bomb of chain-reacting uranium was now fairly begun, and this time it had found the right — fast — track.
Szilard chafed. The months after the first Uranium Committee meeting became “the most curious period of my life.” No one called. “We heard nothing from Washington at all… I had assumed that once we had demonstrated that in the fission of uranium neutrons are emitted, there would be no difficulty in getting people interested; but I was wrong.” The Uranium Committee's November 1 report had in fact been languishing in Roosevelt's files; Watson finally decided on his own in early February 1940 to bring it up again. He asked Lyman Briggs if he had anything to add. Briggs reported the transfer, finally, of the $6,000 for Fermi's work on neutron absorption in graphite. That was “a crucial undertaking,” Briggs said; he imagined it would determine “whether or not the undertaking has a practical application.” He proposed to wait for results.
Something other than Briggs' penurious methodology triggered a new burst of activity from Szilard. He had spent the winter preparing a thorough theoretical study, “Divergent chain reactions in systems composed of uranium and carbon” — divergent in this case meaning chain reactions that continue to multiply once begun (the document's first footnote, numbered zero, cited “H. G. Wells, The World Set Free [1913]”). Early in the new year Joliot's group reported a uranium-water experiment that “seemed to come so close to being chain-reacting,” says Szilard, “that if we improved the system somewhat by replacing water with graphite, in my opinion we should have gotten over the hump.” He arranged lunch with Fermi to discuss the French paper. “I asked him, ‘Did you read Joliot's paper?’ He said he did. I asked him, ‘What did you think of it?’ and Fermi said, ‘Not much.’” Szilard was furious. “At which point I saw no reason to continue the conversation and went home.”
He traveled again to Princeton to see Einstein. They worked up another letter and sent it under Einstein's signature to Sachs. It emphasized the secret German uranium research at the Kaiser Wilhelm Institutes, about which they had learned from the physical chemist Peter Debye, the 1936 Nobel laureate in chemistry and director of the physics institute at Dahlem, who had been expelled recently to the United States, ostensibly on leave of absence, when he refused to give up Dutch citizenship and join the Nazi Reich. Sachs sent the Einstein letter on to Pa Watson for FDR. But Watson thought it sensible to check first with the Uranium Committee. Adamson responded, echoing Briggs: everything depended on the graphite measurements at Columbia. Watson proposed to wait for the official report. Sachs may have rebutted; Roosevelt wrote the gadfly economist on April 5 emphasizing that the Briggs committee was “the most practical method of continuing this research” but also calling for another committee meeting that Sachs might attend. Briggs dutifully scheduled it for Saturday afternoon, April 27.
In the meantime another development intervened. Alfred Nier at the University of Minnesota had gone to work, after Fermi wrote urging him again to do so, to prepare to separate measurable samples of U235 and U238. John Dunning sent him uranium hexafluoride, a highly corrosive compound that is a white solid at room temperature but volatilizes to a gas when heated to 140°F. “I worked with this for a couple of months in late 1939,” Nier remembers. Unfortunately the gas was too volatile; it dispersed through Nier's three-foot glass spectrometer tube despite the best efforts of his vacuum pump to clear it and contaminated the collector plates:
Finally I said, “This won't do.” A new instrument was built in about 10 days in February, 1940. Our glass blower bent the horseshoe-shaped mass spectrometer tube for me; I made the metal parts myself. As a source of uranium, I used the less volatile uranium tetrachloride and tetrabromide left over from [his earlier] Harvard experiments. The first separation of U-235 and U-238 was actually accomplished on February 28 and 29, 1940. It was a leap year, and on Friday afternoon, February 29,1 pasted the little samples [collected on nickel foil] on the margin of a handwritten letter and delivered them to the Minneapolis Post Office at about six o'clock. The letter was sent by airmail special delivery and arrived at Columbia University on Saturday. I was aroused early Sunday morning by a long-distance telephone call from John Dunning [who had worked through the night bombarding the samples with neutrons from the Columbia cyclotron]. The Columbia test of the samples clearly showed that U-235 was responsible for the slow neutron fission of uranium.
The demonstration vindicated Bohr's hypothesis, but it also led Briggs to even greater suspicion of the value of natural uranium; it was “very doubtful,” he reported to Watson on April 9 “whether a chain reaction can be established without separating 235 from the rest of the uranium.” Nier, Dunning and their collaborators Eugene T. Booth and Aristide von Grosse had written much the same thing in the
Predictably, then, when the Uranium Committee met on April 27, with Sachs, Pegram, Fermi, Szilard and Wigner in attendance, it listened to the renewed debate, squared its shoulders at Sachs' exhortation to plunge ahead — and never wavered in its adamant conviction that a large-scale uranium-graphite experiment should await the outcome of Fermi's graphite measurements.
Now that the $6,000 had been paid, Columbia was able to buy the graphite Szilard had tracked down for Fermi's use. “Cartons of carefully-wrapped graphite bricks began to arrive at the Pupin Laboratory,” Herbert Anderson remembers, four tons in all. “Fermi returned to the chain reaction problem with enthusiasm. This was the kind of physics he liked best. Together we stacked the graphite bricks in a neat pile. We cut narrow slots in some of the bricks for the rhodium foil detectors we wanted to insert, and soon we were ready to make measurements.”
“So the physicists on the seventh floor of Pupin Laboratories started looking like coal miners,” adds Fermi, “and the wives to whom these physicists came back tired at night were wondering what was happening.”
The arrangement was designed to determine how far neutrons from a radon-beryllium source set in paraffin on the floor under the graphite column would diffuse up the column through the graphite after first slowing down in scattering collisions: the farther the neutrons traveled, the smaller was carbon's absorption cross section and therefore the better moderator it would be. The Pupin seventh floor became a racetrack like the second floor of the institute in Rome. Anderson describes the scene:
A precise schedule was followed for each measurement. With the rhodium in place in the graphite, the source was inserted in its position inside the pile and removed after a one-minute exposure. To get the rhodium foil under the Gei-ger counter in the allotted 20 seconds [because its induced half-life is only 44 seconds] took coordination and some fast legwork. The division of labor was typical. I removed the source on signal; Fermi, stopwatch in hand, grabbed the rhodium and raced down the hall at top speed. He had just enough time to place the foil carefully into position, close the lead shield and, at the prescribed moment, start the count. Then with obvious satisfaction at seeing everything go right, he would watch the flashing lights on the scaler, tapping his fingers on the bench in time with the clicking of the register. Such a display of the phenomenon of radioactivity never failed to delight him.
The absorption cross section, as Fermi and Anderson subsequently calculated it, proved usefully small: 3 ? 10–27 cm2. And could be made smaller still, they thought, with purer graphite. The measurement strongly supported Fermi's and Szilard's plan to attempt to induce a slow-neutron chain reaction in natural uranium.
But while such a plan might demonstrate a potential future source of power, the American scientists and administrators who were advising Briggs could not yet identify any military use. In April the British Thomson committee asked A. V. Hill, a scientific adviser to the British Embassy in Washington, to find out what the Americans were doing about fission. According to the official history of the British atomic energy program, Hill talked to unidentified “scientists of the Carnegie Institution,” whose opinions he reported pungently:
