The Making of the Atomic Bomb

manuscript had set the barium results “against all previous laws of nuclear physics.” He moderated the phrase in proof to “against all previous experience.”

But even with the carbon copy, the missing page and the December 21 letter finally at hand in Kungalv, Meitner hesitated to leap. On January 1, after conveying New Year's greetings to Hahn, she wrote: “We have read your work very thoroughly and consider it perhaps possible energetically after all that such a heavy nucleus bursts.” She veered off to worry about their misbegotten transuranics, “not a good reference for my new start.” Frisch added a New Year's wish of his own and a more genial reservation: “If your new findings are really true, it would certainly be of the greatest interest and I am very curious about further results.”

Meitner returned to Stockholm later that day and Frisch to Copenhagen. He was “keen to submit our speculations — it wasn't really more at that time — to Bohr.” The note of hesitancy in their letter to Hahn suggests they sought the authority of Bohr's blessing. Frisch saw him on January 3: “I had hardly begun to tell him, when he struck his forehead with his hand and exclaimed, ‘Oh what idiots we have all been! Oh but this is wonderful! This is just as it must be!’” Their conversation lasted only a few minutes, Frisch wrote his aunt that day, “since Bohr immediately and in every respect was in agreement with us… [He] still wants to consider this quantitatively this evening and to talk with me again about it tomorrow.”

In Stockholm that day Meitner had received Hahn's revised proofs. Independently they quieted her doubt. She wrote Hahn emphatically: “I am fairly certain now that you really have a splitting towards barium and I consider it a wonderful result for which I congratulate you and Strassmann very warmly… You now have a wide, beautiful field of work ahead of you. And believe me, even though I stand here very empty-handed at the moment, I am still happy about the marvelousness of these findings.”

Now those findings needed interpretation. Aunt and nephew outlined a theoretical paper by long-distance telephone. Frisch drafted it Friday, January 6, and that evening took the trolley to the House of Honor to discuss it with Bohr, who was leaving for the United States the next morning for a term of work at the Institute for Advanced Study. There was time the next morning to type only part of the draft; Frisch delivered two pages to Bohr at the train station from which he and his nineteen-year-old son Erik were departing for Goteborg harbor. On the assumption that Frisch would immediately send the paper along to Nature Bohr promised not to mention it to their American colleagues until he heard from Frisch that it had been received and was in press. Among the notes he brought to that final discussion Frisch mentioned an experiment to confirm by physical means the Dahlem chemistry.

Harm's and Strassmann's article had been published in Berlin on January 6. When it arrived in Copenhagen the next day Frisch thought to go over the whole business with George Placzek. Placzek was characteristically skeptical and characteristically witty about it. Uranium already suffered from alpha decay, Frisch remembers him scoffing; to think that it could be made to burst as well “was like dissecting a man killed by a falling brick and finding that he would have died of cancer.” Placzek suggested that Frisch use a cloud chamber to look for energetic fragments that would prove the nucleus had split. The institute's radium-based neutron sources would fog a cloud- chamber photograph with gamma radiation, Frisch realized. But a simple ionization chamber would do. “One would expect fast-moving nuclei, of atomic number about 40–50 and atomic weight 100–150, and up to 100 MeV energy to emerge from a layer of uranium bombarded with neutrons,” he explained his experiment in a subsequent report. “In spite of their high energy, these nuclei should have a range, in air, of a few millimetres only, on account of their high effective charge… which implies very dense ionization.” In the course of their short passage his highly charged nuclear fragments would strip about 3 million electrons from the nuclei of air gases. They should be easy to find.

His chamber consisted of “two metal plates separated by a glass ring about 1 cm. high.” The charged plates, which would collect the air ions, connected to a simple amplifier, which connected to an oscilloscope. To the bottom plate he attached a piece of uranium-coated foil. He set up the experiment in the basement of the institute and retrieved three of the neutron sources from the covered well. He placed the sources close to the foil and looked for the expected nuclei to emerge. Since they were highly energetic and strongly ionizing they would create quick, sharp, vertical pulses of the sweeping green beam of the oscilloscope.

Frisch started measurements on the afternoon of Friday, January 13, and “pulses at about the predicted amplitude and frequency (one or two per minute) were seen within a few hours.” He ran checks with either the neutron sources or the uranium lining removed. He wrapped the sources with paraffin to slow the neutrons and “enhanced the effect by a factor of two.” He continued measurements “until six in the morning to verify that the apparatus was working consistently.” As had Werner Heisenberg before him, he lived upstairs at the institute; exhausted, he climbed the stairs to bed. He remembers thinking that 13 had proved once again to be his lucky number.

Even luckier than that: “At seven in the morning I was knocked out of bed by the postman who brought a telegram to say that my father had been released from concentration camp.” His parents would move to Stockholm and share an apartment with his aunt, whose possessions, thanks to Hahn, were eventually shipped.

In “a state of slight confusion” Frisch spent the next day repeating the experiment for anyone who cared to see. One who came down in the morning to the basement laboratory was a black-haired, blue-eyed American biologist of Irish heritage named William A. Arnold who was studying on a Rockefeller Fellowship with George de Hevesy. Arnold was thirty-four, Frisch's age, on leave from the Hopkins Marine Station at Pacific Grove, California. He had made his way to Europe from San Francisco the previous September by freighter with his wife and young daughter. He could have gone to Berkeley to pick up radioisotope technique, but would have missed Hving in Copenhagen, learning from de Hevesy — would have missed contributing a coinage to the gamble that is history. Frisch showed the American the experiment and pointed out the pulses on the oscilloscope. “From the size of the spikes,” Arnold recalls, “it was clear that they must represent 100–200 MeV, very much larger than the spikes from [uranium's natural background of] alpha particles.”

Later that day Frisch looked me up and said, “You work in a microbiology lab. What do you call the process in which one bacterium divides into two?” And I answered, “binary fission.” He wanted to know if you could call it “fission” alone, and I said you could.

Frisch the sketch artist, good at visualizing as his aunt was not, had metamorphosed his liquid drop into a dividing living cell. Thereby the name for a multiplication of life became the name for a violent process of destruction. “I wrote home to my mother,” says Frisch, “that I felt like someone who has caught an elephant by the tail.”

Aunt and nephew conferred by telephone further over the weekend to prepare not one but two papers for Nature: a joint explanation of the reaction and Frisch's report of the confirming evidence of his experiment. Both reports — “Disintegration of uranium by neutrons: a new type of nuclear reaction” and “Physical evidence for the division of heavy nuclei under neutron bombardment” — used the new term “fission.” Frisch finished the two papers on Monday evening, January 16, and posted them airmail to London the next morning. Since he and Bohr had already discussed the theoretical paper and since the experiment only confirmed the Hahn-Strassmann discovery, he did not hurry to let Bohr know.

Bohr sailed on the Swedish-American liner Drottningholm with his son Erik and the Belgian theoretician L6on Rosenfeld. “As we were boarding the ship,” Rosenfeld recalls, “Bohr told me he had just been handed a note by Frisch, containing his and Lise Meitner's conclusions; we should ‘try to understand it.’” That meant a working voyage; a blackboard was duly installed in Bohr's stateroom. The North Atlantic was stormy in that season; it made him “rather miserable, all the time on the verge of seasickness” but hardly stopped the work. The first question he wanted to answer was why, if the nucleus oscillated more or less randomly when it was bombarded, it seemed to prefer splitting into two parts rather than some other number. He was satisfied when he saw that the heaviest nuclei, because of their instability, require no more energy to split than they do to emit a single particle. It was a question of probabilities and two fragments were greatly more probable than a crowd.

The Fermis had arrived in New York on January 2, Laura feeling distinctly alien, Enrico announcing with his usual mock solemnity, “We have founded the American branch of the Fermi family.” They put up temporarily at the King's Crown Hotel, opposite Columbia University, where Szilard was also living. George Pegram, the tall, soft- spoken Virginian who was chairman of the physics department and dean of graduate studies at Columbia, had met the Fermis as they debarked the Franconia; now in turn they waited at dockside to meet Bohr. The American theoretician John Archibald Wheeler, then twenty-nine years old, who had worked with Bohr in