When Fermi understood the cause of the crash, he smiled with relief and announced to the group, “I’m hungry, it’s time for lunch.” All the control rods were reinserted in the pile, locked in, and the group braved the freezing cold to walk to the main university dining room at Hutchinson Commons. In the splendor of a glorious Gothic replica of the Great Hall at Oxford’s Christ Church College, they had a quiet lunch and spoke of anything except what they had just witnessed.
They returned to the squash court at about two o’clock, and Fermi asked the team to return the safety rods up to their positions prior to lunch. Over the next hour or so, Weil withdrew the control rod gradually, according to Fermi’s instructions. Each time, the instruments would chatter away and then level off. At about 3:25 p.m. Fermi ordered a full foot of additional withdrawal. As Weil followed Fermi’s instructions, Fermi turned to Compton. “This is going to do it,” he assured Compton, who joined the group after lunch, with a wide-eyed Crawford Greenewalt in tow. For Greenewalt, this was a moment he would remember for the rest of his life. “Now it will become self-sustaining,” Fermi explained. “The trace will climb and continue to climb,” he said, referring to the line being drawn across the graph drum attached to the counters. “It will not level off.”
He was right. The counters picked up speed and this time did not level off. The clicking became a high-pitched whine. The line traced on the graph paper moved ever upward. Fermi took some measurements, fiddled with his slide rule again.
“I couldn’t see the instruments,” Weil later said. “I had to watch Fermi every second, waiting for orders. His face was motionless. His eyes darted from one dial to another. His expression was so calm it was hard. But suddenly, his whole face broke into a broad smile.”
“The reaction is self-sustaining,” Fermi announced. “The curve is exponential.” Still he did not order the reactor shut down. Not yet. He continued to study the graph and the instruments, monitoring the exponential production of neutrons. He gave no indication of next steps. Richard Watts, a member of Wilson’s instrumentation team, memorialized the moment in the log book: “We’re cookin’!”
FIGURE 17.1. CP-1 goes critical, December 2, 1942. Fermi is on the balcony overlooking the pile; George Weil is below, operating the control rod. Painting by Gary Sheehan. Courtesy of the Chicago History Museum.
FIGURE 17.2. Logbook note at moment of criticality. Facsimile in Nuclear Science Museum, Argonne Laboratory. Photo by Susan Schwartz. Courtesy of Argonne National Laboratory.
The several dozen witnesses grew increasingly tense, but Fermi was calm. The team atop the reactor, led by Sam Allison, was alert to any sign of danger, ready at a moment’s notice to flood the pile with the cadmium solution. At one point Leona Libby approached Fermi and whispered, “When do we become scared?” Fermi didn’t answer. His attention was entirely on the instruments.
Twenty-eight minutes into criticality, he decided he had witnessed enough. “Zip!” he called out to Zinn, and Zinn dutifully released the safety rods into the pile. At 3:53 p.m., the world’s first controlled fission chain reaction came to a complete halt.
The room was quiet. Fermi was elated, but said little. He was silent even as Leona Libby accompanied him home at the end of the day. Wigner had brought a bottle of chianti for the occasion. Those in attendance shared the wine out of paper cups, Fermi first. Later most of those present signed the straw cover surrounding the bottle. Perhaps the most famous chianti in history, it now resides in the archives at Argonne Lab. No toasts were made, no dramatic speeches offered. History had been made, but the future looked grim. Everyone understood that this was a major step toward the development of a fission weapon. Szilard recounts that he shook Fermi’s hand in congratulations but warned that this would go down as a black day in human history.
FIGURE 17.3. Fourth anniversary reunion of some CP-1 participants, December 1946, at the University of Chicago. Back row, from left: Norman Hilberry, Samuel Allison, Thomas Brill, Robert Nobles, Warren Nyer, and Marvin Wilkening. Middle row: Harold Agnew, William Sturm, Harold Lichtenberger, Leona Libby, and Leo Szilard. Front row: Enrico Fermi, Walter Zinn, Albert Wattenberg, and Herbert L. Anderson. Courtesy of Argonne National Laboratory.
Compton took Greenewalt with him back to his offices. The DuPont executive was sufficiently impressed by Fermi’s performance—according to Compton “his eyes were aglow”—that he swept away the remaining obstacles and the company promptly concluded a contract with the government to build all the project’s reactors. Compton wanted to let Conant and the rest of the Manhattan Project leadership know of the pile’s success, but the two had not agreed on a secure means of communicating the news by phone. Compton rang up Conant and, in a burst of uncharacteristic lyricism, reported, “The Italian navigator has just landed in the new world. The earth was not as large as he had estimated,” Compton continued, “and he arrived at the new world sooner than he had expected.” Conant picked up the reference immediately. “Is that so? And were the natives friendly?” he asked. “Very friendly,” replied Compton. The message had been passed.
LAURA FERMI HAD BEEN PLANNING A COCKTAIL PARTY FOR MEMBERS of the Met Lab that evening at her home. She had no idea what her husband was working on, and when he left for work that morning he gave no indication that anything momentous would be occurring. After he returned for dinner, she asked him to buy some cigarettes for the guests. Enrico—who hated cigarettes—refused, claiming rather absurdly that he did not know how to buy cigarettes and that they left a foul stench after a party that took days to air out.
As soon as the party began, however, Laura noticed something unusual. The Zinns were the first to arrive, and Walter made a point of shaking Enrico’s hand, offering congratulations. His was only the first of many congratulations offered as party guests arrived. Mystified, she knew better than to ask Enrico, who would have put her off with a silent shrug. Instead, she asked Leona Libby, the only woman on the team. Libby, bound by official secrecy, was embarrassed, because she felt close to Laura and did not want to dissemble. Without thinking it through, she blurted out that Laura’s husband helped sink a Japanese admiral—meaning, she later wrote, that it was as if Enrico had done just that. Laura was naturally quite astonished. The Met Lab was a long way from the war in the Pacific. “Are you making fun of me?” she asked, with justifiable irritation. Anderson, sensing perhaps that Leona was digging herself into a hole, joined the conversation. “Do you think that anything is impossible for Enrico?” Leona later wrote that this was perhaps the most embarrassing moment of her life, having been forced to lie to a lovely, intelligent woman, effectively a second mother to her, in order to maintain confidentiality.
After the party, Laura grilled Enrico about the Japanese admiral, and her husband, characteristically, was playfully evasive. Only after the war did she learn of the significance of December 2, 1942, and the part her husband played.
THERE IS SOMETHING A BIT CONTRIVED ABOUT THE EVENTS surrounding December 2, 1942. Many have waxed lyrical about the importance of the moment. It was the first time humans tricked nature into releasing, in a sustained, controlled way, the energy embedded in the nucleus of the atom. Using knowledge painstakingly derived from experiment after exhaustive experiment to achieve this goal, Fermi showed that a controlled chain reaction was possible and helped clear the path to a fission weapon. He also led the way to the exploitation of uranium for peaceful purposes. Compton summarized the impressions of many when he wrote in 1956: “The first self-sustained atomic nuclear chain reaction, achieved on December 2, 1942, did indeed usher in a new age. Henceforth, the vast reserves of energy held in the nucleus of the atom were at the disposal of man.” On the twentieth anniversary of the event, Wigner, always more circumspect, would write:
Do we then exaggerate the importance of Fermi’s famous experiment? I may have thought so at sometime in the past but do not believe it now. The experiment was the culmination of the efforts to prove the chain reaction. The elimination of the l
ast doubts in the information on which our further work had to depend had a decisive influence on our effectiveness in tackling the second problem of the Chicago project: the design and realization of a large scale reactor to produce the nuclear explosive, plutonium.… Even though our hearts were by no means light when we sipped the wine around Fermi’s pile, our fears were undefined, like the vague apprehensions of a man who has done something bigger than he ever expected to. Our forebodings did not concern concrete events. In fact, our hopes, some of them very far-reaching, preponderated.
One is left, however, with the impression that much of Fermi’s performance that day was a show put on by a master to impress and inspire an eager, receptive audience. Consider that he had accumulated enough data, through operating the numerous noncritical piles constructed at Columbia and during the summer months in Chicago, to enable him to predict how neutrons would be produced in a pile with an ellipsoid structure. He had systematically taken data on virtually every step of the final pile’s construction, allowing him to predict the moment the pile would become critical “almost to the exact brick,” in the words of official historians Corbin Allardice and Edward R. Trapnell. At each step of the way on the fateful day he was able to predict what would happen at the next withdrawal of the control rod and then, at a moment of high drama, the unexpected release of the zip rods at 11:35 a.m. that morning, he suggested that the team break for lunch, just as he had in October 1934. When the pile went critical at the moment he predicted it would, he decided to let it run for almost half an hour without a word to those around him, all of whom anxiously awaited his order to shut it down. One is struck by the impression that he could simply have asked Weil to move the control rod to the point at which he predicted criticality, measured it, and called it a day. He could have chosen to go critical with Anderson, Zinn, and Allison in attendance the previous evening, with no audience at all, because the pile was ready to go the night of December 1, 1942. Yet he did none of these things. He may have had some underlying concerns over the safety of the experiment and proceeded slowly to ensure that it would not run out of control. But Fermi also clearly understood the drama of the occasion and rose to it. In this instance he chose to play the role not of the disinterested experimental genius but of the showman his peers wanted him to play. He played with his slide rule, although one wonders if he even really needed to do that. He made sure that he looked the part of the physics genius that day. This is not to disparage him in the slightest. He worked for almost four years for this moment, in what he and others considered a desperate race against time. He was absolutely confident in his ability to make this happen, a confidence he shared with others, notably with Teller early in 1942 when he ventured his idea for a fusion weapon. For Fermi, the results must have been a foregone conclusion, but he had the insight to know that the moment required a great show, and he was happy to oblige.
It was, nevertheless, the culmination of years of hard work and enormous dedication by a large team of talented individuals, a team that found exceptional, inspirational leadership in a brilliant émigré physicist, one who had, in his youth, predicted the possibility of releasing enormous amounts of energy from within the atom. It is instructive to consider that the German effort was at this point already years behind Fermi’s effort. The Germans never constructed a real, working reactor. Part of this difference results from the early decision, by Fermi and Szilard, to use graphite rather than heavy water and to press for greater purification of the graphite when impurities impeded the efficiency of the chain reaction. Graphite was plentiful, cheap, and easy to mill with woodworking tools. When the Germans tried working with graphite, the material they used was far too impure to do the job, but the physicist responsible for much of this work, Walter Bothe, did not believe that the impurities could be removed. Bothe and Heisenberg decided to use heavy water, extremely difficult to make in bulk. This effectively doomed the German project.
Part of the difference, surely, was Szilard’s uncanny ability to squeeze the best-quality uranium and graphite from commercial suppliers, helped along the way by Walter Zinn and Sam Allison. Part of it was Fermi’s extraordinary, intuitive understanding of how neutrons would be produced in various configurations of uranium, an understanding that had as its foundation the two years of solid, lonely work he did in Rome with Amaldi after the discovery of slow neutrons in October 1934, on which they reported in a long and exhaustive 1936 paper. In the words of a Chicago colleague, Fermi learned how to “think like a neutron.” No German had done the exhaustive, painstaking work required to develop this intuition. German physicists spent much of the 1930s either fleeing from Hitler or pondering the meaning of quantum theory, but they simply had not completed the grinding experimental work on neutron physics that Fermi had.
He was also, in his own unassuming, casual way, a brilliant and inspirational leader. He led from the front, easily accessible to senior and junior scientists alike, able to cut through thorny puzzles quickly and decisively, confident in his ability to understand any problem thrown at him, simplifying experimental setups wherever possible. He was utterly unafraid of the “quick and dirty” solution that worked. His confidence was contagious and his example led them to achieve well beyond what they thought they were capable of. He succeeded in the extraordinary challenge of scaling up the kind of team he developed in Rome to a much larger American organization under much greater pressure. It worked and produced an historic success. A direct line can be drawn from those early neutron experiments in Rome to the experiments by Fermi and Szilard at Columbia in early 1939, through the endless subsequent experimentation with different piles, to the events at Stagg Field that cold winter day in December 1942. The Stagg Field experiment capped off a major phase of research, but it was also the beginning of another chapter in the story of the Manhattan Project, a chapter in which Fermi was to continue to play a pivotal role. Plutonium production reactors now needed to be built and the “Italian navigator” would also play a leading role in this new phase of the work.
CHAPTER EIGHTEEN
XENON-135
FROM JANUARY 1943 THROUGH THE SUMMER OF 1944, FERMI divided his time between three places: the newly relocated Met Lab in Argonne Forest, just west of Chicago; Oak Ridge, where the vast isotope separation plants were taking shape and where Fermi and a team from DuPont built the first plutonium research reactor; and the eastern desert of Washington State near the village of Richland, where the large-scale plutonium production reactors began their deadly work. Though he made the occasional trip to other locations—to Berkeley, and to the new village of Los Alamos, New Mexico, soon to become the epicenter of bomb design and construction—the majority of his time was spent with Met Lab colleagues who had worked with him in the squash court under the stands of Stagg Field.
IN FEBRUARY 1943, AFTER THE LABOR PROBLEMS THAT BEDEVILED the construction of the lab facilities were resolved, the entire Met Lab moved to Argonne. In the short period between December 2, 1942, and the February 1943 move, Fermi and the team continued to experiment with the pile, at one point driving the pile up to two hundred watts.
In February, the pile, originally dubbed CP-1 (Chicago Pile 1), was reconstructed at Argonne, in a new and more flexible configuration that became known as CP-2. Fermi reconfigured it so it had a central core that could be removed to test out different lattice structures and different quality materials. When it went critical in May 1943, its reproduction factor was much higher than CP-1. It took a little over one minute for CP-1 to double in power. CP-2 took about five seconds to do the same.
In CP-2, not only had Fermi created a prototype for the eventual production of plutonium, he also created a veritable neutron factory. He would never again have to fuss with delicate neutron sources in glass bulbs. All the neutrons he needed for research existed in the heart of CP-2. Beginning almost immediately, with the enthusiasm of a child with a new toy, he ran systematic experiments to test neutron reactivity on a variety of targets. He found a way to creat
e beams of ultraslow neutrons and studied neutron diffraction and refraction in great detail. He also developed ways to test the purity of graphite and uranium now being produced in unprecedented quantities for the huge plutonium production reactors.
Working closely with Anderson, Zinn, Marshall, Libby, and the rest of the team, Fermi developed new detectors that measured the precise number of neutrons produced in the reactor. These detectors enabled the team to carry out new studies with unprecedented precision.
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