Making of the Atomic Bomb

by Richard Rhodes

  * * *

  Princeton, Einstein reported to his friend Elizabeth, the Queen of Belgium, “is a wonderful little spot, a quaint and ceremonious village of puny demigods on stilts. Yet, by ignoring certain social conventions, I have been able to create for myself an atmosphere conducive to study and free from distraction.”728 Wigner noticed that von Neumann “fell in love with America on the first day. He thought: these are sane people who don’t talk in these traditional terms which are meaningless. To a certain extent the materialism of the United States, which was greater than that of Europe, appealed to him.”729 When Stanislaw Ulam arrived in Princeton in 1935 he found von Neumann comfortably ensconced in a “large and impressive house. A black servant let me in.” The von Neumanns gave two or three parties a week. “These were not completely carefree,” Ulam notes; “the shadow of coming world events pervaded the social atmosphere.”730 Ulam’s own enthusiasm for America, formulated a few years later when he was a Junior Fellow at Harvard, was tempered with a criticism of the extreme weather: “I used to tell my friends that the United States was like the little child in a fairy tale, at whose birth all the good fairies came bearing gifts, and only one failed to come. It was the one bringing the climate.”731

  Leopold Infeld, riding the train through New Jersey from New York to Princeton, “was astonished at so many wooden houses; in Europe they are looked down upon as cheap substitutes which do not, like brick, resist the attack of passing time.” Inevitably on that passage he noticed “old junked cars, piles of scrap iron.” At Princeton the campus was deserted. He found a hotel and asked where all the students had gone. Perhaps to see Notre Dame, the clerk said. “Was I crazy?” Infeld asked himself. “Notre Dame is in Paris. Here is Princeton with empty streets. What does it all mean?” He soon found out. “Suddenly the whole atmosphere changed. It happened in a discontinuous way, in a split second. Cars began to run, crowds of people streamed through the streets, noisy students shouted and sang.”732 Infeld arrived on a Saturday; in those days Princeton played Notre Dame at football.

  His first night in the New World, Hans Bethe walked all over New York.733

  A chemist, Kurt Mendelssohn, vividly recalled the morning after his escape: “When I woke up the sun was shining in my face. I had slept deeply, soundly and long—for the first time in many weeks. [The previous night] I had arrived in London and gone to bed without fear that at 3 a.m. a car with a couple of S.A. men would draw up and take me away.”734

  Before it is science and career, before it is livelihood, before even it is family or love, freedom is sound sleep and safety to notice the play of morning sun.

  8

  Stirring and Digging735

  The seventh Solvay Conference, held in Brussels in late October 1933, was George Gamow’s ticket of escape from a Soviet Union rapidly becoming inhospitable to theoretical physicists who persisted in modern views. The previous summer the tall, blond, powerfully built Odessan and his wife Rho, also a physicist, had tried to escape by paddling a faltboat—a collapsible rubber kayak—170 miles south from the Crimea to Turkey across the Black Sea without benefit of a weather report. They took a pocket compass, carefully hoarded hard-boiled eggs, cooking chocolate, two bottles of brandy and a bag of fresh strawberries, set out in the morning ostensibly on a recreational excursion and paddled hard all day and into the night. The only document they carried was Gamow’s Danish motorcycle-driver’s license, souvenir of the 1930 winter he spent in Copenhagen after working with Rutherford at the Cavendish. Gamow planned to show the Turks the document, announce himself in Danish to be a Dane, head for the nearest Danish consulate and put himself long-distance in Bohr’s capable hands. But the Black Sea is named for its storms. The wind thwarted the Gamows’ escape, drenching them in heavy seas, exhausting them through a long, cold night and finally blowing them back to shore.736

  Back in Leningrad the following year Gamow received notice from his government that he was officially delegated to the Solvay Conference. “I could not believe my eyes,” he writes in his autobiography.737 It was an easy way out of the country—except that Rho had not been included. Gamow determined to acquire a second passport or defiantly stay home. Through the Bolshevik economist Nikolai Bukharin, whom he knew, he arranged an interview with Party Chairman Vyacheslav Molotov at the Kremlin. Molotov wondered that the theoretician could not live for two weeks without his wife. Gamow feigned camaraderie:

  “You see,” I said, “to make my request persuasive I should tell you that my wife, being a physicist, acts as my scientific secretary, taking care of papers, notes, and so on. So I cannot attend a large congress like that without her help. But this is not true. The point is that she has never been abroad, and after Brussels I want to take her to Paris to see the Louvre, the Folies Bergère, and so forth, and to do some shopping.”738

  That Molotov understood. “I don’t think this will be difficult to arrange,” he told Gamow.

  When the time arrived to collect the passports Gamow found that Molotov had changed his mind, preferring not to set an awkward precedent. Gamow stubbornly refused to cooperate. The passport office called him three times to pick up his passport and three times he insisted he would wait until there were two. The fourth time “the voice on the telephone informed me that both passports were ready. And indeed they were!” (After the conference the young defectors sailed to America.739 Gamow taught at the University of Michigan’s summer school in pleasant Ann Arbor and from there moved to accept a professorship at George Washington University in Washington, D.C.)

  The Solvay Conference, devoted for the first time to nuclear physics, drew men and women from the highest ranks of two generations: Marie Curie, Rutherford, Bohr, Lise Meitner among the older physicists; Heisenberg, Pauli, Enrico Fermi, Chadwick (eight men in all from Cambridge and no one from devastated Göttingen), Gamow, Irene and Frédéric JoliotCurie, Patrick Blackett, Rudolf Peierls among the younger. Ernest Lawrence, his cyclotron humming, was the token American that year.

  They debated the structure of the proton. Other topics they discussed may have seemed more far-reaching at the time. None would prove to be. On August 2, 1932, working with a carefully prepared cloud chamber, an American experimentalist at Caltech named Carl Anderson had discovered a new particle in a shower of cosmic rays. The particle was an electron with a positive instead of a negative charge, a “positron,” the first indication that the universe consists not only of matter but of antimatter as well. (Its discovery earned Anderson the 1936 Nobel Prize.) Physicists everywhere immediately looked through their files of cloud-chamber photographs and identified positron tracks they had misidentified before (the Joliot-Curies, who had missed the neutron, saw that they had also missed the positron). The new particle raised the possibility that the positively charged proton might in fact be compound, might be not a unitary particle but a neutron in association with a positron. (It was not; there proved not to be room in the nucleus for electrons positive or negative.)

  After they had identified the positrons they had missed before, the Joliot-Curies had started up their cloud chamber again and looked for the new particle in other experimental arrangements. They found that if they bombarded medium-weight elements with alpha particles from polonium, the targets ejected protons. Then they noticed that lighter elements, including in particular aluminum and boron, sometimes ejected a neutron and then a positron instead of a proton. That seemed evidence for a compound proton. They presented their evidence with enthusiasm as a report to the Solvay Conference.

  Lise Meitner attacked the Joliot-Curies’ report. She had performed similar experiments at the KWI and she was highly respected for the cautious precision of her work. In her experiments, she emphasized, she had been “unable to uncover a single neutron.”740 Sentiment favored Meitner. “In the end, the great majority of the physicists present did not believe in the accuracy of our experiments,” Joliot says. “After the meeting we were feeling rather depressed.” Fortunately the theoreticians intervened. “B
ut at that moment Professor Niels Bohr took us aside . . . and told us he thought our results were very important. A little later Pauli gave us similar encouragement.”741 The Joliot-Curies returned to Paris determined to settle the issue once and for all.

  Husband and wife were then thirty-three and thirty-six years old, with a small daughter at home. They sailed and swam together in summer, skied together in winter, worked together efficiently in the laboratory in the Latin Quarter on the Rue Pierre Curie. Irene had succeeded her mother as director of the Radium Institute in 1932: the long-widowed pioneer was mortally ill with leukemia induced by too many years of exposure to radiation.

  It seemed likely that the appearance of neutrons and positrons rather than protons might depend on the energy of the alpha particles attacking the target. The Joliot-Curies could test that possibility by moving their polonium source away from the target, slowing the alphas by forcing them to batter their way through longer ranges of air. Joliot went to work. Without question he was seeing neutrons. When he shifted the polonium away from the aluminum-foil target “the emission of neutrons [ceased] altogether when a minimum velocity [was] reached.” But something else happened then to surprise him.742 After neutron emission ceased, positron emission continued—not stopping abruptly but decreasing “only over a period of time, like the radiation . . . from a naturally radioactive element.” What was going on? Joliot had been observing the particles with a cloud chamber, catching their ionizing tracks in its supersaturated fog. Now he switched to a Geiger counter and called in Irene. As he explained to a colleague the next day: “I irradiate this target with alpha rays from my source; you can hear the Geiger counter crackling. I remove the source: the crackling ought to stop, but in fact it continues.”743 The strange activity declined to half its initial intensity in about three minutes. They would hardly yet have dared to think of that period as a half-life. It might merely mark the erratic performance of the Geiger counter.

  A young German physicist who specialized in Geiger counters, Wolfgang Gentner, was working at the institute that year. Joliot asked him to check the lab instruments. The couple went off to a social evening they could find no excuse to avoid. “The following morning,” writes the colleague to whom Joliot spoke that day, “the Joliots found on their desk a little hand-written note from Gentner, telling them that the Geiger counters were in perfect working order.”744

  They were nearly certain then that they had discovered how to make matter radioactive by artificial means.

  They calculated the probable reaction. An aluminum nucleus of 13 protons and 14 neutrons, capturing an alpha particle of 2 protons and 2 neutrons and immediately re-emitting 1 neutron must be converting itself into an unstable isotope of phosphorus with 15 protons and 15 neutrons (13 + 2 protons = 15; 14 + 2 - 1 neutrons = 15). The phosphorus then probably decayed to silicon (14 protons, 16 neutrons). The 3-minute period was the half-life of that decay.

  They could not chemically trace the infinitesimal accumulation of silicon. Joliot explained why in 1935, when he and his wife accepted the Nobel Prize in Chemistry for their discovery: “The yield of these transmutations is very small, and the weights of elements formed . . . are less than 10-15 [grams], representing at most a few million atoms”—too few to find by chemical reaction alone.745 But they could trace the radioactivity of the phosphorus with a Geiger counter. If it did indeed signal the artificial transmutation of some of the aluminum to phosphorus, they should be able to separate the two different elements chemically. The radioactivity would go with the new phosphorus and leave the untransmuted aluminum behind. But they needed a definitive separation that could be carried out within three minutes, before the faint induced radioactivity faded below their Geiger counter’s threshold.

  The request perplexed a chemist in a nearby laboratory—“never having envisaged chemistry from that point of view,” says Joliot—but he contrived the necessary procedure.746 The Joliot-Curies irradiated a piece of aluminum foil, dropped it into a container of hydrochloric acid and covered the container. The acid dissolved the foil, producing, by reaction, gaseous hydrogen, which should carry the phosphorus with it out of solution. They drew off the gas into an inverted test tube. The dissolved aluminum fell silent then but the gas made the Geiger counter chatter: whatever was radioactive had been carried along. A different chemical test proved that the radioactive substance was phosphorus. Joliot bounded like a boy.

  The discovery might serve as an offering to Irène’s ailing mother, who had prepared the daughter and sponsored the son-in-law:

  Marie Curie saw our research work and I will never forget the expression of intense joy which came over her when Irene and I showed her the first artificially radioactive element in a little glass tube. I can still see her taking in her fingers (which were already burnt with radium) this little tube containing the radioactive compound—as yet one in which the activity was very weak. To verify what we had told her she held it near a Geiger-Müller counter and she could hear the rate meter giving off a great many “clicks.” This was doubtless the last great satisfaction of her life.747

  The Joliot-Curies reported their work—“one of the most important discoveries of the century,” Emilio Segrè says in his history of modern physics—in the Comptes Rendus on January 15, 1934, and in a letter to Nature dated four days later.748 “These experiments give the first chemical proof of artificial transmutation,” they concluded proudly.749 Rutherford wrote them within a fortnight: “I congratulate you both on a fine piece of work which I am sure will ultimately prove of much importance.” He had tried a number of such experiments himself, he said, “but without any success”—high praise from the master of experiment.750

  They had demonstrated that it was possible not only to chip pieces off the nucleus, as Rutherford had done, but also to force it artificially to release some of its energy in radioactive decay. Joliot foresaw the potential consequences of that attack in his half of the joint Nobel Prize address. Given the progress of science, he said, “we are entitled to think that scientists, building up or shattering elements at will, will be able to bring about transmutations of an explosive type. . . . If such transmutations do succeed in spreading in matter, the enormous liberation of useful energy can be imagined.” But he saw the possibility of cataclysm “if the contagion spreads to all the elements of our planet”:751

  Astronomers sometimes observe that a star of medium magnitude increases suddenly in size; a star invisible to the naked eye may become very brilliant and visible without any telescope—the appearance of a Nova. This sudden flaring up of the star is perhaps due to transmutations of an explosive character like those which our wandering imagination is perceiving now—a process that the investigators will no doubt attempt to realize while taking, we hope, the necessary precautions.

  Leo Szilard received no invitation to the Solvay Conference. By October 1933 he had not accomplished any nuclear physics of note except within the well-equipped laboratory of his brain. In August he had written a friend that he was “spending much money at present for travelling about and earn of course nothing and cannot possibly go on with this for very long.”752 The idea of a nuclear chain reaction “became a sort of obsession” with him. When he heard of the Joliot-Curies’ discovery, in January, his obsession bloomed: “I suddenly saw that the tools were on hand to explore the possibility of such a chain reaction.”753

  He moved to a less expensive hotel, the Strand Palace, near Trafalgar Square, and settled in to think. He had “a little money saved up” after all, “enough perhaps to live for a year in the style in which I was accustomed to live, and therefore I was in no particular hurry to look for a job”—the excitement of new ideas thus relieving his August urgency.754 The bath was down the hall. “I remember that I went into my bath . . . around nine o’clock in the morning. There is no place as good to think as a bathtub. I would just soak there and think, and around twelve o’clock the maid would knock and say, ‘Are you all right, sir?’ Then I usually got ou
t and made a few notes, dictated a few memoranda.”755

  One of the “memoranda” took the form of a patent application, filed March 12, 1934, relating to atomic energy.756, 757 It was the first of several, that year and the next, all finally merged into one complete specification, “Improvements in or Relating to the Transmutation of Chemical Elements.” (The same day Szilard applied for a patent, never issued, proposing the storage of books on microfilm.758) Szilard had already realized—in September, in the context of inducing a chain reaction—that neutrons would be more efficient than alpha particles at bombarding nuclei. He applied that insight now to propose an alternative method for creating artificial radioactivity:

  In accordance with the present invention radio-active bodies are generated by bombarding suitable elements with neutrons. . . . Such uncharged nuclei penetrate even substances containing the heavier elements without ionization losses and cause the formation of radio-active substances.759

  That was a first step. It was also a cheeky piece of bravado. Szilard had only theoretical grounds for believing that neutrons might induce radioactivity artificially. He had not done the necessary experiments. Only the Joliot-Curies had carried out such experiments so far, and they used alpha particles. Szilard was pursuing more than artificial radioactivity. He was pursuing chain reactions, power generation, atomic bombs. He had not yet found patentable form for these excursions. He wondered which element or elements might emit two or more neutrons for each neutron captured. He decided at some point, he said later, “that the reasonable thing to do would be to investigate systematically all the elements. There were ninety-two of them. But of course this is a rather boring task, so I thought that I would get some money, have some apparatus built, and then hire somebody who would just sit down and go through one element after the other.”760