As the diffusion experiments continued through 1940, Fermi and Szilard began to suspect that impurities in the graphite were absorbing neutrons at a rate that would reduce the probability of a successful chain reaction. A visit to one of the manufacturers, National Carbon in Cleveland, resulted in the identification of trace quantities of boron in the graphite as the culprit. In 1941, Szilard worked closely with the engineers at National Carbon to develop a processed form of graphite with fewer impurities.
A second experimental project took place alongside the diffusion studies, the purpose of which was to test out Fermi’s idea of placing the uranium in lumps throughout the graphite pile, to reduce the likelihood that neutrons would be absorbed by U-238.
These experiments pushed materials science to new frontiers. Industrial graphite had too many impurities to make it useful as a moderator for the chain reaction. Szilard and the engineers at National Carbon worked hard to develop methods to remove boron and produce a suitable graphite moderator. In doing so, they created the world’s first “nuclear graphite,” a form of pure graphite now used throughout the world in graphite-moderated nuclear reactors.
Another aspect of materials science that got a boost through these experiments involved the production of uranium. Before the war, uranium was valuable only insofar as it was used in scientific experiments, and the most readily available form of the element was uranium oxide, not ideal for the purposes of the “exponential” pile.* There being little economic need for uranium metal, production was still relatively primitive. The best that industry could do was to produce the metal in a powder form, which had a tendency to spontaneously combust when exposed to air. Monitoring these experiments with great interest, the Committee on Uranium worked with a variety of manufacturers to improve uranium metal production techniques.
During this period, the team continued to grow, with Wigner, Wheeler, and others from Princeton joining Fermi’s team.
BY SEPTEMBER 1941, FERMI BELIEVED THAT THE TEAM HAD MADE sufficient progress to build a true working exponential pile. The Committee on Uranium allocated the considerable sum of $40,000 for the purchase of massive quantities of uranium and graphite, and Szilard negotiated the purchases with industrial vendors. The Columbia football players were called back into service, and the pile began taking shape in the basement of Schermerhorn Hall. Fermi later spoke with awed amusement at the ease with which the Columbia athletes packed heavy cans of uranium oxide—uranium was and remains one of the heaviest elements found in nature—and hoisted them into a lattice whose structure Fermi determined as he balanced theoretical calculations and educated guesswork with the practicalities of working with the materials at hand. Large-scale electronic computers not having been invented, it was impossible to do a full-blown calculation as to what size the lattice would have to be to produce a self-sustaining chain reaction. What was possible to measure was the performance of a pile of a specific size and specification and extrapolate whether such a structure, if extended infinitely, would produce such a chain reaction.
The actual pile they built grew to a stack of graphite bricks eight feet on each side and eleven feet high. Within the stack of graphite, square tin cans of uranium oxide, eight inches on each side, were distributed in a three-dimensional matrix of some 288 cans. Slits were placed strategically to allow for insertion and removal of iridium foils to measure radioactivity. The neutron source would be placed in a bed of paraffin at the bottom of the pile.
This first pile perfectly reflected Fermi’s experimental style. Its design was partly a product of sophisticated theoretical considerations, in particular the lumping of uranium throughout the graphite in a “lattice” framework. Yet the design also reflected basic practicalities, such as the dimensions of the graphite bricks themselves. Fermi’s design proved easy to build and lent itself to systematic measurement and evaluation through a series of carefully controlled experiments. He played an active part in its construction, piling graphite bricks and cans of uranium oxide alongside the rest of the team. Unfortunately, however, the results of the experiment were disappointing. By Fermi’s calculation, even extending the structure they built into infinity, the performance would be 13 percent below what would be necessary for a self-sustaining chain reaction.
Undaunted, Fermi and his colleagues were convinced that adjustments in the structure of the lattice and improvements in the purity of the materials could squeeze more excess neutrons out of the process and deliver the desired result. The Committee on Uranium seemed to agree, as did the various bodies that were now coordinating and directing all national work on fission. Central to this effort was Vannevar Bush, an MIT-trained engineer with a skeptical, Yankee demeanor. Bush, an extraordinarily energetic administrator, reported directly to the president.
As the project grew in organizational complexity, research into fission became highly secretive. Secrecy in fission research had begun as an informal agreement among physicists working in the United States and Britain to avoid publishing experimental results that might help German scientists, but had morphed into a formal statutory edifice of security classification. Many of the scientists most deeply involved with this sensitive work—men like Fermi, Szilard, and Wigner—were foreign nationals and found themselves excluded from deliberations within the committees organized to guide and develop fission research. Political leaders all solicited their views, but the decisions took place behind closed doors without the foreign-born scientists.
Others may have treated the need for secrecy differently, but Fermi took it quite seriously and never discussed his work with his wife. Throughout the war, from New York to Chicago to Los Alamos, Fermi was silent about his activities and his role. Laura Fermi only learned anything substantive about her husband’s role in the Manhattan Project in August 1945 after the bomb was dropped, when Fermi handed her a copy of an unclassified US government report on the project.
The work at Columbia on the pile, as well as important technical progress elsewhere, gave Bush a sense of optimism that a fission bomb was indeed possible. On December 6, 1941, he announced an “all-out” effort to pursue a fission weapon. The next day Japan attacked Pearl Harbor and within a few days the United States was at war against Japan, Germany, and Fermi’s home country of Italy. Bush reorganized the project’s leadership once again, creating a new independent organization, called S-1, to replace the Committee on Uranium. Others on the executive committee included Harold Urey, Ernest Lawrence, and Arthur Compton. Lawrence, the inventor of the cyclotron and the leader of the experimental physics group at Berkeley, would be responsible for directing research on plutonium and for developing methods of isotope separation based on his cyclotron experience. Urey, the brilliant Columbia physical chemist who befriended Fermi in early 1939, would be responsible for chemical separation issues associated with plutonium production and would also pursue promising lines of work on isotope separation. Compton, who sat on various oversight committees during this period, would direct further research into the properties of uranium and plutonium under the aegis of the newly created Metallurgical Laboratory at the University of Chicago.
Eager to prove his loyalty to his new country, Fermi was in an awkward position. He was an enemy alien working at the heart of the US government’s most sensitive and secret military project. Change was afoot.
* In a “critical” pile the reactions are self-sustaining; in an “exponential” pile neutron production grows geometrically.
CHAPTER SIXTEEN
THE MOVE TO CHICAGO
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 workm
an 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.
The Last Man Who Knew Everything Page 22
