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.”1098 The 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 nucl
ear 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.
Making of the Atomic Bomb Page 40
