THAT CHANGE WOULD INVOLVE A MOVE TO CHICAGO, TO WORK under the supervision of Arthur Compton, a physicist he respected but hardly knew.
In December 1941, Vannevar Bush tasked Compton with an enormous challenge: the coordination and management of more than a dozen uranium research teams across the country. The fear of German progress on a fission weapon exerted enormous pressure. Now that the United States was formally in a war against the Axis powers, time was at a premium.
Compton was a fine choice for the job. He had been involved in science policy for several years prior to the US entry in the war and knew the senior Manhattan Project leadership well. They, in turn, respected his scientific abilities and his sound judgment. Bush hoped that a disparate team of physicists, many of whom had never before worked in large teams under a single leader, would follow his lead. Compton came to prominence in the early 1920s with a series of X-ray scattering experiments that supported Einstein’s hypothesis that light consisted of particles, called photons. The importance of the work was immediately recognized, and he was awarded the 1927 Nobel Prize. He was one of the very few Americans invited to the Solvay conferences before the war. Before the 1927 Solvay conference, he also attended Corbino’s 1927 conference at Lake Como, where he first encountered the young Fermi.
Tall, athletic, handsome, with a thick head of dark hair slicked back fashionably, Compton looked like a leader. Born into an Ohio family of academic high achievers, Arthur shared and cultivated his family’s strong Christian faith, eventually serving as a deacon in a local Baptist church. He was one of only a handful of scientists in the Manhattan Project who spoke openly about his religious beliefs, which hardly endeared him to the largely irreligious group he was leading. On one occasion while he was managing the Met Lab, he brought a Bible to a fractious meeting and tried to establish his authority by quoting from it. This was not necessary and did not work. His authority did not come from his adherence to biblical principles but rather from his undoubted scientific achievements, his sense of judgment and fair play, and a direct line to the country’s wartime political leadership.
In January 1942, Compton brought the various teams together for a series of meetings in Chicago and New York to thrash out a strategy for centralizing the research effort. Two key decisions came out of these discussions. One was a timetable for the development of the bomb: determination of the feasibility of a chain reaction no later than July 1, 1942; achievement of a controlled chain reaction by January 1943; production of plutonium for the bomb by January 1944; and a working bomb by January 1945. The second was a decision on how to centralize the work. After considerable debate, he made a Solomon-like decision. Lawrence and his team at UC Berkeley would remain in California, but all other teams, including Fermi’s, would come to Chicago. Once the decision was made, Compton immediately informed Fermi by phone, because Fermi was unable to attend the final Chicago meeting owing to a bad cold he had caught not long after the first Chicago meeting. (He sent Szilard in his place.) Compton reports that Fermi immediately agreed, no doubt unwilling to object, given his enemy alien status. Fermi was not, however, a happy man. His team at Columbia was working well together and he had complete control over the project. The move would be inconvenient personally and professionally. He would be working with some of the same people—Anderson was sent to Chicago immediately to coordinate the project, while Zinn stayed behind with Fermi to begin organizing the move—but he would soon be working with many new people, untested and unknown, under the leadership of Arthur Compton.
His status as an enemy alien is surely one of the most bizarre aspects of the entire Manhattan Project story. Here was an Italian national and a member of the Fascist Party at the very center of one of the most secret projects of the US war effort. Travel restrictions were only part of the story. Those who knew him, who knew how lucky the United States was to have him on the side of the Allies, had no doubts about his loyalty, but many of those involved—particularly military officers who were increasingly important in the organization of the project—did not know him at all. The FBI was suspicious of Fermi, but it was more concerned about Szilard. He traveled widely, had no visible means of support, had a lavish lifestyle that often caused him financial problems, and was a high-profile eccentric—exactly the type to attract the FBI’s attention.
The initial FBI report filed on Fermi, dated August 13, 1940, stated:
He is supposed to have left Italy because of the fact that his wife is Jewish. He has been a Nobel Prize winner. His associates like him personally and greatly admire his intellect. He is undoubtedly a Fascist. It is suggested that, before employing him on matters of a secret nature, a much more careful investigation be made. Employment of this person on secret work is not recommended.
Further FBI investigation at Columbia, dated October 22, 1940, confirmed the view of Fermi as an outstanding scientist and loyal “as long as Fascist Party retains control in Italy,” although Professor LaMer from the Columbia chemistry department felt it best not to grant clearances to any foreign national, irrespective of whether a specific individual might be trustworthy. Fortunately, the government cleared him for secret work. After all, much of what was secret in the Manhattan Project originated in Fermi’s brain.
During this period, Enrico and Laura began to feel sufficiently vulnerable that they dug a hole in the floor of their basement in Leonia and buried a tin can of cash for use in case of an emergency. They never had need for the can, of course, and Fermi’s loyalty was never seriously in question. Nevertheless, his enemy alien status had a powerful psychological impact on him. Combined with his natural inclination to defer to authority figures, this probably explains Fermi’s reluctance to make a fuss when Compton asked him to move to Chicago. It also may explain Laura’s apparent willingness to make the move without complaint, because she believed that the move would be temporary. Later on in Chicago, however, when Fermi discovered that his mail—along with that of everyone else on the project—was being read by US Army censors, he complained vociferously, but with little effect.
In the first years of the war, when the German onslaught made it seem possible that they might actually win the war, the Fermis and their Columbia colleagues Joseph and Maria Mayer talked about what they would do if America became a fascist state. The two couples had met at the Ann Arbor summer session in 1930 and had become fast friends. When the Fermis arrived in New York in 1939, they were delighted to find that the Mayers had also arrived at Columbia. If the Nazis conquered the United States, the four of them agreed they would settle on a desert island in the South Pacific, far from political danger. Their division of labor would reflect their relative strengths: at sea, Joe would captain and Enrico would navigate, and on the island Enrico would farm, Maria would curate a small but essential collection of books, and Laura would keep everyone clothed. A doctor and a few others would join them to fill out the little community. It was a ridiculous fantasy that they may not have taken seriously and yet it can be seen as another example, shared by both refugee couples, of underlying uncertainty and insecurity regarding their new homeland.
THERE WAS JUST ENOUGH TIME BETWEEN THE CALL FROM COMPTON and the scheduled move to Chicago to build one more pile at Columbia. This one was more successful.
Fermi redesigned the structure, using cylindrical blocks of compressed uranium powder provided by a new, young member of the team, John Marshall, who was given the task of “sintering” the uranium powder that was provided for the project. Sintering is the process of compressing powder to such an extent that it becomes a solid, similar to the compression of charcoal briquettes. Marshall’s sintering press produced solid slugs of uranium oxide powder about three inches high and three inches in diameter, weighing about four pounds apiece—still not as dense as pure uranium metal, but denser than previous preparations. All told, some 2,160 slugs of uranium, weighing a total of more than four tons, were embedded in a pile of graphite eight feet on each side and some eleven feet, four inches high. It completely filled the space in the basement of Schermerhorn.
The second modification involved trying to extract as much air out of the pile as possible. Fermi and the team worried that nitrogen in the air between the bricks might be reducing the reproduction factor. Any improvement in that factor, however small, would be important. Fermi used the analogy of canned food—then quite popular because of rationing for the war effort—and searched for someone to build a “can” around the entire pile. He found a workman employed by Columbia who hardly spoke English but whose soldering technique was outstanding. He built a tin can around the pile, with a valve set into one side attached to a vacuum pump. The can held its integrity and the air was vacuumed almost completely out of the pile, replaced by carbon-dioxide.
The results were about 4 percent better than the previous pile, but still well below the absolute level required for a sustained reaction. Fermi was still optimistic, believing that further improvements in size, geometry, and purification would bring about the desired result. He was also able to run a series of tests to determine how impurities in either the uranium or the graphite might affect the reproduction factor. For example, he determined that cadmium was an extremely effective absorber of neutrons. This knowledge came in handy over the next few months. He did not, apparently, test for xenon. This also became relevant later in the project.
During these experiments in early 1942, two accidents occurred, underscoring the hazards of working with the unstable and dangerous materials required to prepare the pile. In one incident, Zinn was working with powdered thorium to test its ability to absorb neutrons and, though he had taken the precaution of wearing goggles and gloves, the powder exploded in his face when he opened its airtight container. He suffered severe burns on his hands and face, but the goggles saved his eyesight.
Another accident involved Pegram, Fermi, and Anderson, but it was Anderson who would suffer the consequences. Like thorium, powdered beryllium has a tendency to catch fire if not handled carefully. The three of them received a shipment of powdered radium and beryllium for use in preparing a neutron source. They found the preparation to be slightly damp, placed it on a hot plate to dry it out quickly, and left the lab room. When they returned, the powder was on fire. Anderson rushed in to put out the blaze and no one seemed to be the worse for the incident. Years later, however, Anderson began to have breathing problems, which were traced back to inhalation of the beryllium powder during this incident. The illness, called berylliosis, eventually killed him at the age of seventy-four.
In April 1942, the pile was dismantled, removed from the Schermerhorn basement, and shipped to Chicago.
AT JUST ABOUT THE TIME COMPTON WAS MAKING THE DECISION TO bring Fermi and his team to Chicago, Fermi was having a conversation with his old friend and colleague Edward Teller.
Teller immigrated to the United States from Europe in 1935, when he was offered a job at George Washington University in Washington, DC. He was in attendance when Fermi and Bohr explained uranium fission to the electrified audience at the March 1939 conference in Washington and then moved to Columbia in the fall of 1941 to help Fermi and the team with their graphite pile experiments. The two dined regularly at the Columbia faculty club. “Walking back to the laboratory after lunch one day,” Teller relates, “Fermi posed the question: ‘Now that we have a good prospect of developing an atomic bomb, couldn’t such an explosion be used to start something similar to the reactions in the sun?’” In what must count as one of the greatest understatements in the history of science, Teller continues, “The problem interested me.”
The idea that the sun is powered by fusion reactions was proposed in the late 1920s by the British physicist Robert Atkinson and a German named Fritz Houtermans, later elaborated upon by the eccentric, fun-loving Russian George Gamow, and finally worked out in detail by Hans Bethe in 1938. By January 1942, physicists had a complete understanding of the basic processes underlying the way the sun and other stars work. In the hot, dense core of a star, protons are moving so fast that they break through the “Coulomb barrier”—the electromagnetic repulsion keeping two protons apart. The protons fuse, creating helium nuclei and eventually a range of other heavier nuclei. Each fusion results in radiant energy, in the form of photons and neutrinos. The physics of the process is quite complex, and Fermi’s beta decay theory helped Bethe work it out in detail. Per unit of mass, the energy released in fusion, particularly of hydrogen, is far higher than that released in uranium fission.
Fermi’s calculations suggested that the temperatures achievable in fission weapons might well be sufficient to set off a fusion reaction in hydrogen. In sharing this idea with Teller, he unwittingly changed the course of Teller’s life. Teller became a man possessed. He realized that because hydrogen was so plentiful and so stable, a fusion weapon could have virtually unlimited destructive power, the only limit being how much hydrogen the bomb might contain. That summer Teller arrived at Berkeley to join J. Robert Oppenheimer’s team exploring the feasibility of a fast-neutron chain reaction for the bomb—the slow-neutron reactions that Fermi was studying in his pile experiments were not appropriate for the bomb itself—but Teller could think of nothing other than a fusion device. When Teller arrived in Los Alamos the following year, his obsessive work continued.
In that fateful walk across the Columbia campus, Fermi revealed something else of great importance, although Teller does not explicitly comment upon it in his memoirs. By early 1942, even though the pile experiments had been less successful than he had hoped, Fermi had come to the conclusion that fission weapons would work. He may from time to time have expressed doubts, but from that point onward his presumption was almost certainly that fission weapons were feasible.
THE PROCESS OF MOVING THE COLUMBIA PILE PROJECT TO CHICAGO took five months, during which time the graphite bricks and uranium slugs were sent by special shipment across country, Zinn in New York, Anderson in Chicago, and Fermi shuttling between the two. Fermi initially took up residence in Chicago at International House, an independently run Gothic-style dormitory a short walk from the center of campus. He brought two young Columbia graduate students with him, Albert Wattenberg and Bernard Feld. Wattenberg recalls playing chess frequently with Fermi during this period. Fermi could beat him in chess, but always lost in tennis. Laura stayed behind with the two children, now eleven and six, to join her husband when school let out in June.
The University of Chicago was an extraordinary place, a center for scholarship and learning that rivaled schools far older and more prestigious. By the time Fermi arrived in mid-1942, Robert Maynard Hutchins, the enfant terrible who became president in 1929, had built a true academic powerhouse in the Hyde Park neighborhood on the south side of Chicago. The physics department was located in two adjacent buildings, Ryerson and Ekhart, almost squarely in the middle of campus. Compton gave Fermi an office in Ekhart, from which the new arrival directed activities.
AS SOON AS THE MATERIAL ARRIVED, WORK BEGAN ON NEW PILE experiments. Joining Fermi’s team were several key individuals who became close colleagues for the rest of his life. One was an accomplished experimental physicist named Samuel Allison. A little older than Fermi, Allison spent much of the 1930s involved in X-ray scattering studies with Compton. The two wrote the standard textbook on the subject and over the years at Chicago Allison became a trusted associate of Compton. Prior to Fermi’s arrival at Chicago, Compton asked Allison to develop a reactor pile working with beryllium as a neutron moderator, and by mid-1942 he had actually achieved better results than Fermi, even though beryllium was a more dangerous substance with which to work. Allison and Fermi became close colleagues during the war and remained so afterward.
Another colleague was a young PhD student working on a thesis under Chicago physicist Robert Mulliken. Her name was Leona Libby.* Tall, athletic, and attractive, she was the only female member of Fermi’s Chicago team. She lived with her sister near the university and early on became friendly with the Fermis. Laura would often cook meals for Leona and Herb Anderson, who took a room on the third floor of Arthur Compton’s spacious home nearby. Anderson and Libby soon discovered a mutual love of swimming and would take time off every afternoon for a dip in the freezing fresh water of Lake Michigan. Fermi, who was a passionate and exceptionally powerful swimmer fond of a peculiar dog-paddle stroke, often joined them. Harold Agnew, then a student working with Fermi at the Met Lab, recounts an outing in which Fermi challenged a group of younger colleagues to a swim from 55th Street north for about a mile. The group set off and Fermi quickly took the lead. Over his shoulder, he could see the members of the group lagging and turned back to give them encouragement, swimming in circles around them, egging them on. They finally reached their destination at 47th Street, and the group climbed ashore exhausted. Fermi gleefully announced he would swim back. His wet and weary colleagues decided to return by foot.
Others from the Columbia team came and settled in Chicago. Szilard spent much time there and with Zinn was instrumental in pressing manufacturers to find new ways of purifying graphite and uranium. John Marshall, who joined Fermi’s team in mid-1941 and worked out the process of sintering uranium powder, also came with the Columbia team, sharing the third floor of Compton’s home with Anderson. Marshall met Leona Libby soon after arriving in Chicago and within a year they were married.
Leona Libby was obviously charmed by Fermi. In later years she would write of this time:
Fermi would like to show superendurance, to swim farther, to walk farther, to climb farther with less fatigue, and he usually could. In the same way he liked to win at throwing the jackknife, pitching pennies, or playing tennis, and he usually did. These qualities of gaiety and informality of his character made it easy for the young members of the laboratory to become acquainted with him. He was an amazingly comfortable companion, rarely impatient, usually calm and mildly amused.
It was a heady experience for her, for Marshall, and for the other young members of the Met Lab. Already a legend among physicists for his work in Europe, already the subject of growing mythology, here was Enrico Fermi in the flesh, and they found him to be unassuming, approachable, informal, and fun. He had a healthy respect for his own abilities, but that was based on an empirical fact—he was just that much better a physicist than anyone else. Fermi had an enormous personal impact on his colleagues, with whom he collaborated and also went swimming and hiking. They would remember these days for the rest of their lives.
For those first few months in Chicago, Fermi enjoyed working with his new young colleagues and was reminded of the early Rome years. He was no longer working under the cloud of fascism, and his eager, positive new colleagues enjoyed his infectious sense of fun, a sense that had been entirely absent in the final years in Rome. He would stay close to these colleagues for the rest of his life.
Over the course of 1942, however, many others joined the Met Lab, working under Compton’s watchful, forceful leadership. Perhaps most important, the Princeton team—Wigner and Wheeler, in particular—arrived in Chicago soon after Fermi. They delved deep into the theory of the pile and were instrumental in the further development of the pile concept at later stages of the Manhattan Project. In the end, some forty physicists were in Chicago working in secret to create the world’s first controlled uranium chain reaction. As Compton grew increasingly confident in Fermi’s abilities, he put Fermi in charge of ever more elements of the project, until Fermi complained privately to Segrè that he felt like he was doing physics “by phone,” perhaps referring to the dramatic increase in his administrative responsibilities, which took him away from the lab and pure physics. Fermi disagreed with some of the directives the team received from project leaders in Washington. Szilard recalls that Fermi once complained, in frustration, “If we brought the bomb to them ready made on a silver platter, there would still be a fifty-fifty chance that they would mess it up.” He sometimes felt that he was a cog—an important cog, but a cog nevertheless—in an increasingly large and unwieldy machine.
NUCLEAR REACTORS TODAY ARE MAJOR ENGINEERING PROJECTS involving careful planning and reams of design drawings, all carefully vetted and reviewed at every step. In contrast, the first operational nuclear reactor was planned in Fermi’s head, based not on extensive engineering drawings but on his sense of how the neutron flow would develop within the heart of the pile and make its way from one uranium slug to the next. He gave general instructions to machinists and his fellow physicists and let them do the rest. He did not have access to computers to calculate what the geometry of the lattice should be, how big the pile would have to be before it went critical, or how hot it might get as it ran. All these calculations were done either in his head or on his ever-ready slide rule, with the more junior physicists at his side providing back up. By the time work actually began on the final Chicago pile in November 1942, Fermi and his team had built twenty-nine experimental piles testing various aspects of material and configuration. These experiments gave Fermi an intuitive sense of how the pile should be constructed and led to the apparatus that took shape in the squash court under the stands at the abandoned Chicago football stadium.
In later years, Fermi told his wife that the overall structure of the pile came to him in May 1942 while walking with colleagues along the Indiana dunes on Lake Michigan’s southern shore. Like the previous piles, it would be modular, constructed of bricks of graphite embedded with uranium and interspersed with bricks of pure graphite. It was basically quite simple. Modern reactors, built with cooling mechanisms, multiple redundant safety mechanisms, elaborate diagnostics and designed to produce electricity, are highly complex. Fermi’s piles were brutally simple, with only two objectives in mind. One was the proof of concept—a controlled, self-sustaining nuclear fission chain reaction. The other was to serve as a machine to produce plutonium.
If natural uranium is exposed to neutron bombardment, the U-238 in the uranium sometimes undergoes a series of transformations through beta decay into a new element, plutonium 239 (Pu-239). Studying Pu-239, physicists concluded that it might also be used as a material for weapons.
In 1940, a UC Berkeley team led by chemist Glenn Seaborg produced a small amount of this new element by bombarding natural uranium in the Berkeley cyclotron. This process by its very nature could produce only minute quantities of the new element, nowhere near enough for a weapon, but enough to study its properties. Experiments at Berkeley subsequently demonstrated that, as theory predicted, it would be a suitable alternative to U-235 for a fission weapon. It might also prove easier to produce in substantial quantities than U-235. If Fermi’s exponential pile could be made to work, perhaps it could be scaled up to become a plutonium factory. Traditional chemistry could be used to separate Pu-239 from reactor by-products. Hence the priority placed on plutonium research.
So the project Fermi undertook to complete when he arrived in Chicago had two distinct purposes. The creation of a self-sustaining fission chain reaction was clearly important to demonstrate the chain reaction concept. If one could be created, then in principle uranium fission weapons could be built, although enormous challenges would remain. It would also create the possibility of plutonium production, providing a second possible route to a fission weapon. At this particular juncture the success of the Manhattan Project depended almost entirely on Fermi’s ability to achieve a self-sustaining reaction. If he felt any pressure at all, he did not show it, perhaps because by this time he felt certain he could make it work.
LAURA ARRIVED WITH THE CHILDREN IN SEPTEMBER 1942 AND they moved into a grand, old, three-story house at 5537 South Woodlawn that had been vacated by its owner, investment manager Sydney Stein Jr., who moved to Washington to help in the war effort at the Bureau of the Budget. As enemy aliens, the Fermis were not allowed to keep the large floor-standing radio that came with the living room furnishings. After consulting with the FBI, the landlord removed it. On the third floor lived two Japanese exchange students, stranded in Chicago when the war broke out. With an Italian family occupying the rest of the house, the landlord decided—presumably also in consultation with the FBI—to evict the students.
The Fermis soon began to entertain at the house on a regular basis. With new physicists arriving in Chicago almost daily, Laura believed that she could help out, in spite of all the secrecy surrounding her husband’s work, by providing an active social life for the newcomers. Libby recalls attending parties with some of the most distinguished scientists of the day, watching as Enrico led the group in some of his favorite parlor games from his Rome days—games he was always determined to win. These frequent parties forged social bonds within the team and gave wives who were not privy to their husbands’ actual work a sense that they were doing something useful for the war effort. The bonds forged in Chicago and cemented during the later period at Los Alamos were to last for decades.
THE IDEA THAT BEGAN TO FORM ON THE INDIANA DUNES WAS different from the idea that drove the geometry of previous piles. The Columbia piles were squared-off towers of graphite and uranium, rising as high as the Schermerhorn ceiling permitted. Now he began to play with another shape: a flattened, roughly spherical, ellipsoid shape. It was clear to Fermi that such a shape allowed for neutron diffusion that would be more optimal for the reproduction factor. Surface area was Fermi’s enemy, because as neutrons escaped from the pile through the surface contact with the air they were lost from future fissions. A cube of a given volume has a greater surface area than a sphere of the same volume. The smaller the surface area for a given volume, the more likely it would be for neutrons to stay inside the pile. Thus, a spherical shape was better than a squared-off shape. Before he could begin to build anything, however, the Chicago team would have to solve two major problems. One of them involved the purity of materials for the new pile. The other was to figure out how to make a spherical shape sit stably on the floor.
The more he thought about it and the more he discussed the matter with Szilard, Wigner, Allison, Wheeler, and others, it was clear that purity of the materials used in constructing the pile would be a central issue. Impurities could absorb neutrons in unpredictable ways, slowing the process and constraining the reproduction factor. Even if they did not absorb neutrons themselves, under neutron bombardment they might transform into new isotopes that would be neutron absorbers. Szilard and Zinn continued working with producers of graphite and uranium to get materials of sufficient purity to increase the reproduction rate to an acceptable level. Throughout the summer and fall, Fermi and the team built one small experimental pile after another, testing newly arrived materials: new graphite, better quality uranium oxide powder, and uranium metal, cast into egg-shaped lumps by a team at the University of Iowa. Fermi began to sense how the different materials reacted, how different lattice structures produced different intensities of neutrons, how the reproduction factor varied with material and configuration. These experiments continued during the late summer and early fall of 1942.
The ellipsoid shape created a problem of its own: how to build it so that it would be absolutely stable on the floor of the lab. The pile would grow layer by layer from the ground up, with the bottom layer laid out in a rough circle. It would gradually grow wider until it reached a certain height above the floor and then begin to reverse its growth symmetrically. From the side it would look like a flattened sphere. How would all these bricks be held in place? The solution was to create a wooden frame that would, they hoped, hold the layers stable as the pile rose from the lab floor. It required strength and stability, because the pile would be quite heavy and the team could ill afford an accident involving the pile sliding into a messy mountain of uranium and graphite on the lab floor. Fermi found a master carpenter employed by the university who was up to the task.
Instrumentation also needed to be considered. To monitor the reactions, Fermi’s favorite iridium foils could be placed deep inside the pile, removed, and tested for radioactivity. The problem with this process was that it was cumbersome and unsuitable for monitoring the reactions while they were taking place. Volney Wilson, a long-time Compton collaborator, would be in charge of the team responsible for developing this new instrumentation. Working with Wilson were Herb Anderson and Leona Libby. The new instruments would click loudly when either a neutron or a gamma ray was detected and would drive an electronic pen to graph the level of neutron activity on paper tacked to a circular drum, much the way an earthquake detector traces seismic motion.
Another matter deeply concerned Fermi: safety. His studies in New York told him that cadmium was a highly efficient neutron absorber. To control the fission reactions and make sure the pile did not run out of control, perhaps leading to an explosion (the term meltdown had not yet been invented), he decided to insert cadmium-covered wooden strips at strategic points throughout the pile. With all the cadmium strips in place, the pile could not go “critical,” their term for a sustained chain reaction. There would also be a fail-safe mechanism: if during the course of operation the pile became too reactive and at risk of blowing up, a rope could be cut that would allow all the cadmium strips to drop back into the pile simultaneously, bringing the reaction to an abrupt halt. This mechanism would come to be called SCRAM. Though debate continues as to what the acronym came from, its meaning is painfully obvious.
Once a critical mass of material had been assembled—once the reactor had a reproduction rate that grew exponentially—the way to turn the reactor on would be to remove all the cadmium strips but one and then slowly pull out the final strip, the “control rod,” just enough to reach criticality. If the reaction looked like it would grow out of control, all that was needed was to slip the control rod back in place. The reaction would die down almost immediately, as the cadmium absorbed neutrons from the chain reaction. As an added precaution there would be a small group of intrepid souls standing on top of the pile with buckets of a cadmium solution, prepared to douse the whole pile if for some reason SCRAM did not work. That would surely stop the reaction instantly but would also render the entire apparatus useless for further research.
By the end of summer 1942, the general plan was clear. The pile would be built out of town, in an area west of Chicago called Argonne Forest where Compton and his wife enjoyed horseback riding on weekends. Its distance from central Chicago made Argonne ideal. If an accident occurred, it would be far away from the densely populated urban area. Its distance also made it easy to isolate and maintain the kind of secrecy that was required. Work began on a facility for the pile using Compton’s engineers of choice, Stone & Webster, a Massachusetts firm that worked under the auspices of the Army Corps of Engineers. In mid-September a colonel in the corps, Leslie Groves, was promoted to brigadier general and assigned to oversee the entire Manhattan Project, effectively putting Compton, Fermi, Lawrence, Oppenheimer, and the hundreds of other physicists who were drawn into the project over the previous year under military authority. The plan was to finish the facility by October 20, 1942, at which point the entire Met Lab would move there and complete the pile.
Beginning in September 1942, Fermi gave a series of lectures to the Met Lab scientists regarding the theory behind the pile, focusing on calculations of the reproduction factor. Notes of these lectures were taken by Anderson, Libby, and others, and they include some of Fermi’s more endearing uses of American slang. In describing what to do if the reproduction factor ended up much greater than one—in other words, if the chain reaction got out of control—he said, “run quick-like behind a hill many miles away.” The lectures were an important part of the program, designed to ensure that those working so hard on the pile maintained confidence in the science underlying it. They also allowed Fermi to indulge yet again in one of his favorite pastimes, teaching.
GROVES, A BULL OF A MAN WITH A GRUFF MANNER AND AN ABILITY to get things done, had just finished successfully overseeing the construction of the Pentagon, the world’s largest office building. He wanted his next assignment to be overseas and only agreed to the Manhattan Project assignment reluctantly in exchange for two assurances: first, that he would be promoted to general officer rank, and second, that he would have first priority for men and materiel, without restriction. He got both.
Groves visited Chicago in early October 1942 and met with the senior scientists on the project. He was impressed by the “crackpots,” as he liked to call them, who were making progress toward the first chain reaction, and the meeting was productive. Owing to the new authority that Groves had extracted from his masters, graphite and uranium of increasingly higher quality now arrived in Chicago in truly massive quantities. Everyone concerned expected to make the move to Argonne and start work on what would become the first working nuclear reactor.
With total control of the program, Groves imposed military secrecy and ordered that key personnel—Fermi included—could no longer travel by air. The risks of losing essential assets in an air accident were simply too great. He also insisted that a handful of scientists—once again, including Fermi—travel at all times with bodyguards. The bodyguard assigned to Fermi was a suitably large former Chicago policeman named John Baudino. The two eventually became good friends, and Fermi joked that Baudino grew into a decent physicist in the process of sticking by Fermi’s side for the duration of the war.
Finally, in the name of military security, key scientists were not to travel under their own names. They were to use code names suitably chosen so that the scientists would have no trouble remembering them. Fermi liked his new name, Henry Farmer. It sounded very American, even though his pronunciation of his new name sounded distinctly Italian.
Fermi had a sense of humor about his code name. All the senior scientists had one—Eugene Wigner was “Gene Wagner,” Niels Bohr was “Nicholas Baker.” One evening at Los Alamos, after a screening of a forgettable 1943 movie about the life of Madame Curie, Fermi could not resist approaching Bohr: “Mr. Baker, I’ve just seen a grand movie, Madam Cooper.”
On Columbus Day 1942, Roosevelt repealed the enemy alien status of Italians in the United States, even though the country remained at war with Italy. Ironically, just as Fermi became free to travel as he wished, he was restricted by Groves to traveling only by train or car and with Baudino following him everywhere. The bodyguard was supposed to chauffeur his ward whenever the need arose, but on this point Fermi stood his ground. No one would be driving him. Baudino might accompany him, but the bodyguard would be in the passenger seat.
THE PREPARATION FOR FERMI’S PILE EXPERIMENT IN CHICAGO WAS not the only, nor even the main, focus of activity for the Manhattan Project during the summer and fall of 1942. For many participants the experiment Fermi was preparing in Chicago was a foregone conclusion. Planning moved ahead under the presumption that the pile would work according to expectations. Such was the priority of getting a workable bomb in the shortest conceivable amount of time that a number of tracks that depended crucially on each other’s success were moving forward simultaneously. One track was theoretical work on fast-neutron fission, conducted by Oppenheimer and a team based in Berkeley.
Related to this was crucial work on ever more accurate initial calculations of what the critical mass of the uranium bomb would be. More work was also being done on plutonium as a possible fission material. Studies at Berkeley and Chicago indicated plutonium could be used to fuel a fission chain reaction, but the more the new element was studied, the more concern there was about its stability in the quantity necessary for a working bomb.
Groves also directed work to begin on a variety of schemes to separate U-235 from U-238. He chose a location fifteen miles west of Knoxville, Tennessee, a site later known as Oak Ridge. Vast isotope separation plants rose on this site as the Manhattan Project progressed. Oak Ridge was also the site of the first small plutonium production reactor, built once the Chicago experiment proved the concept. Oak Ridge was primarily a research reactor to produce just enough plutonium to begin a more serious set of experiments to determine the new element’s physical properties.
As 1942 drew to a close, Groves selected a large, desolate desert area of southeastern Washington State, eventually known as Hanford, for the top secret location of giant plutonium production reactors. The Columbia River would provide cold fresh water for cooling purposes and the area was easily secured because it was so remote from any urban centers. It was by far the largest facility in the Manhattan Project, some 586 square miles in area.
The work at Berkeley and the selection of large sites in Tennessee and Washington proceeded under the assumption that Fermi’s pile would succeed. Though few physicists who knew about the project doubted that in principle it could work, Fermi and the team were aware that unforeseen difficulties might arise, including issues of safety.
SOON AFTER GROVES’S VISIT TO THE MET LAB IN EARLY OCTOBER 1942, a labor dispute arose at the new lab facilities at Argonne, and by mid-October construction work stopped. Compton had a schedule to meet. Fermi had a pile to build. The two discussed the situation and Fermi suggested finding space on the campus in which to build the pile. Compton thought about it and decided on his own authority—without consulting the university’s president, who almost certainly would have vetoed the idea on safety grounds—to authorize a change in plans. They would build the pile in a squash court under the west stands of Stagg Field, the abandoned football stadium.
In retrospect it was a remarkable decision, reflecting both Compton’s sense of urgency and the trust and confidence he had in his extraordinary Italian colleague. Fermi had persuaded Compton not only by outlining all the safety features he had envisioned but also by referring to the oddly comforting fact that some small percentage of the neutrons released in uranium fission would be emitted moments later than the initial prompt neutrons, giving Fermi additional time to put the control rods in place if the reaction looked like it might run out of control.
Compton was convinced. He understood the physics. He believed in Fermi. Now the work began in earnest.
* When she first met Fermi, Leona was unmarried and her maiden name was Woods. She would marry twice during her lifetime—first to physicist John Marshall, and then to chemist Willard Libby. Because her memoir was written during her marriage to Libby, she is referred to hereafter as Leona Libby, to avoid any confusion.