Little-Team Research with Big-Time Consequences
Almost all the Laboratory's work in physical science centered on or developed from the two main concerns with which it began: machine design and nuclear transformations. Few people stayed long enough or became independent enough to use the Laboratory's facilities to pursue significant topics outside this framework. The most successful in making room for himself was Luis Alvarez, whose discovery of K-electron capture won the approval of that otherwise merciless critic of Laboratory life, Maurice Nahmias. "I am very happy for him because he is the most likeable [of the staff] and does not waste his time with 'new periods.'" But the study of K-electron capture lay so close to the Laboratory's established activities that it was immediately assimilated into period hunting. Alvarez's greatest departures from general practice concerned properties of the neutron, involved collaboration with Stanford, and did not open continuing research lines at Berkeley.
Like Alvarez, McMillan had the freedom and the obligation of the young professor to develop his own research lines. He tended to extend, systematize, or make more precise research domains at the center of the Laboratory's work: for precision, his early close studies of the energies of gamma rays emitted in certain nuclear transitions; for systematization, his up-to-date files on nuclear transformations; for extension, his theory of cyclotron focusing.
McMillan's mastery of all aspects of the Laboratory's nuclear physics and much of its chemistry gave him a decisive advantage in following up the discovery of nuclear fission. It was he who secured the first beachhead in the new territory of transurania. He was accompanied by several accomplished investigators, including Segrè and Seaborg, who annexed the field for the Laboratory.
The neutron was a frustrating object in the laboratory. It went everywhere, refusing obedience to the electric and magnetic fields to which other particles submitted; it provoked a wide—indeed, too wide—range of nuclear transformations; and it was dangerous to living things. The danger, as we know, inspired the Laboratory to invent neutron therapy and caused all large-cyclotron laboratories to surround their machines with a protective barrier, usually of water. Most of the neutrons entering the water quickly slowed to thermal velocities by sharing their energy in collisions with hydrogen atoms and either died in their bath or emerged relatively harmless into the experimental space. The promiscuity of neutrons—their easy union with most nuclei—usually increases as their velocity diminishes, as Fermi had found; in order to explore nuclear responses to their wanton behavior, it was very desirable to work with neutrons of uniform speed. Interest in making homogeneous or "monochromatic" beams of neutrons grew strongly in 1936, when Bohr offered his liquid-drop model of the nucleus in explanation of the anomalously high absorption by certain substances of bombarding particles of particular energies. To study his idea further—to find the energies of particle beams with which various nuclei "resonated"—required control of the beam velocity. And just here the chargeless neutron, the most promising tool for probing resonances, eluded the will of the experimenter.
Although neither the cyclotron nor its keepers at Berkeley were temperamentally adapted to the careful study of resonances, control of neutron beams was of central interest there. Shortly after Fermi's discovery of the power of slow neutrons, several members of the Laboratory discussed production of a roughly monochromatic neutron flux by interrupting the deuteron beam to the
cyclotron's beryllium target. The general idea, according to Alvarez's reference to it in 1938, was to shut off the radio frequency potential intermittently and to turn on an amplifier attached to a neutron detector when the accelerating potential cut out. The only neutrons counted by the chamber would be those created in the beryllium during the deuteron bursts (radio frequency on) with such velocities that they could reach the detector during its live time (radio frequency off). The idea was temporarily dropped when the Laboratory learned of a rough-and-ready adaptation of an old technique to the new purpose. Physicists at Columbia ran the output from a Rn-Be source through paraffin and then into a chopper consisting of two disks, each made in alternate sections of duraluminum and cadmium, which are, respectively, transparent and opaque to slow neutrons. When the disks spin around the same axis, only slow neutrons that passed through a transparent section in the first disk with a speed that brought them to a similar section in the second could reach the detector.
The urgency of the study of resonance absorption—"the point of greatest interest today in nuclear physics," to quote British opinion in the fall of 1938—reopened the matter. The neutron chopper did not give monochromatic rays, among other reasons because fast neutrons could pass right through the disks. Two new solutions to the problem were offered that same fall, one by Alvarez, who developed the scheme discussed in the Laboratory in 1935, and the other by a group around G.P. Thomson at Imperial College, London. By then both parties had been working at their projects for some time. A team at the University of Utrecht may
also have hit on a solution independently, although their first published description of their method was prompted by a preliminary report of Alvarez's.
The machines built by Thomson and by Alvarez highlight typical features of British and California approaches to instrumentation. The Imperial College group used as source Oliphant's version of the Cockcroft-Walton machine, capable of 150 to 250 kV; Alvarez used the 37-inch cyclotron, then delivering deuterons at 8 MeV. Alvarez interrupted his deuteron beam by suppressing the current at the plates of the rf tubes supplying the accelerating potential; the method required considerable electrical power and some electronics and yielded long deuteron bursts (4 msec) that were not very well defined. The British modulated the current through the discharge tube that created the deuterium ions by amplifying the current from a photocell activated by a light beam interrupted by a tuning fork or rotating shutter; it required little power and gave bursts of about 0.5 msec.
Both parties sent the neutrons from the beryllium target through a standard paraffin "howitzer" (fig. 9.1) and into a cadmium-lined pipe to remove stray particles (cadmium has an extraordinary appetite for slow neutrons); and both employed as primary detector the by then standard BF3 ionization chamber activated by alpha particles from the reaction B10 (n,a )Li7 . Alvarez fed the output from his chamber into an amplifier regulated by an elaborate electronic timing circuit to process pulses only during a short time after each deuteron burst; by setting the time in accordance with the distance from the beryllium to the detector, he could arrange that only neutrons of (or, rather, around) a selected velocity would be counted. The British continuously applied the output of their chamber and fed the result to an oscilloscope, which indicated the times of the deuteron bursts as well as the times of the associated chamber pulses. A motion picture of the oscilloscope traces preserved them for analysis. The Dutch competition planned to work much in the British manner, obtaining their neutrons from a d-d reacton in a HT tube, modulating the accelerating potential, and synchronizing the detecting amplifier
and oscilloscope with the modulator. They criticized Alvarez's method for permitting the detection of only one neutron velocity at a time.
This property made Alvarez's instrument what might be called a monochromator: although its chamber received an inhomogeneous beam, the amplifier insured that only neutrons of the selected velocity were counted. Did this mean that the experimenter could consider that he had a homogeneous beam? A deep question that, or perhaps only a matter of words: it called forth expression of a philosophy of science, a very rare, and perhaps unique utterance from the Laboratory in the 1930s. "On the operational viewpoint," Alvarez wrote, in reference to the then widely accepted teachings of Percy Bridgman, "one is justified in asserting that the beam is composed solely of thermal neutrons." The British had no need for Bridgman's philosophy. Their instrument was a velocity spectrometer: the intervals between the chamber pulses and the associated deuteron bursts showed the velocity distribution among the slow neutrons reaching the detector. Their cumbersome method of oscilloscope recording and analysis worked only for small fluxes, however. They ordinarily counted about 25 neutrons a minute. Alvarez's Berkeley rate was 1,000/min.
The definitive form of the neutron velocity spectrometer was made by R.F. Bacher and his students at Cornell. It followed the British by modulating the deuteron source and Alvarez by measuring timed neutrons through a system that split the signal from the BF3 detector, routing one half to a continuously active amplifier and the other to one alive only when neutrons of the velocity of interest arrived. They obtained very nice records of resonance that confirmed consequences of Bohr's nuclear theory as calculated by Bethe. And—a matter that proved more important to the common man—they measured neutron absorption in the isotopes of uranium. Their design was further perfected at Illinois and at
Stanford in 1941. By then the British team had long since retired into the scientific war against Germany.
Alvarez continued with neutronics in collaboration with a refugee from Germany's war against science, Felix Bloch, who had joined the physics department at Stanford in 1934. Their experiment—the exact determination of the magnetic moment of the neutron—was a perfect matching of skills, interests, and ambitions. Bloch had given its theory in 1936, after a visit to Heisenberg that convinced him that Stanford's deepest and quickest route to a place in nuclear physics was via an experimental program in neutronics. He discussed the measurement at the periodic joint Stanford-Berkeley physics seminar and interested Laslett and Alvarez, who mentioned it as a possible application of his monochromator. The business did not seem overly promising, however, since earlier attempts to detect magnetic behavior in the neutron had failed and the very concept of a magnetic moment of an uncharged particle smacked of oxymoron. There seemed no other explanation, however, of the results obtained by Otto Stern and his associates O.R. Frisch and Immanuel Estermann with their hydrogen beams in Hamburg.
According to classical theory, the ratio between the magnetic moment µ and the angular momentum p of a spinning sphere of charge e and mass M is µ/p =e /2Mc . One might expect therefore that in quantum theory the magnetic moment of an elementary charged particle with angular momentum sh /2p , where s is its spin quantum number, would be µ=(eh /4pmc )s . But in the case of the electron, spectra required both µe =eh /4pMc and s = 1/2; a contradiction supressed by introducing a numerical factor g into the relationship, µ/p =gee /2mc , and setting ge = 2. The Dirac theory of the relativistic spinning electron produced spontaneously, among many other marvels, the value ge =2. It was therefore assumed that if the theory applied to protons, sp =1/2, gp =2, and the relevant magneton, let us call it µ0 , would be smaller than µe in the ratio of the masses of the electron and proton, m/M . But in 1933 Stern and his associates announced that the proton
moment, µp , amounted to around 5µ0 . A quick measurement on the heavy water sent by Lewis showed that the moment of the deuteron was smaller than the proton's, although its spin was twice as great. By 1936, owing to further work by Stern in Pittsburgh, where he set up after the Nazis closed his institute, and by Rabi, who greatly improved the technique, the magnetic moments of proton and deuteron were established at between 2.5 and 3.0 µ0 and around 0.8 µ0 respectively.
The obvious way to square the numbers was to assume that their difference measured the moment of the neutron. Since the deuteron was known to have a spin of 1 and since indirect evidence made the neutron's spin 1/2, it appeared that the spin, and hence the moment, of the deuteron's constituents added together, whence µn » –2µ0 , the minus sign signifying the relationship between spin and moment characteristic of the electron. But how to explain that µn does not equal zero? Following a suggestion made by G.C. Wick, who drew on Fermi's theory of beta decay, Bloch and other theorists supposed that the neutron spends some of its time dissolved into an electron and a proton and that in this state "it" can interact with a magnet and so show a moment. Since µe» 2000µ0 , the neutron need not spend much of its time in pieces in order to show an average moment of –2µ0 . Since Fermi's theory treats protons and neutrons on the same footing, the excess moment of the proton was supposed to arise from its temporary disaggregation into a positron and a neutron. Since by the symmetry of the theory, this excess should be equal and opposite to the apparent neutron moment, µp + µn » µ0 , which came close to the deuteron moment.
All this was, of course, only inference, an effete proceeding necessary, perhaps, in astrophysics, but unmanly with objects produced by the billions in the laboratory. Bethe supposed that knowledge of the neutron moment would continue to come exclusively from such indirect arguments: "the magnetic moment of the neutron is hardly accessible to direct measurement." Bloch thought otherwise and offered a calculation of the effects of µn on
the scattering of very slow neutrons. The calculation suggested that µn might be deduced by scattering slow neutrons from magnetic atoms and by passing them through thin magnetized plates. (Slow neutrons are required in order that their wavelength have about the same size as the atoms scattering them.) Since the total transmission through the plates should depend upon their relative magnetization, Bloch hoped that with such a setup he could obtain a direct indication of the existence of the neutron's suppositious moment.
Bloch set something going. Bethe and Livingston organized an experiment at Cornell that showed a 2 percent difference in transmission through plates with parallel and antiparallel magnetism and calculated that the result was not incompatible with Bloch's theory and µn = –2µ0 . Julian Schwinger then published a long calculation treating neutron scattering by Dirac theory that predicted a larger effect in the transmission experiment than Bloch had computed. In commenting on a draft of Schwinger's paper, Bethe, fresh from his try at µn , reaffirmed his view that direct measurements were not likely soon to improve upon subtraction of µp from µd : "It will be a long time before the direct determination will give the neutron moment to anywhere near this accuracy," which he reckoned at 0.15 µ0 . Next, Rabi proposed a refinement in his method of spin flipping that J.R. Dunning and his students at Columbia immediately adapted to Bloch's transmission experiment. They showed that the transmission of neutrons through a thin magnetized plate increases with the magnetization and thickness of the plate and with the slowness of the neutrons and that Rabi's method could partially unpolarize the partially polarized beam emerging from the first plate (fig. 9.2b).
Then, with the help of further calculations by Schwinger, they deduced from the amount of transmission with and without spin flipping that µn is negative and lies between 1 and 3 µ0 . No revelation that; and, because they could not determine with any accuracy the strength and variation of the magnetic field causing the flips, they could say no more. "Further refinements are in progress."
While Dunning's associates were flipping neutrons Rabi's way, a group at Copenhagen led by Frisch, who had settled into Bohr's institute in 1934, independently suggested the use of a depolarized field in an experiment of Bloch's type. They worked with small fields and smaller effects; they could count only about 100 transmitted neutrons/minute from their weak Rn-Be source; they
learned nothing more than that a value of µn = –2µ0 was not incompatible with their experiments, and they gave up. "It would be hopeless to discuss these results any further and to try and enclose the magnetic moment of the neutron between definite limits." Meanwhile Bloch, who did not agree that Schwinger's calculation came closer to reality than his, had proposed to Laslett that he try to find effects of µn by scattering off nickel and iron. Laslett went to work at about the same time the Cornell group did; unlike them, he found no positive results, nothing that would permit any useful quantitative statement about µn . Bloch himself tried to detect something useful in the scattering of neutrons (obtained from d-d synthesis) on cobalt. But in the fall of 1937 many, perhaps most, nuclear physicists shared the "pessimism about the moment of the neutron"—the belief that experimental difficulties of direct determination of µn would not soon be overcome—expressed by Bloch's friend Egon Bretscher.
In June 1938 Bloch and Rabi, then visiting Stanford, went to Berkeley to watch Alvarez demonstrate his neutron monochromator (plate 9.1). Bloch returned to Stanford, to have a try at µn with a Rn-Be source; but he did no better than others, and came to think that useful quantitative results could only be obtained "when an intense neutron source is available that will make it possible to use monochromatic neutrons." In September, he and another émigré, Hans Staub, decided to build a high-tension apparatus for the purpose by exploiting a disused 170 kV x-ray outfit and the d-d reaction; but before they had finished, as Staub tells the tale, Bloch "quite unexpectedly [!] got the apparatus" to work with Alvarez and the Berkeley cyclotron. Was the surprise
Lawrence's willingness to allow a lengthy bit of physics to tie up the machine? Alvarez and Bloch began to work together in the late fall of 1938. The final report of the Columbia measurements (µn is "probably" around –2 ± 0.5µ0 ), published about that time, gave them an easy mark to better. It is doubtful that the news that Frisch planned another go, with the enthusiastic support of Bohr, would have caused Alvarez and Bloch any anxiety. They had the barren field of exact neutron magnetonics to themselves.
Their great advantage over earlier investigators was the neutron flux from the 37-inch cyclotron. Whereas the Columbia and Copenhagen groups had counted a few million neutrons, Alvarez and Bloch counted two hundred million in their year of experimenting. The hundredfold increase in beam allowed them to detect and correct many subtle instrumental effects that menaced their measurement; although for their definitive value of µn they counted only four million neutrons, a number readily obtainable from a good Rn-Be source, they needed the previous two hundred million, which could have been obtained only from an accelerator. The final experimental design is shown schematically in figure 9.3.
Neutrons from a beryllium target, struck by deuterons and slowed down by passage through paraffin, ran down a cadmium-lined tube stuck through the water shielding around the cyclotron. The
paraffin so successfully removed unwanted fast neutrons that it proved unnecessary to use Alvarez's modulated beam as originally intended; and the cadmium swallowed slow neutrons so effectively that the entire arrangement introduced a strong collimated beam consisting primarily of slow neutrons to the thin piece of iron that, when magnetized, acted as a polarizer. The design profited perhaps from the contemporaneous work of Aebersold on the collimation of neutron beams for therapy.
The partially polarized neutrons entered the flip space, where they felt two magnetic fields: a constant one, H0 , to provide direction for the orientation of the magnetic moments, and an oscillating field, H1 , to incite the flips. The analyzer consisted of a second piece of iron in a second powerful electromagnet. The detector was the standard BF3 chamber. The electromagnets at either end were borrowed, one from a colleague who used it for Zeeman spectroscopy, the other from Shell Development Company; the coil energizing the field H0 had been cannibalized from the 11-inch cyclotron and the solenoid that made H1 was wound from flat copper strips through which the neutrons passed in entering and leaving the flip region. The currents energizing H0 and H 1 were kept regular by tapping them from the cyclotron's automatically stabilized supply.
A measurement consisted of reading the number of neutrons registering in the detector as H 0 swept through the value H n that maximized the number of flips. (This occurs when fn , the frequency of the oscillating field, equals the frequency with which the neutron moments process around H0 .) The underlying concept: with H 1 off and polarizer and analyzer magnetized in parallel, a certain flux of neutrons will be counted; with H1 on, a fraction of the neutrons will have their moments reversed, somewhat fewer than before will pass the analyzer, and a smaller flux will register. When this reaches a minimum, resonance obtains and µn comes immediately from the value of the precessional frequency: µn = fnh /2Hn . Or, rather, it comes immediately after the values of fn and Hn have been determined. Alvarez and Bloch invented a method of measuring these quantities that allowed greater
accuracy than standard methods would have given. Write µn = gn µ0 , gn being the value of the neutron moment in nuclear magnetons. From the resonance condition of the experiment, fn = 2Hngn µ0 /h . But from the resonance of protons in the cyclotron, fp = eHp /2pMc , we have fp = 2Hp µ0 /h ; hence gn = (f n /fp )(Hp /H n ). The measurement reduces to finding the ratios of frequencies and fields, which was not difficult, and requires no knowledge of the values of the absolute constants. In effect, the cyclotron supplies the values of e/Mc .
The counting itself was done electronically via a clever circuitry that monitored the unsteady neutron output of the cyclotron. (At best the cyclotron ion beam could be held constant to 1 percent; Alvarez and Bloch worked to an accuracy of a tenth of a percent.) The circuit divided the counting time into intervals of a few seconds during which the flipping fields were alternately on and off. When off, the amplified output of the BF3 chamber was routed to one of a pair of counters; when on, to the other; comparison of the two allowed correction for fluctuations in the initial beam strength. After making this correction, Alvarez and Bloch got the nice sharp dip of resonance indicated by figure 9.4. With the values of frequency and field thus implied, they obtained g n = –1.935, which they judged to be good to about 1 percent. At the time the latest Columbia values for µp and µd were 2.785µ0 and 0.855µ0 respectively, both to an accuracy of 0.7 percent. Consequently, to within experimental error the old relation, µd = µp + µn , still held, although it seemed very unlikely that the intrinsic moments of the constituents should not be altered to some extent by their combination. It was primarily to achieve these measures and amplify their effects that Bloch and his co-workers undertook to build a cyclotron at Stanford. His subsequent
development of a technique for precise measurement of nuclear magnetic moments, and the work he did with it, brought Bloch the Nobel prize for physics for 1952, a handsome payoff for his switch to experimental neutronics sixteen years earlier.
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." 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." 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.
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. 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.
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. 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
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. 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."
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. 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."
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 . 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.
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
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. 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.
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." 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."
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.
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." 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.
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). 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."
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. 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! —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.
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." 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]?"
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." 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.
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. 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.
Following up Fission
Back in Berkeley
When news of fission reached the Laboratory through the daily press about January 29, the staff could not be kept at the assembly of the 60-inch cyclotron, which had been its preoccupation. "All of us couldn't resist our curiosity to look into the matter a bit."
There were various approaches. Oppenheimer proved that fission is impossible. Alvarez wired East for further information, learned that Tuve's group had detected fission fragments, and immediately did the same, for the benefit of Oppenheimer and others who would not accept "so revolutionary [an effect] . . . until it had been confirmed in several laboratories." Alvarez and his collaborator, G.K. Green, in Berkeley as a National Research Fellow, saw the effects of thermal and fast neutrons and went beyond other hasty confirmers by estimating the time delay between irradiation and fission. They obtained an upper limit of 0.003 sec using a pulsed neutron beam modulated in Alvarez's manner; the limit was soon lowered to 0.001 sec by the intermittent neutron generator at Imperial College. They all missed the so-called "delayed neutrons," emitted a few seconds after cessation of the irradiation in a small percentage of fissions. These dilatory particles, ejected during the rearrangement of fission fragments, give a handle for the control of the fission process.
Green and Alvarez used Alvarez's preferred detector, a fast electronic affair, a thin-walled ionization chamber (on one of whose plates the uranium sat) connected to a linear amplifier and oscilloscope. Dale Corson, then in charge of the Laboratory's cloud chamber, teamed with Thornton to see fission fragments in a slower, but more evocative, way. They put uranium oxide on a collodion foil in a chamber filled with air, water vapor, and alcohol; irradiating the whole with neutrons procured from the 37-inch cyclotron, they obtained the nice result shown in plate 9.2. Assuming the short side tracks to belong to recoiling ions from the chamber gases, they inferred that the lower fission fragment must have had an atomic weight of at least 75 to have continued its course without apparent deviation. Two weeks after Corson and Thornton sent off the best of their picutures to the Physical Review
(they could choose among 25 obtained in 885 exposures to cyclotron neutrons), Joliot, a past master of cloud chamber technique, exhibited at the Paris Academy of Sciences the single photograph of a recoiling fission product that he had got in 902 exposures to Rn-Be neutrons. To physicists, the several quick observations of strongly charged, heavy particles constituted "conclusive evidence" of the phenomenon brought to light by Hahn and Strassmann after months of tedious, masterful analytical chemistry.
Seaborg heard about fission at the Journal Club meeting on January 30. "The news excited me very much. After the seminar, I spent hours walking the streets in Berkeley, chagrined that I had not recognized that the 'transuranic' elements . . . were not really 'transuranic' elements; I felt stupid." (He had judged the results of Curie and Savitch to be "rather strange.") His follow-up, or, rather, that of Joseph Kennedy, to whom he assigned the task, was not a chemical but a physical investigation, a search for the fast beta particles that they thought might be released in the successive transformations of the fission products. Kennedy spent some weeks at the quest, but to no avail. Nor could fast beta particles be detected concurrent with fission. They gave up and notified their colleagues of their failure.
In March, reinspired by the "elegant chemical separations" and "startling conclusions" of Hahn and Strassmann's definitive demonstration that uranium can give birth to barium, Kennedy and Seaborg tried another way to obtain information about the fission fragments. Once the chemical nature of one partner in a fission is known, the nature of the other follows from subtraction of atomic numbers; but a similar inference from an isotope of the one to an isotope of the other is not possible if free neutrons come out from a bursting uranium nucleus along with the fission partners. Kennedy and Seaborg looked for evidence of these neutrons by placing a Ra-Be source on the axis of a cylinder,
surrounding it with two concentric shells, the inner of uranium oxide and the outer of water, and counting the neutrons reaching the water's surface. They could not confirm their expectation that more neutrons should reach their detectors with the uranium in place than without it.
There was another and more urgent reason for desiring to know not only whether, but also how many, free neutrons may be emitted during fission. Should the number be large enough, the process of fission, once instituted by the energy of the physicist and his machines, might continue on its own, perhaps with explosive violence. That had been plain to the Columbia group the moment they had confirmed Frisch's experiment. "Here is real atomic energy!" Dunning wrote in his laboratory notebook on confirmation day. And on the morrow, in the same exclamatory style: "Secondary neutrons are highly important! If emitted would give possibility of self perpetuating neutron reaction." Many others saw the same: the omniscient Fermi; the busy Szilard and Szilard's sour partner Arno Brasch, who twitted him about the now worthless patents on chain reactions with which he had hoped to direct the course of events. At Berkeley they also understood. "We are trying to find out whether neutrons are generally given off in the splitting of uranium; and if so, prospects for useful nuclear energy become very real!" "It may be that the day of useful nuclear energy is not so far distant after all." Alvarez spearheaded the hunt, though not with his customary thoroughness. He directed his beam of slow neutrons into a bottle of uranium oxide and looked for fast neutrons arising from fission. He saw none. He also tried, with the help of a chemist, Kenneth Pitzer, to detect an increase in the temperature of uranium irradiated by neutrons. Again the experiments were inconclusive. They were resumed by Malcolm Henderson, back in Berkeley for the
summer, who detected heat enough to indicate that the fission fragments recoiled with an average energy of around 175 MeV, in agreement with other estimates made in other ways. No further efforts to detect fission neutrons at the Laboratory have come to light, although Lawrence said publicly that the possibility of obtaining energy from uranium depended upon the number of neutrons released in fission.
His lethargy in prosecuting this possibility, with which he had been flirting for several years, may be accounted for as follows. For a change, time was more precious than money. Lawrence preferred not to delay completion of the 60-inch cyclotron for the chance that the Laboratory would be the first to find and advertise fission neutrons. Nor was a cyclotron the best tool for the search. The groups who first established the multiplication of free neutrons during fission worked with Ra-Be sources. The earliest in the field was Joliot's. They compared the neutron intensity at various distances from their source of photoneutrons (Rag -Be) when it sat in tanks containing plain water and water strewn with uranium oxide (hence they measured primarily the effects of slow neutrons). Fermi and two of his associates at Columbia used the same technique, which was adapted from one the Rome group had invented to measure neutron absorption; while another pair at Columbia, Szilard and Walter Zinn, observed fast neutrons from uranium struck by slow ones. In Paris they overestimated the average number of neutrons emitted during uranium fission as 3.5; in New York they made it 2, and then, in a more careful measurement by a combination of the two Columbia groups, who pooled their radium and uranium, 1.5. One preferred the higher or
lower figure according to whether one hoped for nuclear power or feared a nuclear bomb.
The only important immediate contribution of the Laboratory to the unravelling of the fission process was made by a junior member, Philip Abelson, who, by combining an undergraduate chemistry major with graduate training in physics, constituted an untried one-man interdisciplinary team. In 1937, in the course of the customary search for new activities, he had confirmed the general scheme of transuranics then recently put forward by Meitner, Hahn, and Strassmann, and added a new activity, of 17 hours, whose place he could not find. Then, under the inspiration of Alvarez's work on K-electron capture, he thought to identify "transuranics" by their characteristic x rays. In March 1938, in "transuranics" prepared in quantity at the cyclotron, Abelson isolated an activity of 77 hours, which he identified with the 66-hour body that the Berlin group had fixed as element 95, and whose characteristic x rays he managed to detect with a cheap spectrometer he had constructed. By extrapolating beyond uranium the increase with atomic number of the penetrating power of L rays—characteristic radiation involving the second or L shell of the atom—Abelson convinced himself that the x rays he detected belonged to an atom of nuclear charge 95. The revelation of fission dumbfounded him.
After a day in the dumps, Abelson looked again and learned that the "L rays" of 77-hr element 95 were in fact K rays of 2.4-hr iodine (atomic number 53). The connection, as Abelson worked it out: the iodine, Meitner et al.'s 2.5-hr element 96, descends from the 77-hr body, which must accordingly be tellurium; tellurium's beta decay can knock out a K electron, setting up the production of iodine's K ray. The process, taken all together, constituted "an unambiguous and independent proof of Hahn's hypothesis of the cleavage of the uranium nucleus." Abelson's rays and Dale Corson's pictures caused Oppenheimer to adjust his evaluation of
fission from "impossible" to "unbelievable." And if the unbelievable should give rise to some extra neutrons? "A ten cm cube of uranium deuteride (one should have something to slow the neutrons without capturing them) might very well blow itself to Hell."
Abelson carried on. In three months of hard work, reprieved from crew duty, he found in the products of uranium irradiated by neutrons no fewer than five activities ascribable to antimony, seven to tellurium, and four to iodine. He thereby not only showed the complexity of the German transuranics, but also demonstrated that fission fragments were ordinary isotopes. Two of his telluriums and one of his iodines had the same periods and beta-ray spectra as activities then recently identified in the Laboratory by Seaborg, Livingood, and Kennedy. Abelson could therefore specify decay sequence, half-life, element, and, in some cases, the isotope of the products he studied. In accomplishing his work he mobilized the great experience of the Laboratory in nuclear chemistry, as represented by Segrè and Seaborg, and, of course, the Laboratory's prop and pride: Abelson attributed his success "in large measure to the huge neutron intensities of the cyclotron." The cyclotron deserved the credit, and more. Its neutrons made fission products strong enough to swamp the activities of the natural descendents of uranium, which regrew during chemical analysis of the irradiated samples. With their "enormously weak preparations," Hahn and Strassmann had great difficulty finding iodine among the uranium products and succeeded only after Abelson's demonstration. Their confirmation resulted in what they called the "definitive suppression" of their long transuranic chains.
And what about the Curie-Savitch body that precipitated the studies that led to fission? As Meitner and Frisch observed, a barium isotope arising from uranium should be accompanied by
an isotope of krypton that might decay through rubidium, strontium, and yttrium to a stable zirconium. Hahn and Strassmann immediately started to look for these elements, of which they soon reported sightings. Now yttrium has chemical properties similar to those of lanthanum, from which Curie and Savitch had not been able to separate R3.5h . The exact investigation of the sequence from strontium on was left to one of Hahn's assistants, who found an yttrium of the necessary period, but declined to pronounce definitively that it decayed to zirconium and constituted the inspirational French "lanthanide." It did, and does.
Utopia or Armageddon
There remain the connections among the processes that the Berlin group had tied up in the problematic isomers of U239 . Recall that the long chains in figure 9.5 arise from fast and thermal neutrons respectively, and the short chain from resonance capture. Meitner and Hahn kept the third heavy uranium, of half-life 23 minutes, as a true U239 and tacitly referred both the long chains to the fission of U238 . They clung to this interpretation despite its odd consequence, that U238 would be fissionable by both fast and thermal neutrons but not by ones with moderate speeds, because the two chains seemed to be present in about equal strengths. They knew that a lighter isotope of uranium existed, but since it makes up less than 1 percent of natural uranium, it did not appear to them to be the ancestor of either long chain. Bohr's highly skeptical associate, George Placzek, adduced the ambiguity of the role ascribed to U238 as a strong argument against the possibility of fission. Thus inspired, Bohr found a way to father thermalneutron fission on U235 . It was only necessary to detach the fissions from the old decay chains: experiment could not distinguish which uranium isotope gave rise to which fragment; the
genetic connections in the first steps of the long chains were illusory.
Bohr argued from the excitation energies of the compound nuclei U239 and U236 . In the former case, the impacting neutrons must bring in the explosive energy, because, as an unpaired nucleon in the odd-numbered isotope 239, it will not excite the compound nucleus very much by its binding energy; in the latter case, the pairing of the neutron in the even isotope 236 contributes enough to the general excitation to create a chance of fission. The slower the neutron, the longer it takes to pass a nucleus and the greater the chance of its absorption. Hence the final interpretation of the "transuranic" chains: one comes from fast-neutron fission of U238 , the other from thermal-neutron fission of U235 . Placzek did not think that Bohr's reasons were very compelling. Others thought them strong enough to undertake the very difficult task of separating enough U235 from natural uranium to investigate its fissile properties directly.
Among the bold were the enterprising pair Seaborg and Kennedy. In the summer of 1939 they began construction of a tube, eventually over twenty feet in length, which they fixed to the outside of the chemistry building. Their scheme adapted a recent serendipitous invention made by Klaus Clusius and Gerhard Dickel, chemists at the University of Munich, who subjected a mixed gas to thermal gradient between a hot wire running down the tube's axis and the cooler tube walls. The resultant motion of the gas is hard to calculate but easy to describe: lighter molecules move radially inward under the thermal stress and axially upward by convection, heavier ones radially outward and axially downward. In the circulation, light molecules concentrate at the top and heavy ones at the bottom of the tube. Clusius and Dickel reported an almost complete separation of the major isotopes of chlorine. Kennedy and Seaborg planned to work with uranium hexafluoride gas, or "hex," as it came to be known from its
unsavory character, and they set up a fluorine generator to cast the spell. A little hex eventuated; a graduate student, Arthur Wahl, joined the project; Clusius's column was installed and Clusius written, in the best and most naive tradition of open science, for advice about uranium separation. Then, on January 12, 1940, the fluorine generator exploded while the three would-be hex splitters were working on it. The next day Kennedy was ill. Seaborg consulted a chemistry book: "Uranium is an extremely powerful, slow-acting poison." Kennedy stayed sick. Although his problem turned out to be mononucleosis, the incident destroyed the group's enthusiasm for hex. They put their columns—eventually they had three in operation—to separating the uncursed isotopes of hydrogen, carbon, and chlorine.
Another reason that Seaborg's group did not pursue uranium splitting was that others were doing it with greater success. By early 1940, the mass spectrograph run by Alfred Nier at the University of Michigan had collected enough U235 for the Columbia group to make possible a positive test of Bohr's conjecture about its fission by thermal neutrons. By the summer of 1940, when Kennedy and Seaborg made a tour of laboratories including Nier's, they learned about several attempts to acquire U235 in bulk: Urey and Tuve by diffusion methods, Beams by centrifugation, all starting with hex. The Columbia group had the help of Aristid von Grosse, a former collaborator of Hahn's and an expert in hex manfacture. That did not exhaust the competition. There were also Frisch, then relocated in England, where he and Otto Blüh, a refugee from Prague, set up a Clusius tube late in 1939; Clusius himself, who had declined to supply the additional information that Seaborg had requested; and Wilhelm Krasny-Ergen, who had established a Clusius distillery in Stockholm, from which, "had his activities not been suspended by the political situation," he expected a sevenfold enrichment in eighty days. Although
Tuve volunteered to send enough hex for preliminary experiments, Kennedy and Seaborg wisely withdrew from uranium separation. Perhaps Lawrence's belief that centrifugation held the greatest promise for large-scale separation of heavy isotopes influenced their decision.
The confirmation that U235 has a large cross-section for fission by slow neutrons gave physicists greater confidence and worry that an explosive chain reaction could be achieved. "Physicists are anxious that there be no public alarm over the possibility of the world being blown to bits by their experiments," Science Service reported in melodramatic ignorance just after the confirmation of fission at the end of January 1939. The world was not alarmed, despite a revelation in the New York Times that a little U235 could wipe out New York City and leave a hole halfway to Philadelphia. During 1939 physicists calculated what might be possible, but in ignorance of the relevant cross-sections and reactions they could only conjecture. An effort to keep pertinent data secret assisted the progress of ignorance and speculation. Although Szilard's novel notion that physicists should censor themselves failed to persuade Joliot and so failed internationally, enough was withheld in the United States that physicists as well placed as Tuve and Frisch were "hard pressed to get some data on uranium fission." The most sanguine discussion of the future of fission came from a colleague of Hahn's at the Kaiser-Wilhelm-Institut für Chemie, Siegfried Flügge, who offered a path to a "uranium machine."
Flügge started from the obvious: for a chain to succeed, enough slow neutrons must be obtained and caused to provoke fissions before they are lost or captured ineffectually. To slow the fast
fission neutrons, it is necessary only to pass them through water, which, to be sure, also captures them; but, according to Flügge's calculations, based on the probability that a slow neutron will cause a fission as measured at Columbia and on his own estimate of the liability of a fast neutron to loss in water while slowing down, a mixture of fifteen kilograms of uranium oxide per liter of water will sustain a chain reaction. That assumed the conservative estimate that an average fission sets free two fast neutrons. It also assumed that the reaction once started could be controlled. Flügge planned a power plant, not a bomb. A method of control had been published by two members of Joliot's team. It rests on the principle that the faster the neutron, the lower its probability of provoking a fission in uranium, and on the fact that the appetite of cadmium nuclei for slow neutrons is almost independent of the temperature. Therefore sprinkle a little cadmium dust in the water along with the uranium oxide. The chain begins, the mixture heats up, the neutrons move faster, the number of fissions goes down; equilibrium will be reached at a temperature fixed by the amount of neutron-removing cadmium present. According to Flügge, with 0.2 gram of cadmium per liter as seasoning, his uranium stew would boil along at a safe and steady 350°. The reactor vessel would require a diameter of a little over a meter. From it heat could be removed and used to make steam to drive a turbine; a cubic meter of uranium thus exploited could generate electricity for eleven years at a rate equivalent to Germany's consumption on the eve of the Depression.
By the time Flügge's design was published, Fermi and his colleagues had decided that water absorbs too many neutrons to serve as decelerator, or moderator, in a uranium machine. Hopes of capitalizing quickly on fission faded; by August 1939, when Tuve managed to learn what had been learned, he concluded that "all indications are that no chain can occur but it is pretty close." At the same time, Bohr was calculating the amount of hydrogen moderator needed to slow the neutrons. He arrived at a ratio of ten atoms of hydrogen to one of uranium, which he thought would prohibit a fast chain or strong explosion. Recalculation reduced
the ratio to one to one, which allowed a possibility. The Paris group was more negative. Their experiments with a homogeneous mixture of three atoms of hydrogen to one of uranium gave evidence of fissions caused by secondary and tertiary neutrons, but not of a divergent multiplication of fissions; and by the end of October 1939, in a paper they withheld from publication, they concluded that "it is almost certainly impossible" to promote a divergent chain reaction in a homogeneous blend of naturally occurring uranium, oxygen, and hydrogen. Of course, a moderator other than ordinary water, or a heterogeneous mixture of uranium and water, might perform better. The published consensus of physicists, as expressed in the several reviews of the year's fission research composed toward the end of 1939, was that exploitation of nuclear energy would not occur in the near future and might not be possible at all.
The announcement on May 1, 1940, of the details of Columbia's fissioning of Nier's latest sample of U235 transformed the discussion. William L. Laurence, a science writer for the New York Times , informed its readers on May 5 that five or ten pounds of U235 could drive an ocean liner or submarine indefinitely. So much light uranium might not be hard to procure. Nier had been able to enlarge his sample two hundred times in a matter of weeks. "It is not impossible that a few months or a year hence may see the realization of this quest." Nothing would be simpler than exploiting the new fuel. "All that is needed to put it to work running motors and steamships is to place it in a tank of water. . . . The water would be turned into steam. . . . New water supplied would keep the process going indefinitely." Furthermore, the business would be automatic and self-regulating, since the
heating of the water speeds up the neutrons and slows or stops the fission process until more water, "the colder the better," is added. Thus utopia. But evil men were trying to suborn the natural good behavior of U235 . "Every German scientist in this field, physicists, chemists, and engineers, it was learned, have been ordered to drop all other researches and devote themselves to this work alone." Fortunately they lacked the essential machine for further investigation, the cyclotron. A race had started, a race with stakes that were incalculable, or almost so. According to Laurence, a pound of U235 improperly treated would have the same explosive power as 15,000 tons of TNT. An effective separation plant, therefore, was "a secret to be given only to the United States government."
The physicists were not pleased with Laurence's mixture of fact and fiction. Nier declared that his handiwork had little present commercial or military value, the amount of U235 so far isolated being "hardly enough to spring a mousetrap." S.K. Allison rated producing utilizable atomic energy as "just as feasible as getting gold out of the ocean." George Pegram, who, as chairman of Columbia's physics department, was constantly pressed by the press, urged his colleagues to "stress what seems to be the fact, namely, that energy from uranium, even if it became available, would apparently not be cheap energy by any means and would not be very explosive energy." Pegram sold this point of view to the informed and responsible chief of the Times 's science section, Waldemar Kaempfert, who squared accounts as follows. To make a pound of U235 would cost more than the expenses of the federal government for an entire year; to make a gram by Nier's improved technique would take over a century. "The prospect of using U235 in the present war is zero."
While Kaempfert calculated, Krasny-Ergen's plan was published in Nature . It set Laurence off again. According to him, 10,000 of Krasny-Ergen's units could make a pound of U235 in forty days; at $100 per unit, a uranium factory would cost only $10 million. "It may be expected, therefore, that Germany will take measures at
once to install such a plant." Kaempfert again flew to the rescue: Krasny-Ergen's method of thermal diffusion would require 17 million kWh to separate a gram of U235 ; to make a kilogram, 34 million tons of coal would have to be burnt, at a cost of $68 million. "The more we think this over the more we are convinced that we would not invest ten cents in a uranium public utility company. . . . We doubt if the Germans have the time or the stupidity to bother much about isolating uranium-235 in large quantities."
Meanwhile Lawrence was examining a plan for a German power plant driven by light uranium. He had this information from Clifford Williams of Shell Development, who had it from Peter Debye, who had recently left the directorship of the Kaiser-Wilhelm-Institut für Physik in Berlin. According to Debye, all his former staff were engaged in developing U235 as a power source (fig. 9.8); Germany was "frantically mining uranium in Czechoslovakia;" and the native metal was to be separated by diffusion. It was perhaps indirectly from this disclosure that the author of the article on Lawrence in Scientific American obtained the information that "the Nazis are trying to lay hands on all the uranium they can find." The least secret bit of science in the United States in the summer of 1940 was that (as the San Francisco Chronicle put it) atomic power "will transform the face of the earth the moment the production of the magic element U-235 can be cheapened."
Pioneering in Transurania
According to Meitner and Frisch, the third of the uranium "isomers" of figure 9.5, the 23-minute activity, was indeed an isotope of uranium. According to accepted theory, it should have reduced its surplus of neutrons by emitting beta particles. Among the first to seek the product of such a transformation, a nucleus heavier than any previously detected on earth, was Edwin McMillan (plate 9.3). He had an advantage over others in having at his disposal the large activating neutron flux from the 37-inch cyclotron and a highly cultivated technique for the study of the radiations from nuclear disintegrations. Although his time had been taken up with cyclotron problems, he retained the independence he had shown in studying gamma rays while most of the rest of the Laboratory were building machines or exploding deuterons.
In his follow up of fission, McMillan characteristically examined the penetration of the recoiling fragments through the standard absorber, aluminum. He obtained a maximum range of about 2.2 cm, in rough agreement with slightly earlier experiments by Joliot. McMillan also examined the decay of the products left with the irradiated uranium, products unable to propel themselves through a single sheet of cigarette paper. He found an activity of 25 minutes, which he conjectured might be the same as the 23-minute body of the Berlin group, and he detected a strong activity of about 2 days' duration. He did not suggest a source for this long period, but promised further absorption measurements to fix the ranges of the recoiling fragments. Segrè took up the study of the nonrecoiling product and refined the periods to 23 minutes and 2.3 days. The first he identified with the heavy uranium of Berlin. Was the second a fission fragment or a transuranic? Segrè did his chemistry, made the 2.3-day body a rare earth (which it was), identified it as a fission fragment (which it was not), and declared it to be a light element. He searched for the alpha emitter that might reasonably be expected to terminate the beta transformations beginning with U239 . No luck. His conclusion, after discussing the matter with McMillan and Seaborg: the 23-
minute U239 becomes a long-lived, undetected element 93. And, underlined: "Transuranic elements have not yet been observed ." Once bitten, twice shy. As a member of Fermi's group, Segrè had erred in claiming a transuranic; as a cautious and institutionally insecure member of the Laboratory, he did not wish to repeat his mistake, and thus missed the transuranic he had. He bent his efforts to straightening out the decay chains of fission fragments.
Segrè's erroneous negative finding was immediately confirmed by John Irvine, a chemist at MIT, who concentrated U239 by taking advantage of the effect on chemical bonding of excited nuclear states. He was unable to detect any activity in a precipitate made by treating his enriched material with a compound of rhenium, which, following the old opinion of the nondiscoverers of the transuranics, element 93 should resemble. In his summary of the state of research on fission, completed in December 1939, Louis Turner of Princeton accepted the straightforward conclusion from the experiments of Segrè and Irvine. The elusiveness of element 93 and the hypothetical terminating alpha emitter nonetheless bothered him. He reduced his bother by the good guess that 94239 is the alpha emitter and U235 the great grandson of U238 . That at least kept the scandal in the uranium family.
Meanwhile Abelson, who had finished up at Berkeley with measurements of the wavelengths of K rays from radioactive substances near the middle of the periodic table, began to doubt the assumption on which Segrè had based his denial of transuranic status to McMillan's 2.3-day activity. The old alternative to likening transuranics to the elements beginning with rhenium and osmium remained: 93 and 94 could well resemble rare earths, and these "actinides," as they were later christened, should have a chemistry like uranium's. In his spare time at Tuve's laboratory, where he went in September 1939 to help with its 60-inch
cyclotron, Abelson showed that the 2.3-day body did not behave consistently like a rare earth. McMillan, too, had his doubts about Segrè's diagnosis. His further tests had shown that the 2.3-day activity remained firmly with the irradiated uranium and that its intensity exceeded that of all the long-lived fission fragments collected by recoil. Furthermore, when cadmium guarded the uranium target from assault by slow neutrons, the intensity of the fission products fell dramatically; whereas the intensities of the 23-minute and 2.3-day activities not only did not change appreciably, but remained in the same ratio, "suggesting a genetic relation between them," and the consequent identification of the longer period with element 93.
Abelson visited Berkeley in May, with orders from Tuve to "make every effort to settle the identity of the 2.3-day substance." Should it come from U235 , the possibility of a chain reaction even in separated uranium appeared doubtful. Abelson and McMillan joined forces and soon found a distinct chemical difference between the 2.3-day activity and rare earths. (The difference, the effect of the presence of an oxidizing agent on certain reactions, explained the erratic results of previous investigators, who had not controlled the oxidizing power of their solutions.) In respect of these reactions, it resembled uranium, and, since it had nothing in common with rhenium, McMillan and Abelson referred it to a possible "second 'rare earth' group of similar elements starting with uranium." It remained to show that the 2.3-day activity grew from the 23-minute U239 . Samples collected from the parent at 20-minute intervals all decayed with a period of 2.3 days. The decay, by beta emission, produced element 94. McMillan and Abelson supposed, with Turner, that 94239 transformed into U235 by alpha emission, which, in fact, it does. Their search for the telltale alpha particles did not succeed, however, and they inferred that, if 94239 were unstable against alpha emission, it must have a half-life of a million years. They overestimated by a factor of 400.
Seaborg knew about the work on element 93 as it progressed, and it made him "eager to work in this exciting field." He assigned Arthur Wahl the task of satisfying his eagerness. While Seaborg and Segrè sought new fission products in uranium struck by very fast neutrons, Wahl perfected chemical means for concentrating 93239 . By the middle of October, both projects were well advanced: Seaborg and Segrè found two chains of decays from Pd through Ag to Cd, and Wahl, following the procedure devised by McMillan and Abelson (oxidation by bromate ion), had isolated the 2.3-day activity from several samples of uranium irradiated with neutrons. But neither he nor his senior colleagues could find the suppositious alpha-emitting 94. They decided to try another route. During the summer of 1940, McMillan had invoked the traditional Berkeley bombardment and sent deuterons against natural uranium. He caught a glimpse of a second isotope of element 93 with a beta activity slightly more energetic than 93239 's; and he also saw a sign of its alpha-emitting descendent. After Kennedy had made a special thin-walled counter to follow this descendent, Seaborg wrote McMillan, who had left Berkeley for war work at MIT, that he, Kennedy, and Wahl would be "very glad to collaborate with you on the isolation of the new isotopes of element 93 and uranium found in the bombardment of uranium with deuterons." McMillan replied that he would be very pleased to have Seaborg continue the work.
McMillan had more than a curiosity about the secrets of nature in the continuation of the search for element 94. His and Abelson's discovery had been announced to the press with the usual fanfare: "A development that may bring man a step closer to the release of atomic force and energy which brought the universe into existence;" "something which may quite conceivably prove more influential in the destiny of the world than any single battle of the current World War." The publicity prompted a
reprimand from Lyman Briggs, director of the National Bureau of Standards, who headed a committee that tried to keep potentially useful information about uranium secret from foreign powers. The potential usefulness lay in the possibility that 94239 might be fissionable like U235 . Although Alvarez had discussed "fishing" 94239 with McMillan in January 1940 and Louis Turner had written Lawrence in July proposing that the 60-inch be put to making enough 94 to test the strong likelihood that 94 could be fished, Abelson and McMillan "did not see any possible connection of our work on element 93 with the fission problem."
Neither the mistake nor the publicity would recur, McMillan wrote Briggs; henceforth all discoveries about 93 and 94 would be submitted to his committee for determination of their sensitivity. As an example of his good behavior and his findings, McMillan disclosed his provisional results about the products of uranium bombarded by deuterons: the unknown isotope 93? produces an alpha-emitting body, presumably an isotope of element 94, with the chemical properties of thorium. Briggs replied that the "most important contribution you could make at this time" was to discover whether the alpha emitter fissioned with slow neutrons. "Even rough data will be valuable, and facilities do not exist elsewhere which will permit attempting the work." Only the 60-inch cyclotron could produce a strong enough sample of 93 to decay into enough 94 to offer the possibility of detecting its fission. Briggs's committee—which probably meant Fermi—estimated that with a sample as strong as the one McMillan and Abelson had used, McMillan should be able to see one fission every ten seconds, provided that 94 was about as fissionable as U235 . That sample had been extremely strong, about 10 mCi of 93, almost certainly, as Alvarez wrote Turner, "the most heavily bombarded substance in history." Its manufacture had so strained the 60-inch that Alvarez doubted that its like would be seen again before the 184-inch started up. "I don't think that we shall be able to fish 94 for some time."
At first fishing may not have been on the agenda of Seaborg's crew. On December 14, 1940, they prepared a modest sample of 93? by irradiating uranium oxide with 175 µAh of deuterons in the 60-inch cyclotron. Wahl took on the purification of 93 as part of his doctoral work. His excellent preparation when interrogated by counters showed a beta and gamma emission sufficiently distinct from those of 93239 to point to the presence of a new isotope. An alpha emitter, apparently the descendent of 93? , also showed itself. Unfortunately, the half-life of 93? fell out too close for comfort to that of 93239 (2.3 days), which was also produced to some extent in the deuteron bombardment. From another sample prepared in January, the half-life of 93? appeared to be 2.1 days, consistent with the increase of the alpha activity that grew from it. On this evidence and some shaky chemistry of the alpha emitter, "Things look[ed] good for element 94," Seaborg wrote McMillan on January 20, 1941. He added: "no one else knows about the most important of these results (the element 94) except Wahl and Kennedy. . . . The Committee [Briggs's] will want us to keep the results VERY SECRET!" On January 28, 1941, Seaborg, McMillan, Kennedy, and Wahl announced the discovery of 94? in a letter to the Physical Review withheld from publication by Briggs's committee. For the "?", they offered the choice of 235, 236, and 238; it was, in fact, 238, made by (d,2n) on U238 , and so identified by Kennedy, Segrè, Wahl, and Perlman in the fall of 1942.
Confirmation that the alpha emitter was an isotope of element 94 required its chemical separation from 93. Wahl tried many methods before he took up with the powerful oxidizing agent, persulfate ion, on the suggestion of a Berkeley chemist, Wendell Latimer, who was not authorized by the Briggs committee to know anything about the matter. (McMillan regarded the introduction of this agent, which promotes the 94 ion to a state soluble in hydrofluoric acid, as the most important contribution of Seaborg, Kennedy, and Wahl to the discovery of element 94.) By the last
week in February, all the 93 had decayed into element 94. Wahl hit what remained with persulfate, dissolved it in the presence of fluoride ion, and purified it. "These experiments," wrote Seaborg, Wahl, and Kennedy, in an understated celebration of Wahl's work, "make it extremely probably that this alpha-radioactivity is due to an isotope of element 94."
Meanwhile another game with 94 was in play at the Laboratory. In December 1940, Segrè, then in New York, talked with Fermi about the possibility that 94 would fission with slow neutrons, a matter then also receiving the attention of Bohr and Wheeler. Lawrence, in New York as usual during the giving season, was persuaded by Fermi and Segrè to order enough 94 from the 60-inch to test the hypothesis. On January 9, Segrè and Seaborg made a little 93239 by neutron bombardment and, by comparing yields, deduced that more 94 could be made by neutrons than by deuterons for the same cyclotron time. Fermi calculated that a kilogram of uranyl nitrate would be target enough; Seaborg and Segrè calculated that the amount of 93 they would have to make to give the 1 µg of 94 they hoped to have would be so radioactive—some 250 mCi—that it would have to be manipulated at long distance or by remote control. After practice runs with uranyl nitrate sent by Fermi—Cooksey had not been very generous with Laboratory funds for the project—Segrè and Seaborg irradiated 1.2 kg of the stuff with neutrons from 3,368 µAh of deuterons from the beryllium target of the 60-inch. For the next four days, from March 3 to March 6 inclusive, they extracted the 93, wearing goggles and lead-impregnated gloves, working with remote controls, carting their improving sample from one piece of apparatus to another in a lead bucket suspended from long poles. The precious product was sealed in a shallow platinum dish under a layer
of duco cement. Kennedy joined the team to monitor the decay of the 93239 into 94239 .
At this moment, early in March 1941, Seaborg's enterprises came together. Wahl's technique for the separation of 94238 , which Seaborg had kept secret from his partner, the alien Segrè, who had not been formally authorized to receive the information by the Briggs committee, was, of course, applicable to the big sample of 94239 . "These results came just in time," Seaborg wrote McMillan, "to be of great help to me [!] in the 94239 project which I [!] am doing for the Uranium Committee." By the end of March, the 93 had decayed to negligibility. Kennedy, Seaborg, and Segrè brought the sample—about 0.5 µg of 94239 mixed with rare earths and other dross—near the beryllium target of the 37-inch cyclotron, irradiated it with neutrons, and had the satisfaction of detecting about one fission per minute per µA of deuterons, from which they guessed that 94239 has a cross-section for slow neutrons about one-fifth that of U235 . The thick sample did not lend itself to such measurements, however: most fission fragments stopped within it or its cement topping. Wahl used the newly found chemistry of 94 to thin it. On May 17, 1941, it again went under the 37-inch. Almost twice as many fission fragments were counted as would have escaped from an equivalent amount of U235 . There was no longer a doubt: 94239 , which the Berkeley team estimated to have a half life of 10,000 years when left to itself, can be fished by slow neutrons. During the summer of 1941, a large sample of 94239 was prepared at the cyclotron and purified by Wahl. Studies of its chemistry by Wahl and Seaborg, of its fissionability under fast neutrons by Segrè and Seaborg, and of its low rate of spontaneous fission by Kennedy and Wahl
confirmed the suspicions that it belonged to a heavy rare-earth group and might make a fine explosive. That proved decisive for the career of Glenn Seaborg and may prove so for the rest of the human race.