Making of the Atomic Bomb

by Richard Rhodes

  The next day, in a letter to George Uhlenbeck at Columbia, “quite something” became “might very well blow itself to hell.”1073 One of Oppenheimer’s students, the American theoretical physicist Philip Morrison, recalls that “when fission was discovered, within perhaps a week there was on the blackboard in Robert Oppenheimer’s office a drawing—a very bad, an execrable drawing—of a bomb.”1074

  * * *

  Enrico Fermi made similar estimates. George Uhlenbeck, who shared an office with him in Pupin Hall, was there one day to overhear him. Fermi was standing at his panoramic office window high in the physics tower looking down the gray winter length of Manhattan Island, its streets alive as always with vendors and taxis and crowds. He cupped his hands as if he were holding a ball. “A little bomb like that,” he said simply, for once not lightly mocking, “and it would all disappear.”1075

  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 w
ith 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:

  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:

  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:

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