Military history

PART TWO

A PECULIAR SOVEREIGNTY

The Manhattan District bore no relation to the industrial or social life of our country; it was a separate state, with its own airplanes and its own factories and its thousands of secrets. It had a peculiar sovereignty, one that could bring about the end, peacefully or violently, of all other sovereignties.

Herbert S. Marks

We must be curious to learn how such a set of objects—hundreds of power plants, thousands of bombs, tens of thousands of people massed in national establishments—can be traced back to a few people sitting at laboratory benches discussing the peculiar behavior of one type of atom.

Spencer R. Weart

10

Neutrons

At the end of January 1939, still ill with a feverish cold that had laid him low for more than a week but determined to prevent information on the possibility of a chain reaction in uranium from reaching physicists in Nazi Germany, Leo Szilard raised himself from his bed in the King’s Crown Hotel on West 116th Street in Manhattan and went out into the New York winter to take counsel of his friend Isador Isaac Rabi.1076 Rabi, no taller than Szilard but always a trimmer and cooler man, who would be the 1944 Nobel laureate in physics, was born in Galicia in 1898 and emigrated to the United States with his family as a small child. Yiddish had been his first language; he grew up on New York’s Lower East Side, where his father worked in a sweatshop making women’s blouses until he accumulated enough savings to open a grocery store. Because his family was Orthodox and fundamentalist in its Judaism, Rabi had not known that the earth revolved around the sun until he read it in a library book. A frightening vision of the vast yellow face of the rising moon seen as a child down a New York street had begun his turn toward science, as had his childhood reading of the cosmological first verses of the Book of Genesis. He was a man of abrupt and honest bluntness who did not easily tolerate fools. One reason for his impatience was certainly that it guarded from harm his deeply emotional commitment to science: he thought physics “infinite,” he told a biographer in late middle age, and he was disappointed that young physicists of that later day, intent on technique, seemed to miss what he had found, “the mystery of it: how very different it is from what you can see, and how profound nature is.”1077, 1078

Szilard learned from Rabi that Enrico Fermi had discussed the possibility of a chain reaction in his public presentation at the Fifth Washington Conference on Theoretical Physics that had met the week before.1079 Szilard adjourned to Fermi’s office but did not find him there. He went back to Rabi and asked him to talk to Fermi “and say that these things ought to be kept secret.” Rabi agreed and Szilard returned to his sickbed.

He was recovering; a day or two later he again sought Rabi out:

I said to him: “Did you talk to Fermi?” Rabi said, “Yes, I did.” I said, “What did Fermi say?” Rabi said, “Fermi said ‘Nuts!’ ” So I said, “Why did he say ‘Nuts!’?” and Rabi said, “Well, I don’t know, but he is in and we can ask him.” So we went over to Fermi’s office, and Rabi said to Fermi, “Look, Fermi, I told you what Szilard thought and you said ‘Nuts!’ and Szilard wants to know why you said ‘Nuts!’ ” So Fermi said, “Well . . . there is the remote possibility that neutrons may be emitted in the fission of uranium and then of course perhaps a chain reaction can be made.” Rabi said, “What do you mean by ‘remote possibility’?” and Fermi said, “Well, ten per cent.” Rabi said, “Ten per cent is not a remote possibility if it means that we may die of it. If I have pneumonia and the doctor tells me that there is a remote possibility that I might die, and it’s ten percent, I get excited about it.”1080

But despite Fermi’s facility with American slang and Rabi’s with probabilities Fermi and Szilard were unable to agree. For the time being they left the discussion there.

Fermi was not misleading Szilard. It was easy to estimate the explosive force of a quantity of uranium, as Fermi would do standing at his office window overlooking Manhattan, if fission proceeded automatically from mere assembly of the material; even journalists had managed that simple calculation. But such obviously was not the case for uranium in its natural form, or the substance would long ago have ceased to exist on earth. However energetically interesting a reaction, fission by itself was merely a laboratory curiosity. Only if it released secondary neutrons, and those in sufficient quantity to initiate and sustain a chain reaction, would it serve for anything more. “Nothing known then,” writes Herbert Anderson, Fermi’s young partner in experiment, “guaranteed the emission of neutrons. Neutron emission had to be observed experimentally and measured quantitatively.”1081 No such work had yet been done. It was, in fact, the new work Fermi had proposed to Anderson immediately upon returning from Washington. Which meant to Fermi that talk of developing fission into a weapon of war was absurdly premature.

Many years later Szilard succinctly summed up the difference between his position and Fermi’s. “From the very beginning the line was drawn,” he said. “ . . . Fermi thought that the conservative thing was to play down the possibility that [a chain reaction] may happen, and I thought the conservative thing was to assume that it would happen and take all the necessary precautions.”1082

Once he was well again Szilard had catching up to do. He cabled Oxford to ship him the cylinder of beryllium he had left behind at the Clarendon when he came to the United States, preliminary to mounting a neutron-emission experiment of his own. At Lewis Strauss’s request he spent a day with the financier discussing the possible consequences of fission, which included, Strauss notes wistfully in his memoirs, making “the performance of our surge generator in Pasadena insignificant.1083 The device had just been completed.”1084 The surge generator in which he had invested some tens of thousands of dollars had been cut down to size. The Strausses were scheduled to leave that evening by overnight train for a Palm Beach vacation; Szilard rode along as far as Washington to continue the discussion. He was massaging his patron: he needed to rent radium to combine with his beryllium to make a neutron source and hoped Strauss might be persuaded to support the expense.

Arriving late at Union Station in Washington, Szilard called the Edward Tellers. They were still recovering from the work of hosting the Washington Conference. Mici Teller protested the surprise visit, her husband remembers: “No! We are both much too tired. He must go to a hotel.” They met Szilard anyway, whereupon to Teller’s surprise Mici invited their countryman to stay with them:1085

We drove to our home, and I showed Szilard to his room. He felt the bed suspiciously, then turned to me suddenly and said: “Is there a hotel nearby?” There was, and he continued: “Good! I have just remembered sleeping in this bed before. It is much too hard.”

But before he left, he sat on the edge of the hard bed and talked excitedly: “You heard Bohr on fission?”

“Yes,” I replied.

Szilard continued: “You know what that means!”

What it meant to Szilard, Teller remembers, was that “Hitler’s success could depend on it.”

The next day Szilard discussed his plan for voluntary secrecy with Teller, then entrained for Princeton to pursue the same subject with Eugene Wigner, who was still drydocked in the infirmary with jaundice. Szilard was thus present in Princeton when yet another momentous insight struck Niels Bohr.

*   *   *   

Bohr and Léon Rosenfeld were staying at the Nassau Club, the Princeton faculty center. On Sunday, February 5, George Placzek joined them at breakfast in the club dining room. The Bohemian theoretician had arrived in Princeton from Copenhagen the night before, another refugee from Nazi persecution. Talk turned to fission. “It is a relief that we are now rid of those transuranians,” Rosenfeld remembers Bohr saying, referring to the confusing radioactivities Hahn, Meitner and Strassmann had found in the late 1930s that Bohr assumed could now be attributed to existing lighter elements—barium, lanthanum and the many other fission products researchers were beginning to identify.

Placzek was skeptical. “The situation is more confused than ever,” he told Bohr.1086 He began then to specify the sources of confusion. He was directly challenging the relevance of Bohr’s liquid-drop model of the nucleus. The Danish laureate paid attention.

Physicists use a convenient measurement they call a “cross section” to indicate the probability that a particular nuclear reaction will or will not happen. The theoretical physicist Rudolf Peierls once explained the measurement with this analogy:

For example, if I throw a ball at a glass window one square foot in area, there may be one chance in ten that the window will break, and nine chances in ten that the ball will just bounce. In the physicists’ language, this particular window, for a ball thrown in this particular way, has a “disintegration cross-section” of 1/10 square foot and an “elastic cross-section” of 9/10 square foot.1087

Cross sections can be measured for many different nuclear reactions, and they are expressed not in square feet but in minute fractions of square centimeters, customarily 10–24, because the diminutive nucleus is the target window of Peierls’ analogy. The cross section that concerned Placzek in his discussion with Bohr was the capture cross section: the probability that a nucleus will capture an approaching neutron. In terms of Peierls’ analogy, the capture cross section measures the chance that the window might be open when the ball arrives and might therefore admit the ball into the living room.

Nuclei capture neutrons of certain energies more frequently than they capture neutrons of other energies. They are naturally tuned, so to speak, to certain specific energy levels—as if Peierls’ window opened more easily to balls thrown at only certain speeds. This phenomenon is known as resonance. The confusion Placzek delighted in reporting concerned a resonance in the capture cross sections of uranium and thorium.

Placzek pointed out that uranium and thorium both exhibit a capture resonance for neutrons with medium-range energies of about 25 electron volts. That meant, first of all, that although fission was one behavior uranium could exhibit under neutron bombardment, capture and subsequent transmutation continued to be another. Bohr was not ever to be rid of those inconvenient “transuranians.” Some of them were real.

If a neutron penetrated a uranium nucleus, for example, the result might be fission. But if the neutron happened to be traveling at the appropriate energy when it penetrated—somewhere around 25 eV—the nucleus would probably capture it without fissioning. Beta decay would follow, increasing the nuclear charge by one unit; the result should be a new, as-yet-unnamed transuranic element of atomic number 93. That was one of Placzek’s points. It would prove in time to be crucial.

The other source of confusion was more straightforward. It was also more immediately relevant to the question of how to harness nuclear energy. It concerned differences between uranium and thorium.

Thorium, element 90, a soft, heavy, lustrous, silver-white metal, was first isolated by the celebrated Swedish chemist Jons Jakob Berzelius in 1828. Berzelius named the new element after Thor, the Norse god of thunder. Its oxide found commercial use beginning in the late nineteenth century as the primary component of the fragile woven mantles of gas lanterns: heat incandesces it a brilliant white. Because it is mildly radioactive, and radioactivity was once considered tonic, thorium was also for some years incorporated into a popular German toothpaste, Doramad. Auer, the company that made German gas mantles, also made the toothpaste. Hahn, Meitner and Strassmann, the Joliot-Curies and others had regularly studied thorium alongside uranium. Its behavior was often similar. Otto Frisch had first demonstrated that it fissioned. He bombarded it next after uranium in the course of his January experiment in Copenhagen, the experiment he had discussed with Bohr after he returned from Kungälv and Bohr had worked so hard in the United States to protect.

Frisch was then also the first to notice that the fission characteristics of thorium differed from those of uranium. Thorium did not respond to the magic of paraffin; it was unaffected by slow neutrons. Richard B. Roberts and his colleagues at the Department of Terrestrial Magnetism of the Carnegie Institution of Washington had just independently confirmed and extended Frisch’s findings. With their 5 million volt Van de Graaff they could generate neutrons of several different, known energies. Continuing their experiments after their Saturday-night show for the Washington Conference group, they had compared uranium and thorium fission responses at varying energies as Frisch with his single neutron source could not. They found to their surprise (Frisch’s paper had not yet appeared in Nature) that while both uranium and thorium fissioned under bombardment by fast neutrons, only uranium fissioned under bombardment by slow neutrons. Some energy between 0.5 MeV and 2.5 MeV marked a lower threshold for fast-neutron fission for both elements. (Bohr and John Wheeler, beginning work at Princeton on fission theory, had estimated the threshold energy to be about 1 MeV.) The slow neutrons that also fissioned uranium were effective at far lower energies. “From these comparisons,” the DTM group concluded in a February paper, “it appears that the uranium fissions are produced by different processes for fast and slow neutrons.”1088

Why, Placzek now prodded Bohr, should both uranium and thorium have similar capture resonances and similar fast-neutron thresholds but different responses to slow neutrons? If the liquid-drop model had any validity at all, the difference made no sense.

Bohr abruptly saw why and was struck dumb. Not to lose what he had only barely grasped, oblivious to courtesy, he pushed back his chair and strode from the room and from the club. Rosenfeld hurried to follow. “Taking a hasty leave of Placzek, I joined Bohr, who was walking silently, lost in deep meditation, which I was careful not to disturb.” The two men tramped speechless through the snow across the Princeton campus to Fine Hall, the Neo-Gothic brick building where the Institute for Advanced Study was then lodged. They went in to Bohr’s office, borrowed from Albert Einstein. It was spacious, with leaded windows, a fireplace, a large blackboard, an Oriental rug to warm the floor.1089 No peripatetic like Bohr, Einstein had judged it too large and moved into a small secretarial annex nearby.

“As soon as we entered the office,” Rosenfeld remembers, “[Bohr] rushed to the blackboard, telling me: ‘Now listen: I have it all.’ And he started—again without uttering a word—drawing graphs on the blackboard.”

The first graph Bohr drew looked like this:

diagram

The horizontal axis plotted neutron energy left to right—low to high, slow to fast. The vertical axis charted cross sections—the probability of a particular nuclear reaction—and served a double purpose. The lazy S that filled most of the frame represented thorium’s cross section for capture at different neutron energies, the steep central peak demonstrating the 25 eV resonance in the middle range. The tail that waved from the horizontal axis on the right side represented a different thorium cross section: its cross section for fission beginning at that high 1 MeV threshold. What Bohr had drawn was thus a visualization of thorium’s changing response to bombardment by neutrons of increasing energy.

Bohr moved to the next section of blackboard and drew a second graph. He labeled it with the mass number of the isotope most plentiful in natural uranium. “He wrote the mass number 238 with very large figures,” Rosenfeld says; “he broke several pieces of chalk in the process.”1090 Bohr’s urgency marked the point of his insight. The second graph looked exactly like the first:

diagram

But a third graph was coming.

Francis Aston had found only U238 when he first passed uranium through his mass spectrograph at the Cavendish. In 1935, using a more powerful instrument, physicist Arthur Jeffrey Dempster of the University of Chicago detected a second, lighter isotope. “It was found,” Dempster announced in a lecture, “that a few seconds’ exposure was sufficient for the main component at 238 reported by Dr. Aston, but on long exposures a faint companion of mass number 235 was also present.”1091 Three years later a gifted Harvard postdoctoral fellow named Alfred Otto Carl Nier, the son of working-class German emigrants to Minnesota, measured the ratio of U235 to U238 in natural uranium as 1:139, which meant that U235 was present to the extent of about 0.7 percent.1092 By contrast, thorium in its natural form is essentially all one isotope, Th232. And that natural difference in the composition of the two elements was the clue that set Bohr off. He drew his third graph. It depicted one cross section, not two:

diagram

Having made a hard copy of his abrupt vision, Bohr was finally ready to explain himself.

Both thorium and U238 could be expected on theoretical grounds to behave similarly, he pointed out to Rosenfeld: to fission only with fast neutrons above 1 MeV. And it seemed that they did. That left U235. It followed as a matter of logic, Bohr said triumphantly, that U235 must be responsible for slow-neutron fission. Such was his essential insight.

He went on to explore the subtle energetics of the several reactions. Thorium was lighter than U235, U238 heavier, but the middle isotope differed more significantly in another important regard. When Th232 absorbed a neutron it became a nucleus of odd mass number, Th233. When U238 absorbed a neutron it also became a nucleus of odd mass number, U239. But when U235 absorbed a neutron it became a nucleus of even mass number, U236. And the vicissitudes of nuclear rearrangement are such, as Fermi would explain one day in a lecture, that “changing from an odd number of neutrons to an even number of neutrons released one or two MeV.”1093 Which meant that U235 had an inherent energetic advantage over its two competitors: it accrued energy toward fission simply by virtue of its change of mass; they did not.

Lise Meitner and Otto Frisch had realized in Kungälv that a certain amount of energy was necessary to agitate the nucleus to fission, but they had not considered in detail the energetics of that input. They were distracted by the enormous 200 MeV output. In fact, the uranium nucleus required an input of about 6 MeV to fission. That much energy was necessary to roil the nucleus to the point where it elongated and broke apart. The absorption of any neutron, regardless of its velocity, made available a binding energy of about 5.3 MeV. But that left U238 about 1 MeV short, which is why it needed fast neutrons of at least that threshold energy before it could fission.

U235 also earned 5.3 MeV when it absorbed a neutron. But it won Fermi’s “one or two MeV” in addition simply by adjusting from an odd to an even mass. That put its total above 6 MeV. So any neutron at all would fission U235—slow, fast or in between. Which was what Bohr’s third graph demonstrated: the probably continuous fission cross section of U235. From slow neutrons on the left only a fraction of an electron volt above zero energy, to fast neutrons on the right above 1 MeV that would also fission U238, any neutron an atom of U235 encountered would agitate it to fission. Natural uranium masked U235’s continuous fissibility; the more abundant U238 captured most of the neutrons. Only by slowing the neutrons with paraffin below the U238 capture resonance at 25 eV had experimenters like Hahn, Strassmann and Frisch been able to coax the highly fissionable U235 out of hiding. In a burst of insight Bohr had answered Placzek’s objections and replenished his liquid drop.

In January Bohr had produced a 700-word paper in three days to protect his European colleagues’ priorities. Now, in his eagerness to spread the news of U235’s special role in fission, he produced an 1,800-word paper in two days, mailing it to the Physical Review on February 7. “Resonance in uranium and thorium disintegrations and the phenomenon of nuclear fission” was nevertheless written with care, more care than it received in the reading.1094 Everyone understood its basic hypothesis—that U235, not U238, is responsible for slow-neutron fission in uranium—though not everyone concurred without the confirmation of experiment. But probably because, as Fermi recalled, isotopes at that time “were considered almost magically inseparable,” everyone overlooked its further implications.1095 Szilard explained to Lewis Strauss that month that “slow neutrons seem to split a uranium isotope which is present in an abundance of about 1% in uranium.”1096 Richard Roberts at the DTM, in a 1940 draft report of considerable significance, asserted that “Bohr . . . ascribed the [slow] neutron reaction to U235 and the fast neutron reaction to U238.”1097 Roberts’ misstatement was probably no more than a rough first approximation that he would have corrected in a polished report. Szilard’s and Roberts’ comments illustrate, however, that the slow-neutron fission of U235 preoccupied the physicists at first to the exclusion of a more ominous potentiality.

Bohr acknowledged it indirectly in his paper for the Physical Review. The slow-neutron fission of U235 occupied the foreground of his discussion because it explained the puzzling difference between uranium and thorium. But Bohr also considered U235’s behavior under fast-neutron bombardment. “For fast neutrons,” he wrote near the end of the paper, “ . . . because of the scarcity of the isotope concerned, the fission yields will be much smaller than those obtained from neutron impacts on the abundant isotope.”1098The statement implies but does not ask a pregnant question: what would the yields be for fast neutrons if U235 could be separated from U238?

*   *   *   

The latest incarnation of Orso Corbino’s garden fish pond in Rome was a tank of water three feet wide and three feet deep that Fermi and Anderson set up that winter in the basement of Pupin Hall.1099 They planned to insert a radon-beryllium neutron source into the center of a five-inch spherical bulb and suspend the bulb in the middle of the tank. Neutrons from the beryllium would then diffuse through the surrounding water, which would slow them down. The neutrons would induce a characteristic 44-second half-life in strips of rhodium foil, Fermi’s favorite neutron detector, set at various distances away from the bulb. Once he established a baseline of neutron activity using the Rn + Be source alone, Fermi intended to pack uranium oxide into the bulb around the source and make a second series of measurements. If more neutrons turned up in the water tank with uranium than without, he could deduce that uranium produced secondary neutrons when it fissioned and could roughly estimate their number. One neutron out for each neutron in was not enough to sustain a chain reaction, since inevitably some would be captured and others drift away: it needed something more than one secondary for each primary, preferably at least two.

Upstairs on the seventh floor Szilard discovered a different experiment in progress. Walter Zinn, a tall, blond Canadian postdoctoral research associate who taught at City College, was bombarding uranium with 2.5 MeV neutrons from a small accelerator. He had reasoned in terms of neutron energy rather than quantity; he was trying to demonstrate secondary neutron production by looking for neutrons faster than the 2.5 MeV’s he supplied. So far he had managed only inconclusive results.

“Szilard watched my experiment with great interest,” Zinn recalls, “and then suggested that perhaps it would be more successful if lower energy neutrons were available. I said, ‘That’s fine, but where do you get them?’ Leo said, ‘Just leave it to me, I’ll get them.’ ”1100

Szilard meant to help Zinn, but he also coveted Zinn’s ionization chamber. “All we needed to do,” he said later, “was to get a gram of radium, get a block of beryllium, expose a piece of uranium to the neutrons which come from the beryllium, and then see by means of the ionization chamber which Zinn had built whether fast neutrons were emitted in the process. Such an experiment need not take more than an hour or two to perform, once the equipment has been built and if you have the neutron source.1101 But of course we had no radium.”

The problem was still money. The Radium Chemical Company of New York and Chicago, a subsidiary of the Union Miniére du Haut-Katanga of Belgium, the dominant source of world radium supplies, was willing to rent a gram of radium for a minimum of three months for $125 a month. Szilard wrote Lewis Strauss at his Virginia farm on February 13 “to see whether you could sanction the expenditures” and presciently briefed the financier on the meaning of the latest developments. The letter’s crucial paragraph addresses Bohr’s new hypothesis that U235 is responsible for slow-neutron fission in natural uranium:1102

If this isotope could be used for maintaining chain reactions, it would have to be separated from the bulk of uranium. This, no doubt, would be done if necessary, but it might take five to ten years before it can be done on a technical scale. Should small scale experiments show that the thorium and the bulk of uranium would not work, but the rare isotope of uranium would, we would have the task immediately to attack the question of concentrating the rare isotope of uranium.1

Strauss’s surge-generator losses had inoculated him against further investment in the nuclear enterprise. He wanted to know, Szilard says, “just how sure I was that this would work.” Since Szilard could offer no guarantees, Strauss offered no support. Szilard turned then to Benjamin Liebowitz. “He was not poor but he was not exactly wealthy. . . . I told him what this was all about, and he said, ‘How much money do you need?’ I said, ‘Well, I’d like to borrow $2,000.’ He took out his checkbook, he wrote out a check, I cashed the check, I rented the . . . radium, and in the meantime the beryllium block arrived from England.”1103

The cylinder of beryllium, which Walter Zinn thought “a strange and unique object” and took for proof of Szilard’s magic ways, arrived on February 18.1104, 1105 The same day Szilard heard from Teller about significant work in Washington at the DTM. Richard Roberts and R. C. Meyer were preparing a letter to the Physical Review reporting the discovery of delayed neutrons from fission. These were not the instantaneous secondary neutrons the Columbia researchers were seeking, but they did confirm that the fission fragments had neutrons to spare and would give them up spontaneously.

The general excitement Teller found at the busy DTM laboratories impressed him more:

As soon as I began taking interest in uranium, sharp discussion started on the practical significance. Tuve, Hafstad, and Roberts are entirely aware of what is involved. They also know of Fermi’s experiments. Of course, I didn’t say anything. The above-mentioned letter [to the Physical Review] cannot cause any harm. . . .1109

I do not know their detailed plans, but I believe that urgent action [to maintain secrecy] is required. Very many people have discovered already what is involved. Those in Washington would like to persuade the Carnegie Institution that it should provide more money for U-research in view of the practical significance of the matter. . . . But right now this has no reality unless the [Carnegie] leadership becomes more interested than it has been so far. . . .

I repeat that there is a chain-reaction mood in Washington. I only had to say “uranium” and then could listen for two hours to their thoughts.

The president of the Carnegie Institution was a New England Yankee, the grandson of two sea captains, an electrical engineer, inventor and former dean of the school of engineering at the Massachusetts Institute of Technology named Vannevar Bush. If Bush was initially less willing to invest in chain-reaction experiments than Teller would have liked him to be, he kept good company; neither Ernest Lawrence at Berkeley nor Otto Hahn in Dahlem nor Lise Meitner, visiting Copenhagen that February to work with Otto Frisch, chose to pursue moonshine. Only Columbia and Paris mounted early experiments, though the DTM would soon follow the Columbia lead.

Frédéric Joliot and two colleagues, a cultivated Austrian named Hans von Halban and a huge, keen Russian named Lew Kowarski, began an experiment similar to Fermi’s the last week in February to identify secondary neutrons from fission. They also used a tank of water with a central neutron source but dissolved their uranium in the water rather than packing it around the source. More important to their priority of research, they had immediate access to the Radium Institute’s ample radium supply.

Because Fermi’s neutron source relied on radon rather than radium it induced an ambiguity into his experiment that Szilard caught and called to his attention: radon ejected much faster neutrons from beryllium than did radium; at least part of any increase in neutrons Fermi found in his tank might therefore result not from fission but from another, competing reaction in beryllium. Fermi thought the ambiguity trivial, but agreed, as Zinn had before, to repeat the experiment using a radium-beryllium source.1110 Szilard generously offered his. But the radium to energize it was not yet in hand; Szilard was still negotiating its rental because his lack of official affiliation made the Radium Chemical Company nervous.

He got his radium, two grams sealed in a small brass capsule, early in March, after he arranged admission to the Columbia laboratories for three months as a guest researcher. He and Zinn immediately set up their experiment. They made an ingenious nest, like Chinese boxes, of its various components: a large cake of paraffin wax, the beryllium cylinder set at the bottom of a blind hole in the paraffin, the radium capsule fitted into the beryllium cylinder; resting on the beryllium, inside the paraffin, a box lined with neutron-absorbing cadmium filled with uranium oxide; pushed into that box, but shielded from the radium’s gamma radiation by a lead plug, the ionization tube itself, which connected to an oscilloscope. With this arrangement, says Szilard, they could measure the flux of neutrons from the uranium with and without the cadmium shield:

Everything was ready and all we had to do was to turn a switch, lean back, and watch the screen of a television tube. If flashes of light appeared on the screen, that would mean that neutrons were emitted in the fission process of uranium and this in turn would mean that the large-scale liberation of atomic energy was just around the corner. We turned the switch and saw the flashes. We watched them for a little while and then we switched everything off and went home.1111

They had made a rough estimate of neutron production: “We find the number of neutrons emitted per fission to be about two.”1112 With radium available merely by picking up the phone, the French team a week earlier had found “more than one neutron . . . produced for each neutron absorbed.”1113 Fermi and Anderson estimated “a yield of about two neutrons per each neutron captured.”1114 Szilard immediately alerted Wigner and Teller. Teller remembers the moment well:

I was at my piano, attempting with the collaboration of a friend and his violin to make Mozart sound like Mozart, when the telephone rang. It was Szilard, calling from New York. He spoke to me in Hungarian, and he said only one thing: “I have found the neutrons.”1115

Szilard also wired Lewis Strauss:

PERFORMED TODAY PROPOSED EXPERIMENT WITH BERYLLIUM BLOCK WITH STRIKING RESULT. VERY LARGE NEUTRON EMISSION FOUND. ESTIMATE CHANCES FOR REACTION NOW ABOVE 50%.1116

Szilard had known what the neutrons would mean since the day he crossed the street in Bloomsbury: the shape of things to come. “That night,” he recalled later, “there was very little doubt in my mind that the world was headed for grief.”1117

*   *   *   

Though he was still recovering from jaundice, Eugene Wigner responded vigorously to Szilard’s disturbing news while a storm of betrayal broke over Central Europe. Hitler ordered the President and the Foreign Minister of Czechoslovakia to Berlin on March 14 and threatened to bomb Prague to rubble unless they surrendered their country. With the Nazi leader’s encouragement the Slovaks formally seceded from the republic that day. Ruthenia, Czechoslovakia’s narrow eastern extension along the Carpathians, also claimed independence as Carpatho-Ukraine, an exercise in grave-robbing abruptly terminated the following morning when the fascist Hungary of Admiral Horthy invaded the new nation with German endorsement. Hitler flew in triumph to Prague. On March 16 he decreed what was left of Czechoslovakia—Bohemia and Moravia—to be a German protectorate. The country that France and Great Britain had abandoned at Munich was partitioned without resistance.

Wigner caught the train to New York. On the morning of March 16 he met with Szilard, Fermi and George Pegram in Pegram’s office. Since at least the end of January Szilard had been promoting a new version of his Bund—he called it the Association for Scientific Collaboration—to monitor research, collect and disburse funds and maintain secrecy, a civilian organization that might guide the development of atomic energy. He had discussed it with Lewis Strauss on the train to Washington, with Teller after the night of the hard bed, with Wigner in Princeton the weekend Bohr drew his graphs. As far as Wigner was concerned, the time for such amateurism was over. He “strongly appealed to us,” says Szilard, “immediately to inform the United States government of these discoveries.”1118 It was “such a serious business that we could not assume responsibility for handling it.”1119

At sixty-three George Braxton Pegram was a generation older than the two Hungarians and the Italian who debated in his office that morning.1120 A South Carolinian who had earned his Ph.D. from Columbia in 1903 working with thorium, he had studied under Max Planck at the University of Berlin and corresponded with Ernest Rutherford when Rutherford was still progressing in fruitful exile at McGill. Pegram was tall and athletic, a champion at tennis well into his sixties, a canoeist when young who enjoyed paddling and sailing an eighteen-foot sponson around Manhattan Island. His interest in radioactivity may have been aroused by his father, a chemistry professor; “probably the most important problem before the physicist today,” the senior Pegram told the North Carolina Academy of Sciences in 1911, “is that of making the enormous energy [within the atom] available for the world’s work.”1121 The next year, as an associate professor of physics at Columbia, Pegram had written Albert Einstein encouraging him to come to New York to lecture on relativity theory. Pegram had brought Rabi and Fermi to Columbia, building the university’s international reputation for nuclear research. He was gray now, with thinning hair, wirerimmed glasses, protuberant ears, a strong, square, wide-chinned jaw. Radioactivity intrigued him still, but a university dean’s well-worn conservatism counseled him to caution.

He knew someone in Washington, he told Wigner: Charles Edison, Undersecretary of the Navy. Wigner insisted Pegram immediately call the man. Pegram was willing to do so, but first the group should discuss logistics. Who would carry the news? Fermi was traveling to Washington that afternoon to lecture in the evening to a group of physicists; he could meet with the Navy the next day. His Nobel Prize should give him exceptional credibility. Pegram called Washington. Edison was unavailable; his office directed Pegram to Admiral Stanford C. Hooper, technical assistant to the Chief of Naval Operations. Hooper agreed to hear Fermi out. Pegram’s call was the first direct contact between the physicists of nuclear fission and the United States government.

The next topic on the morning’s agenda was secrecy. Fermi and Szilard had both written reports on their secondary-neutron experiments and were ready to send them to the Physical Review. With Pegram’s concurrence they decided to go ahead and mail the reports to the Review, to establish priority, but to ask the editor to delay publishing them until the secrecy issue could be resolved. Both papers went off that day.

Pegram prepared a letter of introduction for Fermi to carry along to his appointment. It stated a hesitant case dense with hypotheticals:

Experiments in the physics laboratory at Columbia University reveal that conditions may be found under which the chemical element uranium may be able to liberate its large excess of atomic energy, and that this might mean the possibility that uranium might be used as an explosive that would liberate a million times as much energy per pound as any known explosive. My own feeling is that the probabilities are against this, but my colleagues and I think that the bare possibility should not be disregarded.1122

Thus lightly armed, Fermi departed to engage the Navy.

The debate was hardly ended, nor Wigner’s long day done. He returned to Princeton with Szilard in tow for an important meeting with Niels Bohr. It had been planned in advance; John Wheeler and Léon Rosenfeld would attend and Teller was making a special trip up from Washington. If Bohr could be convinced to swing his prestige behind secrecy, the campaign to isolate German nuclear physics research might work.

They met in the evening in Wigner’s office. “Szilard outlined the Columbia data,” Wheeler reports, “and the preliminary indications from it that at least two secondary neutrons emerge from each neutron-induced fission. Did this not mean that a nuclear explosive was certainly possible?” Not necessarily, Bohr countered.1123 “We tried to convince him,” Teller writes, “that we should go ahead with fission research but we should not publish the results. We should keep the results secret, lest the Nazis learn of them and produce nuclear explosions first. Bohr insisted that we would never succeed in producing nuclear energy and he also insisted that secrecy must never be introduced into physics.”1124

Bohr’s skepticism, says Wheeler, concerned “the enormous difficulty of separating the necessary quantities of U235.”1125 Fermi noted in a later lecture that “it was not very clear [in 1939] that the job of separating large amounts of uranium 235 was one that could be taken seriously.”1126 At the Princeton meeting, Teller remembers, Bohr insisted that “it can never be done unless you turn the United States into one huge factory.”1127

More crucial for Bohr was the issue of secrecy. He had worked for decades to shape physics into an international community, a model within its limited franchise of what a peaceful, politically united world might be. Openness was its fragile, essential charter, an operational necessity, as freedom of speech is an operational necessity to a democracy. Complete openness enforced absolute honesty: the scientist reported all his results, favorable and unfavorable, where all could read them, making possible the ongoing correction of error. Secrecy would revoke that charter and subordinate science as a political system—Polanyi’s “republic”—to the anarchic competition of the nation-states. No one was more anguished than Bohr by the menace of Nazi Germany; Laura Fermi remembers of this period, “two months after his landing in the United States,” that “he spoke about the doom of Europe in increasingly apocalyptic terms, and his face was that of a man haunted by one idea.”1128 If U235 could be separated easily from U238, that misfortune might be cause for temporary compromise with principle in the interest of survival. Bohr thought the technology looked not even remotely accessible. The meeting dragged on inconclusively past midnight.

The next afternoon Fermi turned up at the Navy Department on Constitution Avenue for his appointment with Admiral Hooper. He had probably planned a conservative presentation. The contempt of the desk officer who went in to announce him to the admiral encouraged that approach. “There’s a wop outside,” Fermi overheard the man say.1129 So much for the authority of the Nobel Prize.

In what Lewis Strauss, by now a Navy volunteer, calls “a ramshackle old board room” Hooper assembled an audience of naval officers, officers from the Army’s Bureau of Ordnance and two civilian scientists attached to the Naval Research Laboratory.1130 One of the civilians, a bluff physicist named Ross Gunn, had watched Richard Roberts demonstrate fission in the target room of the 5 MV Van de Graaff at the DTM not long after Fermi passed through at the time of the Fifth Washington Conference. Gunn worked on submarine propulsion; he was eager to learn more about an energy source that burned no oxygen.

Fermi led his auditors through an hour of neutron physics. If the notes of one of the participants, a naval officer, are comprehensive, Fermi emphasized his water-tank measurements rather than Szilard’s more direct ionization-chamber work. New experiments in preparation might confirm a chain reaction, Fermi explained. The problem then would be to assemble a sufficiently large mass of uranium to capture and use the secondary neutrons before they escaped through the surface of the material.

The officer taking notes interrupted.1131 What might be the size of this mass? Would it fit into the breech of a gun?

Rather than look at physics down a gun barrel Fermi withdrew to the ultramundane. It might turn out to be the size of a small star, he said, smiling and knowing better.

Neutrons diffusing through a tank of water: it was all too vague. Except to alert Ross Gunn, the meeting came to nothing. “Enrico himself . . . doubted the relevance of his predictions,” says Laura Fermi.1132 The Navy reported itself interested in maintaining contact; representatives would undoubtedly visit the Columbia premises. Fermi smelled the condescension and cooled.

March 17 was a Friday; Szilard traveled down to Washington from Princeton with Teller; Fermi stayed the weekend. They got together, reports Szilard, “to discuss whether or not these things”—the Physical Review papers—“should be published. Both Teller and I thought that they should not. Fermi thought that they should. But after a long discussion, Fermi took the position that after all this was a democracy; if the majority was against publication, he would abide by the wish of the majority.”1133 Within a day or two the issue became moot. The group learned of the Joliot/von Halban/Kowarski paper, published in Nature on March 18.1134 “From that moment on,” Szilard notes, “Fermi was adamant that withholding publication made no sense.”1135

The following month, on April 22, Joliot, von Halban and Kowarski published a second paper in Nature concerning secondary neutrons.1136 This one, “Number of neutrons liberated in the nuclear fission of uranium,” rang bells. Calculating on the basis of the experiment previously reported, the French team found 3.5 secondary neutrons per fission. “The interest of the phenomenon discussed here as a means of producing a chain of nuclear reactions,” the three men wrote, “was already mentioned in our previous letter.” Now they concluded that if a sufficient amount of uranium were immersed in a suitable moderator, “the fission chain will perpetuate itself and break up only after reaching the walls limiting the medium. Our experimental results show that this condition will most probably be satisfied.”1137 That is, uranium would most probably chain-react.

Joliot’s was an authoritative voice. G. P. Thomson, J.J.’s son, who was professor of physics at Imperial College, London, heard it. “I began to consider carrying out certain experiments with uranium,” he told a correspondent later. “What I had in mind was something rather more than a piece of pure research, for at the back of my thoughts there lay the possibility of a weapon.” He applied forthwith to the British Air Ministry for a ton of uranium oxide, “ashamed of putting forward a proposal apparently so absurd.”1138

More ominously, two initiatives originated simultaneously in Germany as a result of the French report.1139 A physicist at Göttingen alerted the Reich Ministry of Education. That led to a secret conference in Berlin on April 29, which led in turn to a research program, a ban on uranium exports and provision for supplies of radium from the Czechoslovakian mines at Joachimsthal. (Otto Hahn was invited to the conference but arranged to be elsewhere.) The same week a young physicist working at Hamburg, Paul Harteck, wrote a letter jointly with his assistant to the German War Office:

We take the liberty of calling to your attention the newest development in nuclear physics, which, in our opinion, will probably make it possible to produce an explosive many orders of magnitude more powerful than the conventional ones. . . . That country which first makes use of it has an unsurpassable advantage over the others.1140

The Harteck letter reached Kurt Diebner, a competent nuclear physicist stuck unhappily in the Wehrmacht’s ordnance department studying high explosives. Diebner carried it to Hans Geiger. Geiger recommended pursuing the research. The War Office agreed.

A public debate in Washington on April 29 paralleled the secret conference in Berlin. The New York Times account accurately summarizes the divisions in the U.S. physics community at the time:

Tempers and temperatures increased visibly today among members of the American Physical Society as they closed their Spring meeting with arguments over the probability of some scientist blowing up a sizable portion of the earth with a tiny bit of uranium, the element which produces radium.1141

Dr. Niels Bohr of Copenhagen, a colleague of Dr. Albert Einstein at the Institute for Advanced Study, Princeton, N.J., declared that bombardment of a small amount of the pure Isotope U235 of uranium with slow neutron particles of atoms would start a “chain reaction” or atomic explosion sufficiently great to blow up a laboratory and the surrounding country for many miles.

Many physicists declared, however, that it would be difficult, if not impossible, to separate Isotope 235 from the more abundant Isotope 238. The Isotope 235 is only 1 per cent of the uranium element.

Dr. L. Onsager of Yale University described, however, a new apparatus in which, according to his calculations, the isotopes of elements can be separated in gaseous form in tubes which are cooled on one side and heated to high temperatures on the other.

Other physicists argued that such a process would be almost prohibitively expensive and that the yield of Isotope 235 would be infinitesimally small. Nevertheless, they pointed out that, if Dr. Onsager’s process of separation should work, the creation of a nuclear explosion which would wreck as large an area as New York City would be comparatively easy. A single neutron particle, striking the nucleus of a uranium atom, they declared, would be sufficient to set off a chain reaction of millions of other atoms.

The Times story assumes the truth of Bohr’s argument in favor of U235, although even Bohr was apparently still emphasizing only a slowneutron reaction. Fermi and others were not yet convinced of U235’s role. The two uranium isotopes might not easily be separated in quantity, but it had occurred to John Dunning earlier in the month that they could be separated in microscopic amounts in Alfred Nier’s mass spectrograph. Dunning had immediately written Nier a long, impassioned letter asking him, in effect, to resolve the dispute between Fermi and Bohr and push chain-reaction research dramatically forward. Nier, Dunning and Fermi all attended the American Physical Society meeting. In person Dunning urged Nier to try for a separation much as he had urged him in the key paragraph of his letter:

There is one line of attack that deserves strong effort, and that is where we need your cooperation. . . . It is of the utmost importance to get some uranium isotopes separated in enough quantities for a real test. If you could separate effectively even tiny amounts of the two main isotopes [a third isotope, U234, is present in natural uranium to the trace extent of one part in 17,000], there is a good chance that [Eugene T.] Booth and I could demonstrate, by bombarding them with the cyclotron, which isotope is responsible. There is no other way to settle this business. If we could all cooperate and you aid us by separating some samples, then we could, by combining forces, settle the whole matter.1142

The important point for Dunning, the reason for his passion, was that if U235 was responsible for slow-neutron fission, then its fission cross section must be 139 times as large as the slow-neutron fission cross section of natural uranium, since it was present in the natural substance to the extent of only one part in 140. “By separating the 235 isotope,” Herbert Anderson emphasizes in a memoir, “it would be much easier to obtain the chain reaction. More than this, with the separated isotope the prospect for a bomb with unprecedented explosive power would be very great.”1143

Fermi urged Nier in similar terms; Nier recalls that he “went back and figured out how we might soup up our apparatus some in order to increase the output. . . . I did work on the problem, but at first it seemed like such a farfetched thing that I didn’t work on it as hard as I might have. It was just one of a number of things I was trying to do.”1144

Fermi in any case was more interested in pursuing a chain reaction in natural uranium than in attempting to separate isotopes. “He was not discouraged by the small cross-section for fission in the natural [element],” comments Anderson. “ ‘Stay with me,’ he advised, ‘we’ll work with natural uranium. You’ll see. We’ll be the first to make the chain reaction.’ I stuck with Fermi.”1145

By mid-April Szilard had managed to borrow about five hundred pounds of black, grimy uranium oxide free of charge from the Eldorado Radium Corporation, an organization owned by the Russian-born Pregel brothers, Boris and Alexander. Boris had studied at the Radium Institute in Paris; Eldorado speculated in rare minerals and owned important uranium deposits at Great Bear Lake in the Northwest Territories of Canada.

Like Fermi’s and Anderson’s previous experiment, the new project involved measuring neutron production in a tank of liquid. For a more accurate reading the experimenters needed a longer exposure time than their customary rhodium foils activated to 44-second half-life would allow. They planned instead simply to fill the tank with a 10 percent solution of manganese, an ironlike metal that gives amethyst its purple color and that activates upon neutron bombardment to an isotope with a nearly 3-hour half-life. “The [radio]activity induced in manganese,” they explained afterward in their report, “is proportional to the number of [slow] neutrons present.”1146 So the hydrogen in the water would serve to slow both the primary neutrons from the central neutron source and any secondary neutrons from fission, and the manganese in the water would serve to measure them—a nice economy of design.

Atoms on the surface of a mass of uranium are exposed to neutrons more efficiently than atoms deeper inside. Fermi and Szilard therefore decided not to bulk their five hundred pounds of uranium oxide into one large container but to distribute it throughout the tank by packing it into fifty-two cans as tall and narrow as lengths of pipe—two inches in diameter and two feet long.

Packing cans and mixing manganese solutions, which had to be changed and the manganese concentrated after each experimental run, was work. So was staying up half the night taking readings of manganese radioactivity. Fermi accepted the challenge with gusto. “He liked to work harder than anyone else,” Anderson notes, “but everyone worked very hard.” Except Szilard. “Szilard thought he ought to spend his time thinking.”1147 Fermi was insulted. “Szilard made a mortal sin,” Segrè remembers, echoing Fermi. “He said, ‘Oh, I don’t want to work and dirty my hands like a painter’s assistant.’ ”1148 When Szilard announced that he had hired a standin, a young man whom Anderson remembers as “very competent,” Fermi acceded to the arrangement without comment.1149But he never again pursued an experiment jointly with Szilard.

The arrangement as finally consummated looked like this:

diagram

Szilard’s Ra + Be source stands in the center of the tank, which holds 143 gallons of manganese solution; the fifty-two cans of UO2 gather around.

It worked. The three physicists found neutron activity “about ten percent higher with uranium oxide than without it. This result shows that in our arrangement more neutrons are emitted by uranium than are absorbed by uranium.”1150 But the experiment raised puzzling questions. Resonance absorption, for example, was clearly a problem, capturing neutrons that might otherwise serve the chain reaction. The report estimates “an average emission [of secondary neutrons] of about 1.2 neutrons per thermal neutron” but notes that “this number should be increased, to perhaps 1.5,” because some of the neutrons had obviously been captured without fissioning—demonstrating the big capture resonance around 25 eV that Bohr had attributed on his graphs to U238.1151

Another problem was the use of water as a moderator. As Fermi’s team had discovered in Rome in 1934, hydrogen was more efficient than any other element at slowing down neutrons, and slow neutrons avoided the parasitic capture resonance of U238. But hydrogen itself also absorbed some slow neutrons, reducing further the number available for fission. And it was already clear that every possible secondary neutron would have to be husbanded carefully if a chain reaction was to be initiated in natural uranium. George Placzek came down from Cornell, where he had found a new home, for a visit, looked over the arrangement and insightfully foreclosed its future. As Szilard tells it:

We were inclined to conclude that . . . the water-uranium system would sustain a chain reaction. . . . Placzek said that our conclusion was wrong because in order to make a chain reaction go, we would have to eliminate the absorption of [neutrons by the] water; that is, we would have to reduce the amount of water in the system, and if we reduced the water in the system, we would increase the parasitic absorption of [neutrons by] uranium [because with less water fewer neutrons would be slowed]. He recommended that we abandon the water-uranium system and use helium for slowing down the neutrons. To Fermi this sounded funny, and Fermi referred to helium thereafter invariably as Placzek’s helium.1152

In June the Columbia team wrote up its experiment and sent the resulting paper, “Neutron production and absorption in uranium,” to the Physical Review, which received it on July 3.1153 Fermi left for the Summer School of Theoretical Physics at Ann Arbor, his attention diverted, says Anderson, “by an interesting problem in cosmic rays.”1154 Either Fermi did not share Szilard’s sense of the urgency of chain-reaction research or he was withdrawing for a time from the Navy’s indifference and Placzek’s persuasive criticism of his uranium-water system; probably both. Anderson settled down to study resonance absorption in uranium, a project that would evolve into his doctoral dissertation.

Szilard remained in the steamy city: “I was left alone in New York. I still had no position at Columbia; my three months [of laboratory privileges] were up, but there were no experiments going on anyway and all I had to do was to think.”1155

*   *   *   

Szilard thought first about an alternative to water. The next common material up the periodic table that might work—that had a capture cross section considerably smaller than hydrogen’s, that was cheap, that would be thermally and chemically stable—was carbon. The mineral form of carbon, chemically identical to diamond but the product of a different structure of crystallization, is graphite, a black, greasy, opaque, lustrous material that is the essential component of pencil lead. Although carbon slows neutrons much less rapidly than hydrogen, even that difference might be put to advantage by careful design.

Lewis Strauss was leaving for Europe the week of July 2. Hoping that the financier might coax support for uranium research from Belgium’s Union Miniére, Szilard sent Strauss a last-minute letter arguing that a chain reaction in uranium “is an immediate possibility” but chose not to mention his new uranium-graphite conception.1156 Apparently he wanted to discuss it first with Fermi; the same day, July 3, he wrote the Italian laureate at length. “It seems to me now,” he reported, “that there is a good chance that carbon might be an excellent element to use in place of hydrogen, and there is a strong temptation to gamble on this chance.” He wanted to try “a large-scale experiment with a carbon-uranium-oxide mixture” as soon as they could acquire enough material. In the meantime he thought he would set up a small experiment to measure more accurately carbon’s capture cross section, only the upper limit of which was then known. If carbon should prove unsuitable their “next best guess might be heavy water,” rich in deuterium, though they would need “a few tons” of that scarce and expensive liquid. (Deuterium, H2, has a much smaller cross section for neutron capture than ordinary hydrogen.)

Across the one hundred sixty-third anniversary of the Declaration of Independence Szilard’s ideas evolved rapidly. On July 5 he visited the National Carbon Company of New York to look into purchasing graphite blocks of high purity (because impurities such as boron with large capture cross sections would soak up too many neutrons). He wrote Fermi his finding the same day: “It seems that it will be possible to get sufficiently pure carbon at a reasonable price.”1157 He also mentioned arranging the uranium and carbon in layers.

Fermi sat down in Ann Arbor at the end of the week to respond to Szilard’s first report. Independently he had arrived at a similar plan:

Thank you for your letter. I was also considering the possibility of using carbon for slowing down the neutrons. . . . According to my estimates a possible recipe might be about 39,000 kg of carbon mixed with 600 kg of uranium. If it were really so the amounts of materials would certainly not be too large.1158

Since however the amount of uranium that can be used, especially in a homogeneous mixture, is exceedingly small . . . perhaps the use of thick layers of carbon separated by layers of uranium might allow use of a somewhat larger percentage of uranium.

The idea of layering or in some other way separating the uranium from the graphite originated in calculations Fermi made in June for the manganese water-tank experiment. Fermi’s calculations led both men to consider partitioning the oxide from the graphite in the new design they were independently evolving. Partitioning would give the fast secondary neutrons room to slow down, bouncing around in the moderator, before they encountered any U238 nuclei. Szilard’s next letter, on July 8, mentions that “the carbon and the uranium oxide would not be mixed but built up in layers, or in any case used in some canned form.”1159 Both the July 5 and July 8 letters apparently crossed with Fermi’s letter in the mail.

By the time he heard from Fermi, Szilard had seen still farther and realized that small spheres of uranium arranged within blocks of graphite would be “even more favorable from the point of view of a chain reaction than the system of plane uranium layers which was initially considered.”1160 The arrangement Szilard had in mind he called a “lattice.” (A geodesic dome would represent such a lattice arrangement schematically if it were a complete sphere and if all its interior volume were filled like its surface with evenly spaced points.) His calculations indicated somewhat larger volumes of material than had Fermi’s: “perhaps 50 tons of carbon and 5 tons of uranium.”1161 The entire experiment, he thought, would cost about $35,000.1162

If a chain reaction would work in graphite and uranium, Szilard assumed, then a bomb was probable. And if he had managed these conclusions, he further assumed, then so had his counterparts in Nazi Germany. He sought out Pegram in those early July days and tried to convince him of the urgent need for a large-scale experiment to settle the question. The dean resisted the assault: “He took the position that even though the matter appeared to be rather urgent, this being summer and Fermi being away there was really nothing that usefully could be done until the fall.”1163

For several weeks Szilard had been trying on his own to raise funds from the U.S. military. In late May he had asked Wigner to contact the Army’s Aberdeen Proving Ground, its weapons-development facility in Maryland. While he was thinking through the possibilities of a uraniumgraphite system he had talked to Ross Gunn about Navy support. Now Fermi’s letter of July 9 and a July 10 letter from Gunn arrived to discourage him. Fermi wrote of layering the carbon and uranium but calculated in terms of a homogeneous system—of graphite and uranium oxide crushed and mixed together. Szilard concluded he was being mocked: “I knew very well that Fermi . . . computed the homogeneous mixture only because it was the easiest to compute. This showed me that Fermi did not take this really seriously.”1164 Gunn in turn reported that “it seems almost impossible . . . to carry through any sort of an agreement [with the Navy] that would be really helpful to you. I regret this situation but see no escape.”1165

Despite his Olympian ego not even Leo Szilard felt capable of saving the world entirely alone. He called on his Hungarian compatriots now for moral support. Edward Teller had moved to Manhattan for the summer to teach physics at Columbia; Eugene Wigner came up from Princeton to conspire with them. In later years Szilard would recount several different versions of how their conversation went, but a letter he wrote on August 15, 1939, offers reliable contemporary testimony: “Dr. Wigner is taking the stand that it is our duty to enlist the cooperation of the [Roosevelt] Administration. A few weeks ago he came to New York in order to discuss this point with Dr. Teller and me.”1166 Szilard had shown Wigner his uraniumgraphite calculations. “He was impressed and he was concerned.”1167 Both Teller and Wigner, Szilard wrote in a background memorandum in 1941, “shared the opinion that no time must be lost in following up this line of development and in the discussion that followed, the opinion crystallized that an attempt ought to be made to enlist the support of the Government rather than that of private industry. Dr. Wigner, in particular, urged very strongly that the Government of the United States be advised.”1168

But the discussion slipped away from that project into “worry about what would happen if the Germans got hold of large quantities of the uranium which the Belgians were mining in the Congo.” Perhaps Szilard emphasized the futility of the government contacts that he and Fermi had already made. “So we began to think, through what channels could we approach the Belgian government and warn them against selling any uranium to Germany?”1169

It occurred to Szilard then that his old friend Albert Einstein knew the Queen of Belgium. Einstein had met Queen Elizabeth in 1929 on a trip to Antwerp to visit his uncle; thereafter the physicist and the sovereign maintained a regular correspondence, Einstein addressing her in plainspoken letters simply as “Queen.”

The Hungarians were aware that Einstein was summering on Long Island. Szilard proposed visiting Einstein and asking him to alert Elizabeth of Belgium. Since Szilard owned no car and had never learned to drive he enlisted Wigner to deliver him. They called Einstein’s office at the Institute for Advanced Study and learned he was staying at a summer house on Old Grove Pond on Nassau Point, the spit of land that divides Little from Great Peconic Bay on the northeastern arm of the island.

They called Einstein to arrange a day. At this time Szilard also furthered Wigner’s proposal to contact the United States government by seeking advice from a knowledgeable emigré economist, Gustav Stolper, a Berliner resettled in New York who had once been a member of the Reichstag.1170 Stolper offered to try to identify an influential messenger.

Wigner picked up Szilard on the morning of Sunday, July 16, and drove out Long Island to Peconic.1171 They reached the area in early afternoon but had no luck soliciting directions to the house until Szilard thought to ask for it in Einstein’s name. “We were on the point of giving up and going back to New York”—two world-class Hungarians lost among country lanes in summer heat—“when I saw a boy aged maybe seven or eight standing on the curb. I leaned out of the window and I said, ‘Say, do you by any chance know where Professor Einstein lives?’ The boy knew that and he offered to take us there.”1172

C. P. Snow had visited Einstein at the same summer retreat two years before, also losing his way, and makes the scene familiar:

He came into the sitting room a minute or two after we arrived. There was no furniture apart from some garden chairs and a small table. The window looked out on to the water, but the shutters were half closed to keep out the heat. The humidity was very high.1173

At close quarters, Einstein’s head was as I had imagined it: magnificent, with a humanizing touch of the comic. Great furrowed forehead; aureole of white hair; enormous bulging chocolate eyes. I can’t guess what I should have expected from such a face if I hadn’t known. A shrewd Swiss once said it had the brightness of a good artisan’s countenance, that he looked like a reliable old-fashioned watchmaker in a small town who perhaps collected butterflies on a Sunday.

What did surprise me was his physique. He had come in from sailing and was wearing nothing but a pair of shorts. It was a massive body, very heavily muscled: he was running to fat round the midriff and in the upper arms, rather like a footballer in middle-age, but he was still an unusually strong man. He was cordial, simple, utterly unshy. The large eyes looked at me, as though he was thinking: what had I come for, what did I want to talk about?

 . . . The hours went on. I have a hazy memory that several people drifted in and out of the room, but I do not remember who they were. Stifling heat. There appeared to be no set time for meals. He was already, I think, eating very little, but he was still smoking his pipe. Trays of open sandwiches—various kinds of wurst, cheese, cucumber—came in every now and then. It was all casual and Central European. We drank nothing but soda water.

Similarly settled, Szilard told Einstein about the Columbia secondaryneutron experiments and his calculations toward a chain reaction in uranium and graphite. Long afterward he would recall his surprise that Einstein had not yet heard of the possibility of a chain reaction. When he mentioned it Einstein interjected, “Daran habe ich gar nicht gedacht!”—“I never thought of that!” He was nevertheless, says Szilard, “very quick to see the implications and perfectly willing to do anything that needed to be done.1174He was willing to assume responsibility for sounding the alarm even though it was quite possible that the alarm might prove to be a false alarm. The one thing most scientists are really afraid of is to make fools of themselves. Einstein was free from such a fear and this above all is what made his position unique on this occasion.”1175

Einstein hesitated to write Queen Elizabeth but was willing to contact an acquaintance who was a member of the Belgian cabinet. Wigner spoke up to insist again that the United States government should be alerted, pointing out, Szilard goes on, “that we should not approach a foreign government without giving the State Department an opportunity to object.” Wigner suggested that they send the Belgian letter with a cover letter through State. All three men thought that made sense.

Einstein dictated a letter to the Belgian ambassador, a more formal contact appropriate to their State Department plan, and Wigner took it down in longhand in German.1176 At the same time Szilard drafted a cover letter. Einstein’s was the first of several such compositions—they served in succession as drafts—and the origin of most of the statements that ultimately found their way into the letter he actually sent.

Wigner carried the first Einstein draft back to Princeton, translated it into English and on Monday gave it to his secretary to type. When it was ready he mailed it to Szilard. Then he left Princeton to drive to California on vacation.

A message from Gustav Stolper awaited Szilard at the King’s Crown. “He reported to me,” Szilard wrote Einstein on July 19, “that he had discussed our problems with Dr. Alexander Sachs, a vice-president of the Lehman Corporation, biologist and national economist, and that Dr. Sachs wanted to talk to me about this matter.”1177, 1178 Eagerly Szilard arranged an appointment.

Alexander Sachs, born in Russia, was then forty-six years old. He had come to the United States when he was eleven, graduated from Columbia in biology at nineteen, worked as a clerk on Wall Street, returned to Columbia to study philosophy and then went on to Harvard with several prestigious fellowships to pursue philosophy, jurisprudence and sociology. He contributed economics text to Franklin Roosevelt’s campaign speeches in 1932; beginning in 1933 he worked for three years for the National Recovery Administration, joining the Lehman Corporation in 1936. He had thick curls and a receding chin and looked and sounded like the comedian Ed Wynn. His associates at the NRA used to point him out to visiting firemen under that nom de guerre as ultimate proof, if the NRA itself was not sufficient, of Roosevelt’s gift for radical innovation. Sachs communicated in dense, florid prose (he had been thinking that spring of writing a book entitled The Inter- War Retreat from Reason as Exemplified in the Mis-history of the Recent Past and in the Contemporaneous Conduct of International Political and Economic Affairs by the United States and Great Britain) but could coruscate in committee.

Sachs heard Szilard out. Then, as Szilard wrote Einstein, he “took the position, and completely convinced me, that these were matters which first and foremost concerned the White House and that the best thing to do, also from the practical point of view, was to inform Roosevelt. He said that if we gave him a statement he would make sure it reached Roosevelt in person.”1179 Among those who valued Sachs’ opinions and called him from time to time for talks, it seems, was the President of the United States.

Szilard was stunned. The very boldness of the proposal won his heart after all the months when he had confronted caution and skepticism: “Although I have seen Dr. Sachs once,” he told Einstein, “and really was not able to form any judgment about him, I nevertheless think that it could not do any harm to try this way and I also think that in this regard he is in a position to fulfill his promise.”1180

Szilard met Sachs shortly after returning from Peconic—between Sunday and Wednesday. Unable at midweek to reach Wigner en route to California, he tracked down Teller, who thought Sachs’ proposal preferable to the plan they had previously worked out.1181 Drawing on the first Einstein draft, Szilard now prepared a draft letter to Roosevelt. He wrote it in German because Einstein’s English was insecure, added a cover letter and mailed it to Long Island. “Perhaps you will be able to tell me over the telephone whether you would like to return the draft with your marginal comments by mail,” he proposed in the cover letter, “or whether I should come out to discuss the whole thing once more with you.” If he visited Peconic again, Szilard wrote, he would ask Teller to drive him, “not only because I believe his advice is valuable but also because I think you might enjoy getting to know him.1182 He is particularly nice.”

Einstein preferred to review a letter to the President in person. Teller therefore delivered Szilard to Peconic, probably on Sunday, July 30, in his sturdy 1935 Plymouth.1183 “I entered history as Szilard’s chauffeur,” Teller aphorizes the experience.1184 They found the Princeton laureate in old clothes and slippers. Elsa Einstein served tea. Szilard and Einstein composed a third text together, which Teller wrote down.1185 “Yes, yes,” Teller remembers Einstein commenting, “this would be the first time that man releases nuclear energy in a direct form rather than indirectly.”1186 Directly from fission, he meant, rather than indirectly from the sun, where a different nuclear reaction produces the copious radiation that reaches the earth as sunlight.

Einstein apparently questioned if Sachs was the best man to carry the news to Roosevelt. On August 2 Szilard wrote Einstein hoping “at long last” for a decision “upon whom we should try to get as middle man.”1187 He had seen Sachs in the interim; the economist, who certainly coveted the assignment of representing Albert Einstein to the President, had generously listed the financier Bernard Baruch or Karl T. Compton, the president of MIT, as possible alternates. On the other hand, he had strongly endorsed Charles Lindbergh, though he must have known that Roosevelt despised the famous aviator for his outspoken pro-German isolationism. Szilard wrote that he and Sachs had discussed “a somewhat longer and more extensive version” of the letter Einstein had written with Szilard at their second Peconic meeting; he now enclosed both the longer and shorter versions and asked Einstein to return his favorite along with a letter of introduction to Lindbergh.

Einstein opted for the longer version, which incorporated the shorter statement that had originated with him but carried additional paragraphs contributed by Szilard in consultation with Sachs. He signed both letters and returned them to Szilard in less than a week with a note hoping “that you will finally overcome your inner resistance; it’s always questionable to try to do something too cleverly.”1188 That is, be bold and get moving. “We will try to follow your advice,” Szilard rejoined on August 9, “and as far as possible overcome our inner resistances which, admittedly, exist. Incidentally, we are surely not trying to be too clever and will be quite satisfied if only we don’t look too stupid.”1189

Szilard transmitted the letter in its final form to Sachs on August 15 along with a memorandum of his own that elaborated on the letter’s discussion of the possibilities and dangers of fission. He had not given up contacting Lindbergh—he drafted a letter to the aviator the following day—but he seems to have decided to try Sachs in the meantime, probably in the interest of moving the project on; he pointedly asked Sachs either to deliver the letter to Roosevelt or to return it.1190

One of the discussions Szilard had added to the longer draft that Einstein chose concerned who should serve as liaison between “the Administration and the group of physicists working on chain reactions in America.”1191 In his letter of transmittal to Sachs, Szilard now tacitly offered himself for that service. “If a man, having courage and imagination, could be found,” he wrote, “and if such a man were put—in accordance with Dr. Einstein’s suggestion—in the position to act with some measure of authority in this matter, this would certainly be an important step forward. In order that you may be able to see of what assistance such a man could be in our work, allow me please to give you a short account of the past history of the case.”1192 The short account that followed, an abbreviated and implicit curriculum vitae, essentially outlined Szilard’s own role since Bohr’s announcement of the discovery of fission seven crowded months earlier.

Szilard’s offer was as innocent of American bureaucratic politics as it was bold. It was surely also the apotheosis of his drive to save the world. By this time the Hungarians at least believed they saw major humanitarian benefit inherent in what Eugene Wigner would describe in retrospect as “a horrible military weapon,” explaining:1193

Although none of us spoke much about it to the authorities [during this early period]—they considered us dreamers enough as it was—we did hope for another effect of the development of atomic weapons in addition to the warding off of imminent disaster. We realized that, should atomic weapons be developed, no two nations would be able to live in peace with each other unless their military forces were controlled by a common higher authority. We expected that these controls, if they were effective enough to abolish atomic warfare, would be effective enough to abolish also all other forms of war. This hope was almost as strong a spur to our endeavors as was our fear of becoming the victims of the enemy’s atomic bombings.

From the horrible weapon which they were about to urge the United States to develop, Szilard, Teller and Wigner—“the Hungarian conspiracy,” Merle Tuve was amused to call them—hoped for more than deterrence against German aggression.1194 They also hoped for world government and world peace, conditions they imagined bombs made of uranium might enforce.

*   *   *   

Alexander Sachs intended to read aloud to the President when he met with him. He believed busy people saw so much paper they tended to dismiss the printed word. “Our social system is such,” he told a Senate committee in 1945, “that any public figure [is] punch-drunk with printer’s ink. . . .1195 This was a matter that the Commander in Chief and the head of the Nation must know. I could only do it if I could see him for a long stretch and read the material so it came in by way of the ear and not as a soft mascara on the eye.” He needed a full hour of Franklin Delano Roosevelt’s time.

History intervened to crowd the President’s calendar. Having won the Rhineland, Austria and Czechoslovakia simply by taking them, having signed the Pact of Steel with Italy on May 22 and a ten-year treaty of nonaggression and neutrality with the USSR on August 23, Adolf Hitler ordered the invasion of Poland beginning at 4:45 A.M. on September 1, 1939, and precipitated the Second World War. The German invasion fielded fifty-six divisions against thirty Polish divisions strung thinly across the long Polish frontier; Hitler had ten times the aircraft, including plentiful squadrons of Stuka dive-bombers, and nine divisions of Panzer tanks against Polish horse cavalry armed with swords and spears. The assault was “a perfect specimen of the modern Blitzkrieg,” writes Winston Churchill: “the close interaction on the battlefield of army and air force; the violent bombardment of all communications and of any town that seems an attractive target; the arming of an active Fifth Column; the free use of spies and parachutists; and above all, the irresistible forward thrusts of great masses of armour.”1196

The mathematician Stanislaw Ulam had just returned from visiting Poland, bringing with him on a student visa his sixteen-year-old brother, Adam:

Adam and I were staying in a hotel on Columbus Circle. It was a very hot, humid, New York night. I could not sleep very well. It must have been around one or two in the morning when the telephone rang. Dazed and perspiring, very uncomfortable, I picked up the receiver and the somber, throaty voice of my friend the topologist Witold Hurewicz began to recite the horrible tale of the start of war: “Warsaw has been bombed, the war has begun,” he said. This is how I learned about the beginning of World War II. He kept describing what he had heard on the radio. I turned on my own. Adam was asleep; I did not wake him. There would be time to tell him the news in the morning. Our father and sister were in Poland, so were many other relatives. At that moment, I suddenly felt as if a curtain had fallen on my past life, cutting it off from my future. There has been a different color and meaning to everything ever since.1197

One of Roosevelt’s first acts was to appeal to the belligerents to refrain from bombing civilian populations. Revulsion against the bombing of cities had grown in the United States since at least the Japanese bombing of Shanghai in 1937.1198 When Spanish Fascists bombed Barcelona in March 1938, Secretary of State Cordell Hull had condemned the atrocity publicly: “No theory of war can justify such conduct,” he told reporters. “ . . . I feel that I am speaking for the whole American people.”1199 In June the Senate passed a resolution condemning the “inhuman bombing of civilian populations.”1200 As war approached, revulsion began to give way to impulses of revenge; in the summer of 1939 Herbert Hoover could urge an international ban on the bombing of cities and still argue that “one of the impelling reasons for the unceasing building of bombing planes is to prepare reprisals.”1201 Bombing was bad because it was enemy bombing. Scientific American saw through to a darker truth: “Although . . . aerial bombing remains an unknown, indeterminate quantity, the world may be sure that the unwholesome atrocities which are happening today are but curtain raisers on insane dramas to come.”1202

So although Roosevelt had asked Congress for increased funds for long-range bombers nine months before, in appealing to the belligerents on September 1, 1939, he could still articulate the moral indignation of millions of Americans:

The ruthless bombing from the air of civilians in unfortified centers of population during the course of the hostilities which have raged in various quarters of the earth during the past few years, which has resulted in the maiming and in the death of thousands of defenseless men, women and children, has sickened the hearts of every civilized man and woman, and has profoundly shocked the conscience of humanity.1203

If resort is had to this form of inhuman barbarism during the period of the tragic conflagration with which the world is now confronted, hundreds of thousands of innocent human beings who have no responsibility for, and who are not even remotely participating in, the hostilities which have now broken out, will lose their lives. I am therefore addressing this urgent appeal to every Government which may be engaged in hostilities publicly to affirm its determination that its armed forces shall in no event, and under no circumstances, undertake the bombardment from the air of civilian populations or of unfortified cities, upon the understanding that these same rules of warfare will be scrupulously observed by all of their opponents. I request an immediate reply.

Great Britain agreed to the President’s terms the same day. Germany, busy bombing Warsaw, concurred on September 18.

The invasion of Poland brought Britain and France into the war on September 3. Abruptly Roosevelt’s schedule filled to overflowing. In early September in particular he was working overtime with a reluctant Congress to revise the Neutrality Act to terms more favorable to Britain; Sachs was unable even to discuss arranging an interview until after the first week in September.

*   *   *   

By September Kurt Diebner’s new War Office department had consolidated German fission research under its authority. Diebner enlisted a young Leipzig theoretician named Erich Bagge and together the two physicists planned a secret conference to consider the feasibility of a weapons project.1204 They had the authority to enlist the services of any German citizen they wished and they used it, sending out papers that left Hans Geiger, Walther Bothe, Otto Hahn and a number of other exceptional older men nervously uncertain if they were being invited to Berlin for consultation or ordered to active military service.

At the conference in Berlin on September 16 the physicists learned that German intelligence had discovered the beginnings of uranium research abroad—meaning, presumably, in the United States and Britain. They discussed the long, thorough theoretical paper by Niels Bohr and John Wheeler, “The mechanism of nuclear fission,” that had been published in the September Physical Review and especially its conclusion, which Bohr and Wheeler had elaborated from Bohr’s Sunday-morning graph work, that U235 was probably the isotope of uranium responsible for slow-neutron fission.1205 Hahn like Bohr argued that isotope separation was difficult to the point of impossibility. Bagge proposed calling in Werner Heisenberg, his superior at Leipzig, to adjudicate.

Heisenberg therefore attended a second Berlin conference on September 26 and discussed two possible ways to harness the energy from fission: by slowing secondary neutrons with a moderator to make a “uranium burner” and by separating U235 to make an explosive. Paul Harteck, the Hamburg physicist who had written the War Office the previous April, traveled to the second conference armed with a paper he had just finished on the importance of layering uranium and moderator to avoid the U238 capture resonance—the same insight that had come independently to Fermi and Szilard in early July. Harteck’s study, however, considered using heavy water as moderator, even though Harteck had worked with Rutherford at the Cavendish and knew from personal experience how expensive heavy-water production could be—water in which deuterium replaced hydrogen had to be tediously distilled from tons of ordinary H20.

Diebner and Bagge had outlined for the second conference a “Preparatory Working Plan for Initiating Experiments on the Exploitation of Nuclear Fission.”1206 Heisenberg would head up theoretical investigation. Bagge would measure deuterium’s cross section for collision to establish how effectively heavy water might slow secondary neutrons. Harteck would look into isotope separation. Others would experiment to determine other significant nuclear constants. The War Office would take over the Kaiser Wilhelm Institute of Physics, finished in 1937 and beautifully equipped. Adequate funds would be forthcoming.

The German atomic bomb project was well begun.

It may have been no less complicated by humanitarian ambiguities than the project the Hungarians in the United States proposed. One young but highly respected German physicist involved in the work from near the beginning was Carl Friedrich von Weizsäcker, the son of the German Undersecretary of State. In a 1978 memoir von Weizsäcker remembers discussing the possibility of a bomb with Otto Hahn in the spring of 1939. Hahn opposed secrecy then partly on the grounds of scientific ethics but also partly because he “felt that if it were to be made, it would be worst for the entire world, even for Germany, if Hitler were to be the only one to have it.” Like Szilard, Teller and Wigner, von Weizsäcker remembers realizing in discussions with a friend “that this discovery could not fail to radically change the political structure of the world”:1207

To a person finding himself at the beginning of an era, its simple fundamental structures may become visible like a distant landscape in the flash of a single stroke of lightning. But the path toward them in the dark is long and confusing. At that time [i.e., 1939] we were faced with a very simple logic. Wars waged with atom bombs as regularly recurring events, that is to say, nuclear wars as institutions, do not seem reconcilable with the survival of the participating nations. But the atom bomb exists. It exists in the minds of some men. According to the historically known logic of armaments and power systems, it will soon make its physical appearance. If that is so, then the participating nations and ultimately mankind itself can only survive if war as an institution is abolished.

Both sides might work from fear of the other. But some on both sides would be working also paradoxically believing they were preparing a new force that would ultimately bring peace to the world.

*   *   *   

As September extended its violence Szilard grew impatient. He had heard nothing from Alexander Sachs. Pursuing Sachs’ previous suggestions and his own leads, he arranged for Eugene Wigner to give him a letter of introduction to MIT president Karl T. Compton; recontacted a businessman of possible influence whom he had once interested in the Einstein-Szilard refrigerator pump; read a newspaper account of a Lindbergh speech and reported to Einstein that the aviator “is in fact not our man.”1208 Finally, the last week in September, he and Wigner visited Sachs and found to their dismay that the economist still held Einstein’s letter. “He says he has spoken repeatedly with Roosevelt’s secretary,” Szilard reported to Einstein on October 3, “and has the impression that Roosevelt is so overburdened that it would be wiser to see him at a later date. He intends to go to Washington this week.” The two Hungarians were ready to start over: “There is a distinct possibility that Sachs will be of no use to us. If this is the case, we must put the matter in someone else’s hands. Wigner and I have decided to accord Sachs ten days’ grace. Then I will write you again to let you know how matters stand.”1209

But Alexander Sachs did indeed travel to Washington, not that week but the next, and on Wednesday, October 11, presented himself, probably in the late afternoon, at the White House.1210 Roosevelt’s aide, General Edwin M. Watson, “Pa” to Roosevelt and his intimates, sitting with his own executive secretary and military aide, reviewed Sachs’ agenda.1211 When he was convinced that the information was worth the President’s time, Watson let Sachs into the Oval Office.

“Alex,” Roosevelt hailed him, “what are you up to?”1212

Sachs liked to warm up the President with jokes. His sense of humor tended to learned parables. Now he told Roosevelt the story of the young American inventor who wrote a letter to Napoleon.1213 The inventor proposed to build the emperor a fleet of ships that carried no sail but could attack England in any weather. He had it in his power to deliver Napoleon’s armies to England in a few hours without fear of wind or storm, he wrote, and he was prepared to submit his plans. Napoleon scoffed: ships without sails? “Bah! Away with your visionists!”1214

The young inventor, Sachs concluded, was Robert Fulton. Roosevelt laughed easily; probably he laughed at that.

Sachs cautioned the President to listen carefully: what he had now to impart was at least the equivalent of the steamboat inventor’s proposal to Napoleon. Not yet ready to listen, Roosevelt scribbled a message and summoned an aide. Shortly the aide returned with a treasure, a carefully wrapped bottle of Napoleon brandy that the Roosevelts had preserved in the family for years. The President poured two glasses, passed one to his visitor, toasted him and settled back.

Sachs had made a file for Roosevelt’s reading of Einstein’s letter and Szilard’s memorandum. But neither document had suited his sense of how to present the information to a busy President. “I am an economist, not a scientist,” he would tell friends, “but I had a prior relationship with the President, and Szilard and Einstein agreed I was the right person to make the relevant elaborate scientific material intelligible to Mr. Roosevelt. No scientist could sell it to him.”1215, 1216 Sachs had therefore prepared his own version of the fission story, a composite and paraphrase of the contents of the Einstein and Szilard presentations. Though he left those statements with Roosevelt, he read neither one of them aloud. He read not Einstein’s subsequently famous letter but his own eight-hundred-word summation, the first authoritative report to a head of state of the possibility of using nuclear energy to make a weapon of war.1217 It emphasized power production first, radioactive materials for medical use second and “bombs of hitherto unenvisaged potency and scope” third. It recommended making arrangements with Belgium for uranium supplies and expanding and accelerating experiment but imagined that American industry or private foundations would be willing to foot the bill. To that end it proposed that Roosevelt “designate an individual and a committee to serve as a liaison” between the scientists and the Administration.

Sachs had intentionally listed the peaceful potentials of fission first and second among its prospects.1218 To emphasize the “ambivalence” of the discovery, he said later, the “two poles of good and evil” it embodied, he turned near the end of the discussion to Francis Aston’s 1936 lecture, “Forty Years of Atomic Theory”—it had been published in 1938 as part of a collection, Background to Modern Science, which Sachs had brought along to the White House—where the English spectroscopist had ridiculed “the more elderly and apelike of our prehistoric ancestors” who “objected to the innovation of cooked food and pointed out the grave dangers attending the use of the newly discovered agency, fire.”1219 Sachs read the entire last paragraph of the lecture to Roosevelt, emphasizing the final sentences:1220

Personally I think there is no doubt that sub-atomic energy is available all around us, and that one day man will release and control its almost infinite power. We cannot prevent him from doing so and can only hope that he will not use it exclusively in blowing up his next door neighbor.

“Alex,” said Roosevelt, quickly understanding, “what you are after is to see that the Nazis don’t blow us up.”1221

“Precisely,” Sachs said.

Roosevelt called in Watson. “This requires action,” he told his aide.

Meeting afterward with Sachs, Watson went by the book. He proposed a committee consisting initially of the director of the Bureau of Standards, an Army representative and a Navy representative. The Bureau of Standards, established by Act of Congress in 1901, is the nation’s physics laboratory, charged with applying science and technology in the national interest and for public benefit. Its director in 1939 was Dr. Lyman J. Briggs, a Johns Hopkins Ph.D. and a government scientist for forty-three years who had been nominated by Herbert Hoover and appointed by FDR. The military representatives were Lieutenant Colonel Keith F. Adamson and Commander Gilbert C. Hoover, both ordnance experts.

“Don’t let Alex go without seeing me again,” Roosevelt had directed Watson.1222 Sachs met the same evening with Briggs, briefed him and proposed he and his committee of two get together with the physicists working on fission. Briggs agreed. Sachs saw the President again and declared himself satisfied. That was good enough for Roosevelt.

Briggs set a first meeting of the Advisory Committee on Uranium for October 21 in Washington, a Saturday. Sachs proposed to invite the emigrés; to counterbalance them Briggs invited Tuve, who found a schedule conflict and deputized Richard Roberts as his stand-in.1223 Fermi, still nursing his Navy grievance, refused to attend but was willing to allow Teller to speak in his behalf. On the appointed day the Hungarian conspiracy breakfasted with Sachs at the Carleton Hotel, the out-of-towners having arrived the night before.1224 From the hotel they proceeded to the Department of Commerce. The meeting then counted nine participants: Briggs, a Briggs assistant, Sachs, Szilard, Wigner, Teller, Roberts, Adamson for the Army and Hoover for the Navy.

Szilard began by emphasizing the possibility of a chain reaction in a uranium-graphite system.1225 Whether such a system would work, he said, depended on the capture cross section of carbon and that was not yet sufficiently known. If the value was large, they would know that a large-scale experiment would fail. If the value was extremely small, a large-scale experiment would look highly promising. An intermediate value would necessitate a large-scale experiment to decide. He estimated the destructive potential of a uranium bomb to be as much as twenty thousand tons of high-explosive equivalent. Such a bomb, he had written in the memorandum Sachs carried to Roosevelt, would depend on fast neutrons and might be “too heavy to be transported by airplane,” which meant he was still thinking of exploding natural uranium, not of separating U235.1226

Adamson, openly contemptuous, butted in. “In Aberdeen,” Teller remembers him sneering, “we have a goat tethered to a stick with a ten-foot rope, and we have promised a big prize to anyone who can kill the goat with a death ray. Nobody has claimed the prize yet.”1227 As for twenty thousand tons of high explosive, the Army officer said, he’d been standing outside an ordance depot once when it blew up and it hadn’t even knocked him down.1228

Restraining himself, Wigner spoke after Szilard, supporting his compatriot’s argument.

Roberts raised serious objection.1229 He was convinced that Szilard’s optimism for a chain reaction was premature and his notion of a fast-neutron weapon made of natural uranium misguided. Roberts had co-authored a review of the subject just one month before. It agreed with Szilard that “there are not yet sufficient data to say definitely whether or not a uranium powerhouse is a possibility.”1230 But it also assessed—because the DTM had begun assessing—the question of the fast-neutron fission of natural uranium and found, because of resonance capture and extensive scattering of fast neutrons, that it was “very unlikely that the fast neutrons can produce a sufficient number of fissions to maintain a [chain] reaction.”1231, 1232

The DTM physicist also pointed out that other lines of research might be more promising than a slow-neutron chain reaction in natural uranium. He meant isotope separation. At the University of Virginia Jesse Beams, formerly Ernest Lawrence’s colleague at Yale, was applying to the task the high-speed centrifuges he was developing there. Roberts thought answers to these questions might require several years of work and that research should be left in the meantime to the universities.

Briggs spoke up to defend his committee.1233 He argued vigorously that any assessment of the possibilities of fission at a time when Europe was at war had to include more than physics; it had to include the potential impact of the development on national defense.

Szilard was “astonished,” as he told Pegram the next day, at Sachs’ “active and enthusiastic” participation in the meeting.1234 Sachs seconded Briggs and the Hungarians. “The issue was too important to wait,” he recalled his argument, “and the important thing was to be helpful because if there was something to it there was danger of our being blown up. We had to take time by the forelock, and we had to be ahead.”1235

Then it was Teller’s turn. For himself, he announced in his deep, heavily accented voice, he strongly supported Szilard. But he had also been given the task of serving as messenger for Fermi and Tuve, who had discussed these issues in New York and had come to some agreement about them. “I said that this needed a little support. In particular we needed to acquire a good substance to slow down the neutrons, therefore we needed pure graphite, and this is expensive.”1236 Jesse Beams’ centrifuge work also required support, Teller added.

“How much money do you need?” Commander Hoover wanted to know.1237

Szilard had not planned to ask for money. “The diversion of Government funds for such purposes as ours appears to be hardly possible,” he explained to Pegram the next day, “and I have therefore myself avoided to make any such recommendation.”1238 But Teller answered Hoover promptly, probably speaking for Fermi: “For the first year of this research we need six thousand dollars, mostly in order to buy the graphite.” (“My friends blamed me because the great enterprise of nuclear energy was to start with such a pittance,” Teller reminisces; “they haven’t forgiven me yet.”1239 Szilard, who would write Briggs on October 26 that the graphite alone for a largescale experiment would cost at least $33,000, must have been appalled.1240)

Adamson had anticipated just such a raid on the public treasury. “At this point,” says Szilard, “the representative of the Army started a rather longish tirade”:

He told us that it was naive to believe that we could make a significant contribution to defense by creating a new weapon. He said that if a new weapon is created, it usually takes two wars before one can know whether the weapon is any good or not. Then he explained rather laboriously that it is in the end not weapons which win the wars, but the morale of the troops. He went on in this vein for a long time until suddenly Wigner, the most polite of us, interrupted him. [Wigner] said in his high-pitched voice that it was very interesting for him to hear this. He always thought that weapons were very important and that this is what costs money, and this is why the Army needs such a large appropriation. But he was very interested to hear that he was wrong: it’s not weapons but the morale which wins the wars. And if this is correct, perhaps one should take a second look at the budget of the Army, and maybe the budget could be cut.1241

“All right, all right,” Adamson snapped, “you’ll get your money.”1242

The Uranium Committee produced a report for the President on November 1.1243 It narrowly emphasized exploring a controlled chain reaction “as a continuous source of power in submarines.” In addition, it noted, “If the reaction turns out to be explosive in character, it would provide a possible source of bombs with a destructiveness vastly greater than anything now known.” The committee recommended “adequate support for a thorough investigation.” Initially the government might undertake to supply four tons of pure graphite (this would allow Fermi and Szilard to measure the capture cross section of carbon) and, if justified later, fifty tons of uranium oxide.

Briggs heard from Pa Watson on November 17. The President had read the report, Watson wrote, and wanted to keep it on file. On file is where it remained, mute and inactive, well into 1940.

Even with Szilard and Fermi stalled, fission studies continued at many other American laboratories. Prodded by a late-October letter from Fermi, for example, Alfred Nier at the University of Minnesota finally began preparing to separate enough U235 from U238, using his mass spectroscope, to determine experimentally which isotope is responsible for slow-neutron fission.1244, 1245 But to American physicists and administrators in and out of government a bomb of uranium seemed a remote possibility at best. However intense their sympathies, the war was still a European war.

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