by Gregg Herken
It will not be a calamity if, when we get the answers to the uranium problem, they turn out negative from the military point of view, but if the answers are fantastically positive and we fail to get them first, the results for our country may well be a tragic disaster.… I feel strongly, therefore, that anyone who hesitates on a vigorous, all-out effort on uranium assumes a grave responsibility.6
What Lawrence did not tell Compton was that he had already decided to proceed on his own in the bomb project—with Oppenheimer’s help.7
* * *
Thus far, Nier’s spectrograph had produced only minute traces of U-235. At the current rate, it would take some 25 million years to produce the amount of uranium that Oppenheimer thought necessary for an atomic bomb.
In early November, Lawrence went to see Nier in Minnesota, “to see if he could not ‘steam up’ his output and produce the needed quantities in a hurry.”8 His visit convinced Ernest that Nier was not up to the task, and that the “only immediate recourse was to undertake the job ourselves at home.” Indeed, Nier himself arrived at the Rad Lab soon afterward to help with the conversion of the 37-inch. The boys began dismantling the cyclotron under Brobeck’s direction later that month.
Originally content to wait for the British to determine the fissionability of U-235, Ernest was no longer willing to let others set the pace.9 Since his return from Schenectady, the cyclotroneers had begun working late into the night and on weekends.
On November 27, 1941, Lawrence received a further push from the final report of Compton’s panel, whose conclusion this time was brief and unequivocal: “A fission bomb of superlative destructive power will result from bringing quickly together a sufficient mass of element U-235.”10 The chief uncertainty remaining was how much uranium would be necessary for a bomb. Here Compton had hedged his bets, citing a range of estimates—from less than 5 to more than 220 pounds—an answer which reflected the persisting ignorance about the properties of U-235.
While Compton’s brief summary noted that both the centrifuge and gaseous diffusion were approaching practical tests, there was only a vague allusion to “other methods … which may ultimately prove superior, but are now farther from the engineering stage.”11 No specific mention was made of the electromagnetic method on which Lawrence had begun to pin his hopes.
Sick in bed with a bad cold on November 27, Lawrence dictated a congratulatory letter to Compton. But, in Ernest’s eyes, the panel’s final report hardly mattered. “The boys here already have their 37-inch mass spectrograph between the poles of the magnet,” he exulted.12
* * *
While Lawrence understood that transforming a cyclotron into a working uranium separator was not a simple matter, the concept behind the mass spectrograph, at least, was straightforward enough—relying upon the slight but unvarying difference in mass between isotopes to carry out the separation, atom by atom.13
In principle, it was not unlike pitching rocks into buckets. In the spectrograph, an electrically charged beam of ionized uranium atoms bent by a magnet would split into two, carrying U-238 farther from the source than the lighter isotope, U-235, because of the heavier isotope’s greater mass. When the uranium hit the metal collector “bucket,” the ions gave up their charge and the vapor condensed as microscopic flakes of metal. By repeating the process—putting the contents of the closer bucket back into the beam—further concentration, or “enrichment,” of the U-235 would result.14
While the theory was simple, the practical difficulties were many. Since the difference in mass between U-238 and U-235 was barely 1 percent, this translated—even under ideal circumstances—into a separation of 3/10 inch between collectors for a beam with a 4-foot arc. In reality, this distance was almost vanishingly small.
Yet even before the practical problems of separating uranium by this method had been given serious consideration, a seemingly insurmountable theoretical barrier threatened to put an end to the work. The prevailing wisdom was that a powerful, narrowly focused beam would be impossible in a mass spectrograph because of an elemental fact of physics: electrostatic repulsion would force any stream of ions bearing the same charge to spread out, defocusing the beam and making the process unworkable.15
The “space charge” problem had persuaded Nier and Britain’s M.A.U.D. committee that gaseous diffusion remained the most promising method of separating uranium. The Uranium Committee told Princeton physicist Henry Smyth early in 1941 that electromagnetic separation on an industrial scale “had been investigated and was considered impossible.”16 Among physicists, only Lawrence persisted in believing that intuition and an empirical approach could, once again, defeat the pessimism of the theorists.
That faith was to be tested in the coming days and weeks, as Ernest and the boys working on the 37-inch cyclotron oscillated between hope and despair.
But in the early morning hours of Monday, December 1, the rebuilt cyclotron produced the first beam in its incarnation as a mass spectrograph. “Got ions in the 37-inch,” Cooksey laconically recorded in his diary. Berkeley’s beam was already ten times more powerful than that of Nier’s machine.17
* * *
On Saturday, December 6, 1941, Ernest was back in Washington, summoned to OSRD headquarters by Bush to discuss reorganization of what was being called the “S-1 Project.” That morning in Berkeley the new spectrograph had defied the theorists by separating uranium, albeit an almost-infinitesimal amount. Receiving the news by telephone from Cooksey, Lawrence boasted to the new S-1 Committee that the Rad Lab spectrograph was producing a microgram of enriched uranium every hour.18
In truth, the tiny green speck of U-235 left in the near bucket was almost too small to see. Most of the uranium had been smeared around the inside of the machine by the beam. But with the future at stake, Lawrence touted even this meager success as a triumph.
Largely on the strength of Lawrence’s claim, Bush and Conant assigned to him the responsibility for providing the first samples of enriched uranium for later experiments.19
When the meeting broke up at noon, Bush, Conant, and Compton went around the corner for lunch to the Cosmos Club. Realizing that he now had to make good on his commitment, Ernest promptly left for the airport and the next flight west.
By Sunday morning, he was back at the 37-inch. Lawrence and the boys learned of the Japanese attack on Pearl Harbor from the radio that was always left on at the lab. Elsewhere on campus, word spread quickly. Kamen was startled when the normally unflappable Seaborg burst into the Faculty Club reading room in an agitated state. Cooksey, enjoying a weekend sail on the Bay, did not hear of the attack until he returned to the marina. Oppenheimer received the news at home, where he was sleeping late after attending a benefit for Spanish civil war veterans the night before.20
At the Rad Lab that evening, the first visible traces of shiny uranium metal began accumulating in the spectrograph’s collectors. The uranium in the near bucket was five times richer in U-235 than the far collector. That night, after most of the cyclotroneers had gone home to be with their families, Lawrence stayed behind, filled with what he later described as a mixture of hope and foreboding, walking the perimeter fence until nearly dawn.21
Bleary-eyed, he called the boys together on Monday morning to announce that henceforth any work not directly related to defense would be immediately suspended.22 Later, he sent telegrams and worked the phones, pleading with Rad Lab alumni at colleges and universities across the country to return to Berkeley. A great many not only answered the call but brought their own acolytes with them. Ernest persuaded the campus representative of Realsilk Hosiery, a company that hired student salesmen, to become the lab’s recruiter for nonscientific personnel. For the first time, a guard—a young law student—was stationed at the bottom of the road up to Cyclotron Hill, armed with a .410 shotgun borrowed from Cooksey.23
Back in Washington by mid-December, Lawrence asked for $400,000 to explore electromagnetic separation for the next six months. His request was the first made t
o the S-1 Committee in wartime. In a measure of how much things had changed since Pearl Harbor, it was approved promptly and virtually without discussion.24
The funds would be used to build the prototype of a production mass spectrograph five times the 37-inch in size. Even as he returned, discouraged, from Nier’s lab the previous month, Lawrence had already mentally resolved to take the next logical step: converting the still-uncompleted 184-inch to separate uranium. Not yet born as an instrument of science, the great cyclotron was to be transformed while still in the womb into a weapon of the war.25
As Lawrence was aware, the race was no longer only with the Axis. Berkeley’s immediate rival was not Germany but Princeton; his challenger the former student he had fired twice from the Rad Lab.
* * *
Robert Wilson was a twenty-one-year-old experimentalist from Frontier, Wyoming, whom Ernest had dismissed the first time for losing a rubber seal in the 37-inch. Since the seal prevented the machine from running on the very day that John had scheduled a demonstration of his neutron-ray therapy to a prospective funder, both brothers had flown into a rage.26 Rehired at Alvarez’s instigation, Wilson had later been fired a second time for melting a pair of pliers while welding a probe onto the cyclotron vacuum tank.27 Offered his job back again, Wilson not surprisingly decided instead to accept an offer from Princeton, where Henry Smyth was exploring a different approach to electromagnetic separation.
Smyth’s project, subsequently taken over by Wilson, had by early 1941 produced a device dubbed the “Isotron,” which used an electrical rather than a magnetic field to separate uranium.28 Wilson began to suspect that the reason why Lawrence stopped by Princeton on his way home from Washington was not only to check on his rival’s progress but to lure workers away to the Rad Lab.29
* * *
Once the decision to proceed with the bomb had been made, a location for the project had to be found. The leading candidates were Columbia, Princeton, Chicago, and Berkeley. Ernest, of course, remained a strong advocate for the Rad Lab. Operation of the 37-inch spectrograph was steadily improving, and the 60-inch was still the world’s only source of element 94. Moreover, one practical consideration dominated all: there was no possibility of moving the massive machines.30
Another compelling argument for Berkeley was Oppenheimer. Shortly after Pearl Harbor, Compton had asked Oppie to take over the theoretical calculations on bomb physics from Gregory Breit.31
Despite Ernest’s earlier promise to Bush and Conant, approaching year’s end the Rad Lab had produced a mere 25 micrograms of uranium metal, enriched to barely 3 percent U-235.32 Lawrence was nonetheless in a triumphant mood by Christmas Eve, when the regents approved his plan to convert the 184-inch into a uranium separator.33 His race with Princeton remained neck and neck. That day, when Smyth cabled Lawrence with the latest results from the Isotron, Ernest’s competitive spirit overruled the holiday mood in his reply: “Three cheers. How about lower temperatures? Merry Christmas.”34 On New Year’s Day, Lawrence wrote to Smyth suggesting that he and Wilson consider shifting their workers to Berkeley—“since you are having difficulties recruiting there.”35
A week later, Lawrence was once again on his way east by train, this time in the company of Alvarez and Alfred Loomis. Compton had declared the project’s deadlines: January 1943 to achieve the first self-sustaining atomic chain reaction; January 1944 to extract the first samples of element 94 from an atomic reactor; January 1945 for a bomb.36
From New York, Lawrence and Alvarez took the train to Chicago, where they found Compton at home, sick with the flu. Sitting on the edge of the bed, Ernest presented the case for moving the entire bomb project to Berkeley. Compton had recently decided upon Chicago as the most practical site.37
Lawrence challenged the decision, declaring that, compared to Berkeley, the “tempo of the University of Chicago” was too slow.38 Bristling, Compton countered that he would have an atomic chain reaction going by the end of the year. “I’ll bet you a thousand dollars you won’t,” replied Lawrence with some heat.
When tempers cooled, Ernest dropped the stakes to a nickel cigar.39 His wager aside, Lawrence remained unwilling to accept that Berkeley had lost out to Chicago. In a telegram, he notified Conant that Compton’s choice was “acceptable only as temporary arrangement.”40
Within six weeks, however, there was an entirely new enterprise on the Chicago campus, the Metallurgical Laboratory, which would oversee research into the atomic chain reaction. Work already under way at Columbia, where Fermi had been designing a so-called atomic pile to produce element 94, was unceremoniously moved to the Windy City. Lawrence consented, reluctantly, to lend Seaborg and Segrè to the new “Met Lab.” Seaborg, a codiscoverer of the mysterious element, had meanwhile decided to call it plutonium.41 If Ernest’s uranium enrichment plans failed, plutonium still offered a promising candidate for the bomb. But wartime secrecy would keep it off the periodic chart.42
* * *
At Berkeley, the war had brought a halt to normal academic routine. Called upon to carry out chemical warfare experiments for the army, Martin Kamen and Sam Ruben released small quantities of isotope-tagged gas on Marin County beaches late at night. Ruben’s adviser, chemist Kenneth Pitzer, had already left for Washington, where he led an OSRD-funded project—headquartered at the Congressional Country Club—to develop secret weapons for Allied spies and saboteurs.43
At the Donner Laboratory, the exigencies of war had literally crowded out the healing arts.44 The brand-new third floor, added to the lab to accommodate John’s radiophosphorous clinic, was commandeered by Ernest for the bomb project before the first patient arrived.45
By mid-January 1942, the winding of the upper core of the magnet for the 184-inch was completed. A week later, the final shipment of copper arrived. Lawrence’s latest plan—approved by Sproul—was to fit a number of individual mass spectrographs between the poles of the giant magnet. Originally, the magnet was not expected to be finished until November. After Lawrence obtained emergency funds from the Rockefeller Foundation, however, work was begun around the clock; the completion date was moved up to spring.
Yet even as conversion of the 184-inch proceeded at a breakneck pace, the feasibility of electromagnetic separation remained unproven on an industrial scale. One recent arrival at the Rad Lab was surprised to hear Lawrence shout with joy after a particular run on the converted 37-inch. The fact that a Geiger counter showed the uranium in the bucket to be only very slightly enriched did not seem to dull Lawrence’s enthusiasm.46
But things were steadily improving. By February, the “stockpile” of uranium metal at the Rad Lab exceeded 200 micrograms, enriched to an average 35 percent U-235.47 The converted 37-inch had begun producing 2 micrograms an hour.
During a quick visit to Berkeley that month to check on progress, Bush found the upbeat mood at the Rad Lab not only “refreshing” but contagious.48 Bush wrote Roosevelt a few days later that the electromagnetic method could conceivably provide a shortcut to the bomb, delivering enough material for a weapon by the summer of 1943. A finished bomb might then be ready as early as 1944.
In other encouraging news, Oppenheimer’s latest calculations indicated that the amount of U-235 needed for a weapon was at the low end of the original estimates: closer to 2 kilograms than 100.49 Still, even that goal—a mere 4 to 5 pounds of silvery metal, 80 to 90 percent pure—lay far in the future.
Lawrence remained enough of a realist to want to hedge his bets. Unlike Conant, who was now willing to stake everything on the electromagnetic method, Lawrence urged that the government also back the other “horsemen” in the race: the centrifuge, gaseous diffusion, and Fermi’s plutonium-producing atomic pile.50
In letters to Conant that winter, Lawrence’s mood seesawed between jubilation and despair.51 But in the spring, when the output of the 37-inch hit a new peak, Ernest’s confidence returned.
While designing the spectrographs to go between the poles of the big magnet, Brobeck had
invented a C-shaped vacuum chamber and increased the power of the beam by another factor of ten. The Rad Lab’s personnel director proposed a name for the new machine that would forever link it to the University of California: the “Calutron.”52
Two weeks after the first new vacuum tank was installed between the poles of the 37-inch magnet, the prototype Calutron was already exceeding expectations. “It, therefore, seems clear that we should proceed immediately with the design and construction of a multiple mass spectrograph using the giant cyclotron magnet,” Lawrence wrote to Conant—requesting an additional $200,000 for the purpose.53 “We are rapidly learning the art, and things look better all the while,” Ernest boasted at the end of March.54
Since the magnet of the 184-inch was due to be finished in another week, Lawrence also judged the time propitious to bring Berkeley’s foremost theoretical physicist closer to the project.
Whereas iron shims had been adequate for focusing the beam of the 37-inch, separating enough uranium for a bomb required a more fundamental understanding of the physics behind the spectrograph. Mindful that he had been scolded before for prematurely revealing secrets, Lawrence this time approached Conant cautiously:
One other matter I should like to bring to your attention is the desirability of asking Oppenheimer to serve as a member of S-1. I think he would be a tremendous asset in every way. He combines a penetrating insight of the theoretical aspects of the whole program with solid common sense, which sometimes in certain directions seems to be lacking, and I am sure that you and Dr. Bush would find him a useful adviser.55
* * *
Oppenheimer, of course, had already been introduced to the bomb project, as Conant had rueful reason to know. What Lawrence proposed to do was make Oppie a full-time consultant to the S-1 Committee, laying bare all of its secrets. Unmentioned by Lawrence in his letter was a concern that had already given Conant pause, and that remained a worry even with Ernest: Oppenheimer’s politics.
