13
The New World
Enrico Fermi’s team at Columbia University had been hard at work through 1941 while the government deliberated. Fermi, Leo Szilard, Herbert Anderson and the young physicists who had joined them may never have known how close they came to orphanhood. The isolation of plutonium at Berkeley added a potential military application to their reasons for pursuing a slow-neutron chain reaction in uranium and graphite, but given the necessary resources Fermi at least would certainly have pursued the chain reaction anyway as a physical experiment of fundamental and historic worth. He had missed discovering fission by the thickness of a sheet of aluminum foil; he would not willingly leave to someone else the demonstration of atomic energy’s first sustained release. Thanks largely to Arthur Compton his work found continued support, which may help explain why he admired the pious Woosterite’s intelligence so extravagantly.
Szilard had finally gone on the Columbia payroll on November 1, 1940, when the $40,000 National Defense Research Committee contract came through for physical-constant measurements. To help Fermi without the friction the two men generated when they worked side by side, Szilard undertook to apply his special talent for enlightened cajolery to the problem of procuring supplies of purified uranium and graphite. The record is thick with his correspondence with American graphite manufacturers dismayed to discover that what they thought were the purest of materials were in fact hopelessly contaminated, usually with traces of boron. The cross section for neutron absorption of that light, ubiquitous, silicon-like element, number 5 on the periodic table, was tremendous and poisonous. “Szilard at that time took extremely decisive and strong steps to try to organize the early phases of production of pure materials,” says Fermi. “ . . . He did a marvelous job which later on was taken over by a more powerful organization than was Szilard himself. Although to match Szilard it takes a few able-bodied customers.”1534
In August and September the Columbia team prepared to assemble the largest uranium-graphite lattice yet devised. A slow-neutron chain reaction in natural uranium, like its fast-neutron counterpart U235, requires a critical mass: a volume of uranium and moderator sufficient to sustain neutron multiplication despite the inevitable loss of neutrons from its outer surface. No one yet knew the specifications of that critical volume, but it was obviously vast—on the order of some hundreds of tons. One way to create a self-sustaining chain reaction might be simply to continue stacking uranium and graphite together. But so crude an experiment, if it worked at all, would teach the experimenter very little about controlling the resulting reaction and might culminate in a disastrous and lethal runaway. Fermi proposed to approach the problem by the more circumspect route of a series of subcritical experiments designed to determine the necessary quantities and arrangements and to establish methods of control.
As always, he built directly on previous experience. He and Anderson had calculated the absorption cross section of carbon by measuring the diffusion of neutrons from a neutron source up a column of graphite. The new experiments would enlarge that column to take advantage of the increased stocks of graphite available and to make room for regularly spaced inclusions of uranium oxide: simplicity itself, but in physical form a thick, black, grimy, slippery mass of some thirty tons of extruded bars of graphite confining eight tons of oxide.1535 Fermi named the structure a “pile.” “Much of the standard nomenclature in nuclear science was developed at this time,” Segrè writes.1536 “ . . . I thought for a while that this term was used to refer to a source of nuclear energy in analogy with Volta’s use of the Italian term pila to denote his own great invention of a source of electrical energy [i.e., the Voltaic battery]. I was disillusioned by Fermi himself, who told me that he simply used the common English word pile as synonymous with heap” The Italian laureate was continuing to master the plainsong of American speech.
The exponential pile Fermi proposed to build (so called because an exponent entered into the calculation of its relationship to a full-scale reactor) would be too big for any of the laboratories in Pupin. He sought larger quarters:
We went to Dean Pegram, who was then the man who could carry out magic around the university, and we explained to him that we needed a big room. And when we say big we meant a really big room. Perhaps he made a crack about a church not being the most suited place for a physics laboratory . . . but I think a church would have been just precisely what we wanted. Well, he scouted around the campus and we went with him to dark corridors and under various heating pipes and so on to visit possible sites for this experiment and eventually a big room, not a church, but something that might have been compared in size with a church was discovered in Schermerhorn [Hall].
There, Fermi goes on, they began to build “this structure that at that time looked again in order of magnitude larger than anything that we had seen before. . . . It was a structure of graphite bricks and spread through these graphite bricks in some sort of pattern were big cans, cubic cans, containing uranium oxide.”1537 The cans, 8 by 8 by 8 inches, 288 of them in all, were made of tinned iron sheet; each could hold about 60 pounds of uranium oxide.1538 Each cubic “cell” of the uranium-graphite lattice—a can and its surrounding graphite—was 16 inches on a side. Spheres of uranium in an arrangement of spherical cells would have been more efficient. In these beginning experiments, with materials of doubtful purity, Fermi was pursuing order-of-magnitude estimates, a first rough mapping of new territory. “This structure was chosen because of its constructional simplicity,” the experimenters wrote afterward, “since it could be assembled without cutting our graphite bricks of 4” by 4” by 12”. Although we did not expect that the structure would approach too closely the optimum proportions, we thought it desirable to obtain some preliminary information as soon as possible.”1539 Promising results might also win further NDRC support.
“We were faced with a lot of hard and dirty work,” Herbert Anderson recalls. “The black uranium oxide powder had to be . . . heated to drive off undesired moisture and then packed hot in the containers and soldered shut. To get the required density, the filling was done on a shaking table. Our little group, which by that time included Bernard Feld, George Weil, and Walter Zinn, looked at the heavy task before us with little enthusiasm. It would be exhausting work.”1540 Then Pegram to the rescue in Fermi’s telling:
We were reasonably strong, but I mean we were, after all, thinkers. So Dean Pegram again looked around and said that seems to be a job a little bit beyond your feeble strength, but there is a football squad at Columbia that contains a dozen or so of very husky boys who take jobs by the hour just to carry them through college.1541 Why don’t you hire them?
And it was a marvelous idea; it was really a pleasure for once to direct the work of these husky boys, canning uranium—just shoving it in—handling packs of 50 or 100 pounds with the same ease as another person would have handled three or four pounds.
“Fermi tried to do his share of the work,” Anderson adds; “he donned a lab coat and pitched in to do his stint with the football men, but it was clear that he was out of his class. The rest of us found a lot to keep us busy with measurements and calibrations that suddenly seemed to require exceptional care and precision.”1542
For this first exponential experiment and the many similar experiments to come, Fermi defined a single fundamental magnitude for assessing the chain reaction, “the reproduction factor k.” k was the average number of secondary neutrons produced by one original neutron in a lattice of infinite size—in other words, if the original neutron had all the room in the world in which to drift on its way to encountering a uranium nucleus.1543 One neutron in the zero generation would produce k neutrons in the first generation, k2 neutrons in the second generation, k3 in the third generation and so on. If k was greater than 1.0, the series would diverge, the chain reaction would go, “in which case the production of neutrons is infinite.” If k was less than 1.0, the series would eventually converge to zero: the chain reaction would die out.
k would depend on the quantity and quality of materials used in the pile and the efficiency of their arrangement.
The cubical lattice that the Columbia football squad stacked in Schermerhorn Hall in September 1941 extrapolated to a disappointing first k of 0.87. “Now that is by 0.13 less than one,” Fermi comments—13 percent less than the minimum necessary to make a chain reaction go—“and it was bad. However, at the moment we had a firm point to start from, and we had essentially to see whether we could squeeze the extra 0.13 or preferably a little bit more.” The cans were made of iron, and iron absorbs neutrons. “So, out go the cans.” Cubes of uranium were less efficient than spheres; next time the Columbia group would press the oxide into small rounded lumps. The materials were impure. “So, now, what do these impurities do?—-clearly they can do only harm. Maybe they make harm to the tune of 13 percent.” Szilard would continue his quest for materials of higher purity. “There was some considerable gain to be made . . . there.”1544
“Well,” concludes Fermi, “this brings us to Pearl Harbor.”
* * *
Arthur Compton had less than two weeks to throw together a program between his discussion with Vannevar Bush and James Bryant Conant at the Cosmos Club luncheon on December 6 and the first meeting on December 18 of the new leaders of what was now to be called the S-l program. (S-l for Section One of the Office of Scientific Research and Development: Conant would administer S-l, but the National Defense Research Committee was no longer directly involved; the bomb program had advanced from research into development.) On December 18, Conant notes in the secret history of the project he wrote in 1943, “the atmosphere was charged with excitement—the country had been at war nine days, an expansion of the S-l program was now an accomplished matter. Enthusiasm and optimism reigned.”1545 Compton offered his program to Bush, Conant and Briggs the next day and followed up on December 20 with a memorandum.1546 The projects that had come under his authority were scattered across the country at Columbia, Princeton, Chicago and Berkeley. For the time being he proposed leaving them there.
With the arrival of war, not to breathe a word of the mysteries they were exploring, the project leaders had adopted an informal code: plutonium was “copper,” U235 “magnesium,” uranium generically in the nonsensical British coinage “tube alloy.” “On the basis of the present data,” Compton wrote, optimism reigning, “it appears that explosive units of copper need be only half the size of those using magnesium, and that premature explosions can be ruled out.”1547 Because of the difficulty of engineering a remotely controlled chemical plant to extract plutonium, however, he thought that “the production of useful quantities of copper will take longer than the production of magnesium.” For a timetable he offered:
Knowledge of conditions for chain reaction by June 1, 1942.
Production of chain reaction by October 1, 1942.
Pilot plant for using reaction for copper production, October 1, 1943.
Copper in usable quantities by December 31, 1944.
His schedule was designed to show that plutonium might be produced in time to influence the outcome of the war, the standard which Conant was insisting upon after Pearl Harbor even more vehemently than before. But the uranium-graphite work had not yet won even Compton’s full confidence. If graphite proved impractical and “copper production” had to wait for heavy water (of which Harold Urey was urging the extraction at an existing plant in Canada), Compton’s schedule would slip by “from 6 months to 18 months.” And that might be too late to make a difference.
For the next six months, Compton estimated, the pile studies at Columbia, Princeton and Chicago would cost $590,000 for materials and $618,000 for salaries and support. “This figure seemed big to me,” he remembers modestly, “accustomed as I was to work on research that needed not more than a few thousand dollars per year.”1548
He had met with Pegram and Fermi to prepare this part of his proposal and concluded that when metallic uranium became available the project should be concentrated at Columbia. Over Christmas and through the first weeks of January it fell to Herbert Anderson, the native son, to find a building in the New York City area large enough to house a full-scale chain-reacting pile. Not to be outdone in the matter of informal codes, the Columbia team had named that culmination “the egg-boiling experiment.”1549, 1550 Anderson stumped the wintry boroughs and turned up seven likely locations for boiling uranium eggs. He proposed them to Szilard on January 21; they included a Polo Grounds structure, an aircraft hangar on Long Island that belonged to Curtiss-Wright and the hangar Goodyear used to house its blimps.
But as Compton reviewed the work of the several groups that had come under his authority, bringing their leaders together in Chicago three times during January, their disagreements and duplications made it obvious that all the developmental work on the chain reaction and on plutonium chemistry should be combined at one location. Pegram offered Columbia. They considered Princeton and Berkeley and industrial laboratories in Cleveland and Pittsburgh. Compton offered Chicago. No one wanted to move.
The third meeting of the new year, on Saturday, January 24, Compton conducted from his sickbed in one of the sparsely furnished spare bedrooms on the third floor of his large University Avenue house: he had the flu. Risking infection, Szilard attended, Ernest Lawrence, Luis Alvarez—Lawrence and Alvarez sitting together on the next bed—and several other men. “Each was arguing the merits of his own location,” Compton writes, “and every case was good. I presented the case for Chicago.”1551 He had already won the support of his university’s administration. “We will turn the university inside out if necessary to help win this war,” its vice president had sworn.1552 That was Compton’s first argument: he knew the management and had its support. Second, more scientists were available to staff the operation in the Midwest than on the coasts, where faculties and graduate schools had been “completely drained” for other war work. Third, Chicago was conveniently and centrally located for travel to other sites.
Which convinced no one. Szilard had forty tons of graphite on hand at Columbia and a going concern. The arguments continued. Compton, who was notoriously indecisive, suffered their brunt as long as he could bear it. “Finally, wearied to the point of exhaustion but needing to make a firm decision, I told them that Chicago would be [the project’s] location.”1553
Lawrence scoffed. “You’ll never get a chain reaction going here,” he baited his fellow laureate. “The whole tempo of the University of Chicago is too slow.”
“We’ll have the chain reaction going here by the end of the year,” Compton predicted.
“I’ll bet you a thousand dollars you won’t.”
“I’ll take you on that,” Compton says he answered, “and these men here are the witnesses.”
“I’ll cut the stakes to a five-cent cigar,” Lawrence hedged.
“Agreed,” said Compton, who never smoked a cigar in his life.
After the crowd left, Compton shuffled wearily to his study and called Fermi. “He agreed at once to make the move to Chicago,” Compton writes. Fermi may have agreed, but he found the decision burdensome. He was preparing further experiment. His group was exactly the right size. He owned a pleasant house in a pleasant suburb. He and Laura had buried a cache of Nobel Prize money in a lead pipe under the concrete floor of their basement coal bin against the possibility that as enemy aliens their assets would be frozen. Laura Fermi “had come to consider Leonia as our permanent home,” she writes, “and loathed the idea of moving again.”1554 She says her husband “was unhappy to move. They (I did not know who they were) had decided to concentrate all that work (I did not know what it was) in Chicago and to enlarge it greatly, Enrico grumbled. It was the work he had started at Columbia with a small group of physicists. There is much to be said for a small group. It can work quite efficiently.”1555 But the country was at war. Fermi traveled back and forth by train until the end of April, then camped in Chicago. Laura dug up their buried treasure and followed at the
end of June.
To Szilard, the day after the sickbed meeting—he had returned promptly to New York—Compton sent a respectful telegram: THANK YOU FOR COMING TO PRESENT ABLY COLUMBIA’S SITUATION. NOW WE NEED YOUR HELP IN ORGANIZING THE METALLURGICAL LABORATORY OF O.S.R.D. IN CHICAGO. CAN YOU ARRIVE HERE WEDNESDAY MORNING WITH FERMI AND WIGNER . . . TO DISCUSS DETAILS OF MOVING AND ORGANIZATION.1556 Unlike the Radiation Laboratory at MIT, the new Metallurgical Laboratory hardly disguised its purpose in its name. Who would imagine its goal was the transmutation of the elements to make baseball-sized explosive spheres of unearthly metal?
Before Fermi and his team moved to Illinois they built one more exponential pile, this one loaded with cylindrical lumps of pressed uranium oxide three inches long and three inches in diameter that weighed four pounds each, some two thousand in all, set in blind holes drilled directly into graphite.1557 A new recruit, a handsome, dark-haired young experimentalist named John Marshall, located a suitable press for the work in a junkyard in Jersey City and set it up on the seventh floor of Pupin; Walter Zinn designed stainless steel dies; the powdered oxide bound together under pressure as medicinal tablets pressed from powder—aspirin, for example—do.
Fermi was concerned to free the pile as completely as possible of moisture to reduce neutron absorption. He had canned the oxide before; now he decided to can the entire nine-foot graphite cube. “There are no ready-made cans of the needed size,” Laura Fermi says dryly, “so Enrico ordered one.”1558 That, writes Albert Wattenberg, who joined the group in January, “required soldering together many strips of sheet metal. We were very fortunate in getting a sheet metal worker who made excellent solder joints. It was, however, quite a challenge to deal with him, since he could neither read nor speak English. We communicated with pictures, and somehow he did the job.”1559 Laura Fermi picks up the story: “To insure proper assembly, they marked each section with a little figure of a man: if the can were put together as it should be, all men would stand on their feet, otherwise on their heads.”1560 The Columbia men preheated the oxide lumps to 480°F before loading. They heated the contents of the room-sized can to the boiling point of water and pumped down a partial vacuum. Their heroic efforts reduced the pile’s moisture to 0.03 percent. With the same relatively impure uranium and graphite they had used before but with these improved conditions and arrangements they measured k at the end of April at an encouraging 0.918.
Making of the Atomic Bomb Page 55
