their confidence. He was swept forward by a speech by his protector, O.M. Corbino, who, to Fermi's dismay, told the Accademia dei lincei on June 3, 1934, that his protégé had made element 93. In the style of Millikan and Lawrence, Corbino added that "the power to produce such transformations in sufficient quantity would give mankind not only immediate possession of the rarest elements, but also command of an almost limitless supply of energy."[24] Although warned that the chemical procedures that appeared to show that the "transuranics" could not be known elements might be faulty, the Rome group acquired confidence by repetition, especially when the experienced team of Otto Hahn and Lise Meitner confirmed their findings. In his Nobel lecture in Stockhom, Fermi unreservedly claimed the discoveries of elements 93 and 94, and, as further proof of their chemical individuality he revealed the euphonic names, "ausonium" and "hesperium," by which they were known in Rome.[25]
The world's center of transuranic research from 1935 to 1938 was the Kaiser-Wilhelm-Institut für Chemie in Berlin. There the old team of Meitner and Hahn had been reestablished at Meitner's initiative to follow up the discoveries of the Rome group.[26] By 1937 Hahn, Meitner, and their chemist colleague Fritz Strassmann had found and systematized no fewer than nine activities arising from the irradiation of uranium by neutrons. Three of these activities they ascribed to uranium itself, to a complex U239 capable of decaying in three different modes, each producing a different activity. It was as if U238 could catch three distinct lethal diseases by swallowing a neutron; that the offspring inherited the form of death of its parent; and that the curse could be followed to the fifth generation. The members of this highly degenerate family, all linked by beta decay, are named in figure 9.5. The first two chains were known to be initiated by both fast and thermal neutrons, the third only by fast neutrons of a narrow range of energies, which "resonated" with the uranium nucleus.[27]
This scheme, which dates from 1937, evidently requires U239 to exist in three isomeric states. When Meitner first suggested the possibility in 1936, she could appeal only to the isolated example of Hahn's double isomer in protoactinium; but by 1938 several other examples, among them the isomers of zinc, manganese, and tellurium discovered at the Laboratory, supplied indirect support for the possibility of a multiple U239 . Even the successive decays had a partial analogy around the middle of the periodic table in what Lawrence called an "induced chain reaction," for example, Cd® In® Sn. The apparent inheritability of the decay modes of uranium, however, and the lengths of the chains in two of them, had no analogue among known radioactive processes.[28] Another problem, which haunted all experiments with "transuranics," was their chemical identity. The teams of Rome and Berlin, as well as Irène Curie's in Paris, whose work was to topple the multiple isomers, all worked with natural sources of neutrons. The
Fig. 9.5
The standard misunderstanding of the relations among the "transuranics"
before 1939. After Meitner, Hahn, and Strassman, Zs. f. Phys., 106
(1937), 249.
consequent weakness of their sources, as well as the radiochemical complexity of the material they studied, which included the natural descendents of uranium as well as transuranics and fission products, made it very difficult and tedious to determine the genetic relations of the fleeting products and their likely places in the unknown periods beyond uranium in the table of the elements. Despite the many uncertainties, however, most radiochemists and nuclear physicists throughout the world accepted the transuranics at face value.[29] The common consequence of neutron irradiation of the nucleus was capture followed by beta decay; the most cataclysmic consequence, the release of an alpha particle. Cleavage, the division of a heavy nucleus into approximately equal parts, did not come into serious consideration, even though Bohr's analogy of a nucleus to a liquid drop, which readily accommodated the concept of fission in 1939, was available from 1936 and widely advertised by Bohr on a lecture trip to the United States in the spring of 1937 (fig. 9.6). Whence the blind spot? "Because," as the theoretical physicists understood the matter two years before the discovery of fission, "because the disintegration of an element into two approximately equal halves is so very improbable a process."[30]
Fig. 9.6
Bohr's playful analogy to the capture of a particle by a nucleus. The marbles in
the dish represent the bound nuclear constituents, that on the edge the
incoming particle. The newcomer's energy will quickly be shared by collision
among the constituents. Bohr, Science, 86 (1937), 161.
The Paris group began to fish in the muddy waters of transuranics in 1935. By 1937 they–the fisherfolk were Irène Curie and Paul Savitch—had caught a big one, a 3.5-hour activity formed from uranium by neutron capture; since this R3.5h , as they dubbed it, did not have the chemistry of uranium or any "transuranium," they made it (temporarily) an isotope of thorium, according to U238 (n,a )Th235 . This was to produce still another isomer of U239 with its own special plan of decay. To complicate matters further, Curie and Savitch observed that nothing seemed to prevent cross decays between members of the first two chains of figure 9.5.[31] Hahn and Meitner could not find a radioactive Th235 in their irradiated uranium; Curie and Savitch tried some chemistry on theirs and decided that R3.5h behaved like a rare earth. The only possibilities, they thought, were actinium, which does resemble the rare earths, or a new transuranic quite distinct in chemical properties from the substances investigated by Hahn and Meitner. Either assignment, as they said, faced serious difficulties. In the late spring of 1938, they eliminated one possibility by showing that
R3.5h followed lanthanum rather than actinium. "It seems therefore that this body can only be a transuranic with properties very different from those of other known transuranics."[32]
These properties had been established largely on the supposition that the elements beyond uranium would be homologues of the elements beneath them in the periodic table: 93 was expected to behave like rhenium, 94 like osmium, and so on. This was the scheme generally preferred by chemists. To physicists, however, another alternative lay open: transuranics have the properties of uranium or actinium and become more like rare earths with increasing atomic number. The possibility of a second rare-earth series beyond uranium fitted Bohr's version of the periodic table (fig. 9.7), which had in its favor the prediction, confirmed in 1922 against the opinion of chemists that the then unknown element 72 was not a rare earth. By following the chemists instead of Bohr, Curie and Savitch could find no place in the periodic table near uranium suited to receive R3.5h .[33] No more could Hahn, Meitner, and Strassmann, who, having convinced themselves by chemistry that element 93 did not resemble either of the alternatives suggested by Bohr, actinium or uranium, accepted by default the common opinion of chemists and assimilated it to rhenium.[34]
The analytic chemist of the Berlin team, Strassmann proposed to derive R3.5h from uranium in three steps: two successive alpha emissions to produce radium followed by a beta decay to the nearest rare-earth homologue, actinium. He and Hahn had a look; Meitner had fled Germany in July 1938, just after Curie and Savitch grudgingly made R3.5h transuranic. Hahn and Strassmann found more than they wanted. With barium as a carrier, they identified three sorts of isomeric radium, giving rise to three different heritable decay sequences of the type indicated in figure 9.5. The first product in each sequence had a period of a few days. No doubt, according to Hahn and Strassmann, the incompetent French team had confused the several actinium isotopes in
Fig. 9.7
Two methods of visualizing the chemical allegiance of element 93. 9.7a: as a
homologue of the rare earths, as in the now accepted "actinide" series. 9.7b:
as a homologue of the heavy metals, the view most widely approved in the late
1930s. Quill, Chem. rev., 23 (1938), 101.
their R3.5h activity. But the basis of the Berlin scheme, (n,2a ), seemed scarcely plausible after experiments proposed by Meitner showed that even thermal neutrons of vanishingly small energy provoked it.[35] Several physicists, including Bohr, "expressed their astonishment that slow neutrons should initiate two successive alpha-processes in uranium." Hahn and Strassmann looked again, found another radium and actinium isomer, and struggled to separate their many radiums from barium.[36]
After several weeks of painstaking effort, they decided to quit. We are indebted to the Nazis for the opportunity to follow the dawning discovery of the cause of their failure in detail, in letters from Hahn to his absent partner. On December 19, 1938, he wrote Meitner: "There is in fact something so remarkable about the 'radium-isotopes' that we are now only telling you. . . . The fractionization did not work. Our Ra-isotopes behave like Ba." Their brilliant chemistry had landed them in a situation Hahn had considered desparate. "Perhaps you can suggest some fantastic explanation. We know ourselves that it [uranium] really cannot fall apart into Ba. . . . So, think." It was too much for Meitner. "The assumption of so wide-reaching a disintegration seems very difficult to me at the moment, but there have been so many surprises in nuclear physics that one can not reject anything immediately as impossible." This letter crossed another appeal from Hahn: "We cannot keep silent about our results, even if they are perhaps physically absurd. You see, you would do a good piece of work if you could find a way out."[37] When Hahn wrote these lines, his secretary was typing his and Strassmann's epoch-making announcement of their dilemma. "As chemists, we must rename [our] scheme and insert the symbols Ba, La, Ce in place of Ra, Ac, Th. As nuclear chemists closely associated with physics, we cannot yet convince ourselves to make this leap, which
contradicts all previous experience in nuclear physics."[38]
This cry of schizophrenia contrasts with the smooth interdisciplinarity of Lawrence's Laboratory. Lawrence had asked indifferently, "Shall we call [our new science] nuclear physics or shall we call it nuclear chemistry?" It did not require an answer in Berkeley, where interdisciplinarity was attained by addition—at first by the physicists' acquiring a few techniques of the analytical chemist, subsequently by acquiring chemists, then physicians and physiologists. In Berlin, each member of the team had to adjust to the professional constraints of the others. The peculiar intensity and quality of the collaboration of Hahn, Meitner, and Strassmann, which was more than the sum of its parts, may explain why the discovery of fission occurred in Berlin, and not in Rome, Paris, or Berkeley.[39]
On the proofs of the paper disclosing their conflict of allegiance, Hahn inserted what, in a letter to Meitner, he dubbed "a new fantasy." "We have not proved that the transuranics are not Ma [Tc], Ru, Rh, Pd. . . . Would it be possible for uranium 239 to disintegrate into a Ba and a Ma? A Ba 138 and a Ma 101 would give 239. . . . If there is anything in this, the transuranics, including 'ausonium' and 'hesperium' would be dead. I do not know whether or not that would make me very unhappy."[40] The geographical separation of the group decomposed it into a chemical division (Hahn and Strassmann) and a physical one (Meitner and her nephew Frisch, who came from Bohr's institute to spend the Christmas holidays with her in Sweden). Each side approached the problem from its disciplinary viewpoint; no doubt the menacing and unstable social and political circumstances made freshly attractive the relative security of firm professional indentifications. As a physicist, Meitner could conceive that a splitting of the uranium nucleus might be possible, although not into Ba and Ma; as a former full participant in the team, she could not disbelieve in the sequences (2) and (3) of figure 9.5, and hence in transuranics; and, as a refugee, she feared that "it would be no good
recommendation for my new beginning to have to repudiate three years of work." In any case, she said, and quite rightly, that she had good reasons to suppose that the 23-minute uranium of sequence (3), the one excited by resonance capture, should give rise to a transuranic.[41]
It was Meitner who first raised the problem of the relation between the transuranics and the apparent production of barium from radium. While Hahn and Strassmann worked urgently to confirm by every chemical means that their "radiums" were barium, Meitner and Frisch, who were the only physicists informed of their progress, worked out that "fission," as they called it, could easily [!] be interpreted in a purely classical, nonquantum manner as the sundering of a distended droplet-nucleus. On this assumption, Hahn and Strassmann's bariums (atomic number 56) and their lanthanum descendents should be accompanied by radiokrypton (atomic number 36 = 92 – 56) and its relatives. Frisch had confirmed the general process of fission by irradiating a thin layer of uranium with neutrons from a Rn-Be source and catching the heavily ionizing fission fragments, which he estimated to have an atomic weight of at least 70, in an ionization chamber. The demonstration further menaced the transuranics. Meitner and Frisch put forward as "rather plausible" that the long sequences (1) and (2) of figure 9.5, which "always puzzled us," should be assigned to isotopes of technetium (atomic number 43) decaying through a chain terminating with cadmium (atomic number 48).[42] This interpretation was very materially strengthened in March 1939 when Meitner and Frisch, together using the high-tension set at Bohr's institute, collected enough of the recoiling fission products to follow radioactive decays. They obtained curves very similar to those the united Berlin trio had for sequences (1) and (2) of the transuranics: "the 'transuranium'
periods, too, will have to be ascribed to elements considerably lighter than uranium."[43]
Meitner delayed conveying this news to Hahn in order not to upset his birthday celebration. That was because Hahn and Strassmann, having been forced to recognize fission as chemists, had, on the same professional qualification, declined to relinquish the transuranics. Contrary to their mop-up of the "radium" decays, in which they proved conclusively that they dealt with barium and also found decisive evidence of the presence of the associated radiokrypton, they could not find any sequences of light elements chemically identical with the long transuranic lines.[44] Here the poverty of their means, the limitation of their methods, the complexity of the situation, and the desire to preserve what they could of their earlier results made an impossible barrier. Lacking a strong artificial source—had they only had a cyclotron![45] —they could not produce fission products in sufficient quantities to unscramble their decays or recognize the extent of the problem before them. Looking back in 1960, Hahn and Strassmann could identify with certainty only the 10-sec and 40-sec "uraniums" as mixtures of Xe and Kr; the 66-hr element "95" as 78-hr Te; and the 2.5-hr element "96" as 2.26-hr I. The others, according to a repetition of the experiments of 1938 made in 1971, were quite complicated mixtures, entirely unresolvable by the chemical means earlier available.[46]
By insisting upon ascribing each of their "transuranic" activities to a distinct isotope and clinging as long as they could to the sequence of decays they had worked out in 1938, Hahn and
Strassmann could not identify their "transuranics" with any lighter elements by chemical procedures. And they refused to return to the schizophrenia of interdisciplinarity: "We have in no way touched on physics in the entire business, but have only done chemical separations over and over again. We know our limitations and also of course that in this particular case it makes good sense to do only chemistry."[47] They accordingly continued to hold to the existence of the three isomeric heavy uraniums and the long transuranic sequences for at least four months after their discovery of fission had made all the earlier attributions doubtful. Although they recognized the force of the demonstration by Meitner and Frisch, they preferred not to believe it: "We can find no holes in your interpretation, and on the basis of your results must really kill the transuranics. But for us—Strassmann and myself—this result is completely incomprehensible. For we have not been able to say what these transuranics can be. . . . In any case you have the first result, based on experiments, not on vague conjectures, that will be entirely clear for a physicist. . . . Are you really sure that you have our transuranics in the [material you tested]?"[48]
On January 3 Frisch told Bohr about the "small bomb" he and his aunt were giving physicists as a New Year's gift. "Today I was able to speak to Bohr about the exploding uranium. The conversation had lasted only five minutes when Bohr agreed with us in everything. The only thing he thought remarkable was that he had not thought of the possibility earlier, since it follows so directly from current ideas about the structure of the nucleus."[49] Nine days later Frisch started his search for fission fragments. In four days he had his "conclusive" physicist's proof; in nine days, printer's proofs. Meanwhile Bohr had set sail for Princeton, where he was to stay for several months. Wishing to preserve the confidence of Frisch and Meitner until their papers were published, he did not report either the concept of fission or the detection of the fragments, of which he was informed by cable; but his assistant Léon Rosenfeld, unaware of the confidentiality, disclosed
both. Bohr then made the results public on January 26, at the opening session of the fifth Washington Conference on Theoretical Physics. The newspapers picked up the story and accelerator laboratories from Princeton to Berkeley moved into the new fields at American speed.[50]
Fermi, established at Columbia, turned to "the uranium split business with which half the world seems to be occupied . . . as soon as the cyclotron gave a beam." His search for the fragments found by Frisch was rewarded on January 25. The following day two sets of experimenters, one from the Carnegie Institution and the other from Johns Hopkins, rushed to their laboratories immediately after leaving Bohr and saw the fragments before the Washington Conference ended on January 28. Both used d-d neutrons from high-tension accelerators and both confirmed, by liberal application of paraffin and cadmium, that uranium fission occurs with fast and with thermal neutrons.[51] The Columbia group took more time to publish, employed a Rn-Be source of known intensity in the manner of Amaldi and Fermi, and measured the relative probability of fission by thermal and fast neutrons as 20 to 1.[52]