Making of the Atomic Bomb

by Richard Rhodes

  Apart from the purely scientific interest there may be another aspect of this discovery, which so far does not seem to have caught the attention of those to whom I spoke. First of all it is obvious that the energy released in this new reaction must be very much higher than all previously known cases. . . . This in itself might make it possible to produce power by means of nuclear energy, but I do not think that this possibility is very exciting, for . . . the cost of investment would probably be too high to make the process worthwhile. . . .

  I see . . . possibilities in another direction. These might lead to large-scale production of energy and radioactive elements, unfortunately also perhaps to atomic bombs. This new discovery revives all the hopes and fears in this respect which I had in 1934 and 1935, and which I have as good as abandoned in the course of the last two years. At present I am running a high temperature and am therefore confined to my four walls, but perhaps I can tell you more about these new developments some other time.

  The same day Fermi stepped into the office of John R. Dunning, a Columbia experimentalist whose specialty was neutrons, to propose an experiment. Dunning, his graduate student Herbert Anderson and others at Columbia had built a small cyclotron in the basement of Pupin Hall, the modern thirteen-story physics tower that faces downtown Manhattan from behind the library on the upper campus.1040 A cyclotron was a potent source of neutrons; the two men talked about using it to perform an experiment similar to Frisch’s experiment of January 13–14, of which they were as yet unaware. They discussed arrangements over lunch at the Columbia faculty club and afterward back at Pupin.

  While Fermi was away from his desk Bohr arrived to tell him what he already knew. Finding an empty office, Bohr took the elevator to the basement, to the cyclotron area, where he turned up Herbert Anderson:

  He came right over and grabbed me by the shoulder. Bohr doesn’t lecture you, he whispers in your ear. “Young man,” he said, “let me explain to you about something new and exciting in physics.” Then he told me about the splitting of the uranium nucleus and how naturally this fits in with the idea of the liquid drop. I was quite enchanted. Here was the great man himself, impressive in his bulk, sharing his excitement with me as if it were of the utmost importance for me to know what he had to say.1041

  Bohr was en route to a conference in Washington on theoretical physics that would begin the next afternoon; he left to catch his train without seeing Fermi. As soon as Bohr was gone Anderson hunted up the Italian, who had returned to his office by now. “Before I had a chance to say anything,” Anderson remembers, “he smiled in a friendly fashion and said, ‘I think I know what you want to tell me. Let me explain it to you. . . .’ I have to say that Fermi’s explanation was even more dramatic than Bohr’s.”1042

  Fermi helped Anderson and Dunning begin organizing the experiment he had discussed with Dunning earlier in the day. Anderson happened not long before to have built an ionization chamber and linear amplifier. “All we had to do was prepare a layer of uranium on one electrode and insert it into the chamber. That same afternoon we set up everything at the cyclotron. But the cyclotron was not working very well that day. Then I remembered some radon and beryllium which had been used as a source of neutrons in earlier experiments. It was a lucky thought.”1043 It came too late in the day; Fermi was also attending the Washington conference and had to leave. Anderson and Dunning closed up shop.

  The Washington Conferences on Theoretical Physics, of which the 1939 meeting would be the fifth, were a George Gamow invention. He had stipulated their creation as a condition of his employment at George Washington University in 1934. He took Bohr’s annual gathering in Copenhagen for a model; since there was no comparable assembly in the United States at the time, the Washington Conferences met with immediate success. At the instigation of Merle Tuve, Ernest Lawrence’s boyhood friend and the driving force at the Department of Terrestrial Magnetism of the Carnegie Institution of Washington, the Carnegie Institution co-sponsored the conferences with GWU, though expenses were modest, for travel only, no more in total than five or six hundred dollars a year. People attended because they were interested. Edward Teller recalls the meetings as “in general small and exciting, thoroughly absorbing, and also a little tiring. Somehow, most of the running of the conferences Gamow left to me.”1044 The two men simply chose a topic and made up a list of invitees. Graduate students crowded in to listen. This year’s topic was low-temperature physics.

  Bohr sought out Gamow as soon as he arrived in Washington that evening. Gamow in turn called Teller: “Bohr has just come in. He has gone crazy. He says a neutron can split uranium.” Teller thought of Fermi’s experiments in Rome and the mess of radioactivities they produced and “suddenly understood the obvious.”1045 In Washington Fermi learned to his further disappointment from Bohr that Frisch was supposed to have done an experiment similar to the one left unfinished at Columbia. “Fermi . . . had no idea before that Frisch had made the experiment,” Bohr wrote Margrethe a few days later. “I had no right to prevent others from experimentation, but I emphasized that Frisch had also spoken of an experiment in his notes. I said that it was all my fault that they all heard about Frisch and Meitner’s explanation, and I earnestly asked them to wait [to make a public announcement] until I received a copy of Frisch’s note to Nature, which I hoped would be waiting for me at Princeton [i.e., after the conference].”1046 Fermi, understandably, seems to have argued against further delay.

  Herbert Anderson returned to the basement of Pupin Hall that evening.1047 He retrieved his neutron source. He calculated how many alpha particles the uranium oxide coated on a metal plate inside his ionization chamber would eject spontaneously in its normal process of radioactive decay: three thousand per minute. He calculated the probability of ten of those alphas appearing simultaneously to produce a spurious high-energy kick of the scanning beam of his oscilloscope: “practically never,” he concluded in his laboratory notebook.

  He set the neutron source beside the ionization chamber a little after 9 P.M. and began observing the effect on the oscilloscope. “Most kicks are due to .4 cm range α part[icles] [of approximately] .65 M[e]V,” he noted. Then he saw what he was looking for: “Now large kicks which occur infrequently about 1 every 2 minutes.” He counted them against the clock. In 60 minutes he had counted 33 large kicks. He removed the neutron source. “In 20 min” without a neutron source, he wrote, “0 counts.” It was the first intentional observation of fission west of Copenhagen.

  Dunning showed up later that evening, Anderson remembers, and “was very excited by the result I’d gotten.” Anderson thought Dunning would telegraph Fermi immediately, but he seems not to have done so.1048 Frisch, as he told Bohr later, had cabled no news of his confirming Copenhagen experiment because it seemed to him “just additional evidence of a discovery already made” and “cabling to you would have appeared unmodest to me.”1049 Dunning, despite his excitement at seeing the new phenomenon for himself, may have felt the same way.

  Bohr woke to his dilemma. The conference would begin at two. As recently as three days previously he had written Frisch again, chiding him for not sending a copy of his and Meitner’s Nature note. But he was less concerned now with that delay than he was with protecting the priority of Frisch’s experiment, if any. Reluctantly he acceded to public announcement, stressing, he wrote Frisch afterward, “that no public account . . . could legitimately appear without mentioning your and your aunt’s original interpretation of the Hahn results.”1050

  Fifty-one participants sat for a photograph in the course of the Fifth Washington Conference, and even a partial list of their names confirms the event’s prestige.1051 Otto Stern attended; Fermi; Bohr; Harold Urey of Columbia, who won the 1934 Nobel Prize in Chemistry for isolating a heavy form of hydrogen, deuterium, that carried a neutron in its nucleus; Gregory Breit, a waspish but inspired theoretician; Rabi; George Uhlenbeck, then at Columbia, who had been Paul Ehrenfest’s assistant; Gamow; Teller; Hans Bethe down from Corne
ll; Léon Rosenfeld; Merle Tuve. Conspicuously absent was the Western crowd, probably because the two sponsoring institutions chose not to budget such long-distance travel.

  Gamow opened the meeting by introducing Bohr.1052 His news galvanized the room. A young physicist watching from the back saw an immediate application. Richard B. Roberts, Princeton-trained, worked with Tuve at the Department of Terrestrial Magnetism, the experimental section of the Carnegie Institution, located in a parklike setting in the Chevy Chase area of the capital. Roberts—thin, vigorous, with a strong jaw and wavy dark hair—still remembered the occasion vividly in 1979 in a draft autobiography:

  The Theo. Phys. Conference for 1939 was on the topic of low temperatures and I was not eager to attend. However, I went down to sit in the back row of the meeting. . . . Bohr and Fermi arrived and Bohr proceeded to reveal his news concerning the Hahn and Strassmann experiments. . . . He also told of Meitner’s interpretation that the uranium had split. As usual he mumbled and rambled so there was little in his talk beyond the bare facts. Fermi then took over and gave his usual elegant presentation including all the implications.1053

  Roberts noted in a letter to his father the Monday after the conference ended that “Fermi also . . . described an obvious experiment to test the theory”—Frisch’s experiment, Fermi’s, Dunning’s and Anderson’s experiment. “The remarkable thing is that this reaction results in 200 million volts of energy liberated and brings back the possibility of atomic power.”1054

  Bohr was calling the fission fragments “splitters.” For the time being everyone borrowed that comical usage. Lawrence R. Hafstad, a longtime associate of Tuve, was sitting beside Roberts. When Fermi finished, the two men looked at each other, got up, left the meeting and lit out for the DTM. If “splitters” issued forth from uranium they intended to be among the first to see them.

  * * *

  In New York that day Szilard dragged himself to the nearest Western Union office and cabled the British Admiralty:

  KINDLY DISREGARD MY RECENT LETTER STOP WRITING1055

  The secret patent had revived.

  * * *

  Naturwissenschaften reached Paris about January 16. One of Frédéric Joliot’s associates recalls that “in a rather moving meeting [Joliot] made a report on this result to Madame Joliot and myself after having locked himself in for a few days and not talked to anybody.”1056, 1057 The Joliot-Curies were once again appalled to find they had barely missed a major discovery. In the next few days Joliot independently deduced the large energy release and considered the possibility of a chain reaction, as Szilard had thought he might. He tried to track down the neutrons from fission first, found that approach difficult, then set up an experiment somewhat like Frisch’s. He detected fission fragments on January 26.

  * * *

  The newest building on the DTM grounds was the Atomic Physics Observatory, the working contents of which had just been brought on line two weeks before: a new 5 MV pressure Van de Graaff generator that Tuve, Roberts and their colleagues had built for $51,000 to extend their studies in the structure of the nucleus. The Van de Graaff was named for the Alabama-born physicist who invented it, but Tuve was the first—in 1932—to put it to practical use in experiment. It was essentially a monumental static-electricity generator, an insulated motor-driven pulley belt that picked up ions from discharge needles in its metal base, carried them up through an insulated support cylinder into a smooth metal storage sphere and deposited them on the sphere. As ions accumulated the sphere’s voltage increased. The voltage could then be discharged as a spark—Van de Graaffs discharging lightning-bolt sparks have been staples of madscientist movies—or drawn off to power an accelerator tube. The new machine was built inside a pear-shaped pressure tank, as large as the tank of a water tower, that helped reduce accidental sparking.

  When Tuve had first proposed the Van de Graaff to the zoning board of the prosperous Chevy Chase neighborhood the board had turned him down. Smashing atoms smacked of industrial process and the neighborhood had its property values to consider. Tuve noted the popularity of the Naval Observatory, across Connecticut Avenue a few miles west, and rechristened his project the Atomic Physics Observatory, which it was. As the APO it won approval.1058

  Roberts and Hafstad chose to work with the APO. They had intended to use the old 1 MV Van de Graaff in the building next door to make neutrons for their splitter experiment, but that machine’s ion-source filament was burned out. Although the APO’s vacuum accelerator tube leaked, finding the leak looked to be less tedious than replacing the filament. In fact it needed two days. Hafstad went off Friday night on a ski weekend and another young Tuve protégé, R. C. Meyer, took his place.

  Roberts’ laboratory notebook entries summarize Saturday’s work:

  Sat 4:30 PM1059

  Set up ionization chamber to try to detect

  Neutrons from Li + D [accelerated deuterium nuclei bombarding lithium]

  . . .

  With uranium lined I. C. observed

  α’s [approximately] 1-2 mm and occasional 35 mm kicks (Ba + Kr?)

  The APO’s target room was a small circular basement accessible down a steel ladder, a chilly kiva that smelled pleasantly of oil. As soon as Roberts saw the “tremendous pulses corresponding to very large energy release” he and Meyer ran every test they could think of.1060 “We promptly tried the effect of paraffin (for slow neutrons) and then cadmium to remove the slow neutrons. We also tried all the other heavy elements available [to determine if they would split] and saw the same [i.e., fission] with thorium.”1061 Having made that original discovery (Frisch had made it independently in Copenhagen before them) they stopped to eat. “I told Tuve after supper and he immediately called Bohr and Fermi and they came out Saturday night.”1062

  Not only Bohr and Fermi came, in heavy, dark, pin-striped three-piece suits, Fermi swarthy with a day’s growth of beard, but also Tuve; Rosenfeld; Teller; Erik Bohr, handsome in a heavy overcoat over a decorative Danish sweater; Gregory Breit, owlish in spectacles; and John A. Fleming, the conservative director of the DTM, who had the presence of mind to bring along a photographer. All except Teller posed in the target room with Meyer and Roberts for a historic photograph.1063 The box of the ionization chamber in the foreground is stacked with disks of paraffin; Bohr holds the stub of an after-dinner cigar; Fermi’s grin reveals the gap between his front teeth left by a baby tooth he shed late; Roberts looks into the camera weary but satisfied. Fermi had been amazed at the ionization pulses on the oscilloscope and had insisted they check for equipment malfunctions: he had never seen such pulses in Rome (they were captured by the aluminum foil Amaldi had wrapped around his uranium to block its alpha background).1064 Bohr was still fretting. “I had to stand and look at the first [sic] experiment,” he wrote Margrethe, “without knowing certainly if Frisch had done the same experiment and sent a note to Nature”.1065 Back at Princeton on Sunday he learned from other family letters that Frisch had. “There followed,” Roberts concludes, “several days of excitement, press releases and phone calls.”1066

  Science reporter Thomas Henry had attended the conference; his story appeared in the Washington Evening Star on Saturday afternoon. The Associated Press picked it up. Shortened, it earned a place on an inside page of the Sunday New York Times. Dunning may have seen it there; he finally wired Fermi news that morning of the Columbia experiment. As Herbert Anderson remembers it, “Fermi . . . rushed back to Columbia and straightaway called me into his office. My notebook lists the experiments he felt we should do right away. The date was January 29, 1939.”1067 They had already agreed, says Anderson, that “I would teach him Americana, and he would teach me physics.”1068 Both lessons began in earnest.

  The San Francisco Chronicle picked up the wire-service story. Luis W. Alvarez, Ernest Lawrence’s tall, ice-blond protégé, a future Nobelist whose father was a prominent Mayo Clinic physician, read it at Berkeley sitting in a barber chair in Stevens Union having his hair cut. “So [I told] th
e barber to stop cutting my hair and I got right out of that barber chair and ran as fast as I could to the Radiation Lab . . . where my student Phil Abelson . . . had been [trying to identify] what transuranium elements were produced when neutrons hit uranium; he was so close to discovering fission that it was almost pitiful.”1069 Abelson still remembers the painful moment: “About 9:30 a.m.1070 I heard the sound of running footsteps outside, and immediately afterward Alvarez burst into the laboratory. . . . When [he] told me the news, I almost went numb as I realized that I had come close but had missed a great discovery. . . . For nearly 24 hours I remained numb, not functioning very well. The next morning I was back to normal with a plan to proceed.” By the end of the day Abelson found iodine as a decay product of tellurium from uranium irradiation, another way the nucleus could split (i.e., tellurium 52 + zirconium 40 = U 92).

  Alvarez wired Gamow for details, learned of the Frisch experiment, then tracked down Oppenheimer:

  I remember telling Robert Oppenheimer that we were going to look for [ionization pulses from fission] and he said, “That’s impossible” and gave a lot of theoretical reasons why fission couldn’t really happen. When I invited him over to look at the oscilloscope later, when we saw the big pulses, I would say that in less than fifteen minutes Robert had decided that this was indeed a real effect and . . . he had decided that some neutrons would probably boil off in the reaction, and that you could make bombs and generate power, all inside of a few minutes. . . . It was amazing to see how rapidly his mind worked, and he came to the right conclusions.1071

  The following Saturday Oppenheimer discussed the discovery in a letter to a friend at Caltech, outlining all the experiments Alvarez and others had accomplished during the week and speculating on applications:

  The U business is unbelievable. We first saw it in the papers, wired for more dope, and have had a lot of reports since . . . In how many ways does the U come apart? At random, as one might guess, or only in certain ways? And most of all, are there many neutrons that come off during the splitting, or from the excited pieces? If there are, then a 10 cm cube of U deuteride (one would need the D [deuterium, heavy hydrogen] to slow them without capture) should be quite something.1072 What do you think? It is I think exciting, not in the rare way of positrons and mesotrons, but in a good honest practical way.