The Making of the Atomic Bomb

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 Ubrary on the upper campus. 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.

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.”

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.” 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.” 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.” 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].” Fermi, understandably, seems to have argued against further delay.

Herbert Anderson returned to the basement of Pupin Hall that evening. 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 a 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. 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 unmo-dest to me.” 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.”

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. 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 Cornell; Leon 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. 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.

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.”

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: