by Gino Segrè
Roosevelt rapidly authorized the formation of the Advisory Committee on Uranium, to be headed by Lyman Briggs, the director of the National Bureau of Standards. Its members included an aide of Briggs’s and an army ordnance expert, Merle Tuve from the Carnegie Institution, Sachs, and—at his recommendation—Fermi, plus the three Hungarians.
The newly constituted committee met in Washington on the twenty-first of October. Convinced little would result from the meeting, Fermi decided he would not attend. He asked Teller to represent his point of view. Tuve, who had a conflict, asked to be represented by Richard Roberts, the physicist who had shown Bohr and Fermi the fission-produced pulses at the Washington Conference.
The small meeting began contentiously. Roberts claimed that it was unlikely that a chain reaction could be obtained; his opinion was that neutron absorption effects would simply be too large. The army ordnance officer was openly contemptuous of the ability of physicists to develop new weapons. He insisted it was the morale of the troops and not weapons that won a war. At that point, as Szilard remembered, the mannerly Wigner could no longer restrain himself and interrupted by saying that “it was very interesting to hear this. He [Wigner] had always thought that weapons were very important and that this is what costs money, and this is why the Army needs such a large appropriation. But he was very interested to hear he was wrong … And if this is correct, perhaps one should take a second look at the budget of the Army, and maybe that budget could be cut.” The army officer rapidly backtracked.
In the end Sachs and the three Hungarians prevailed, but only to a minimal extent. The committee recommended purchasing graphite and uranium; the proposed amount was a very modest, almost laughable, six thousand dollars. Even that was not immediately approved. Though the war was unfolding across the Atlantic, the United States felt no urgency about rearming. There continued to be almost no incentive to develop a nuclear weapon.
The German situation was quite different. Under the directorship of the physicist Kurt Diebner, an adviser to the German Army Ordnance Office, a series of meetings in Berlin were held in September 1939 to discuss how nuclear fission might contribute to the war effort. The Berlin-Dahlem Kaiser Wilhelm Physics Institute where Lise Meitner had once worked was placed under Diebner’s command.
A group studying fission and its ramifications, informally known as the Uranverein (Uranium Club), was organized and Heisenberg, who had just returned from the United States, was put in charge of its theoretical division. The Third Reich’s leading nuclear physicists were informed of the latest available findings and told their research would be funded if relevant to advancing the Uranverein’s mission. The German drive to employ fission was scarily already under way. Scientists were not hesitant in pushing for this agenda even though several, including Heisenberg, had experienced difficulties with the Nazi regime.
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NEW AMERICANS
On September 1, 1939, Germany invaded Poland; two days later, France and Britain declared war on Germany. The Fermis had dreaded the outbreak of war; the suddenness with which it began stunned them. In May of that year, the Italian and German foreign ministers, Galeazzo Ciano and Joachim Ribbentrop, had signed the Pact of Steel, in which the two nations pledged to support each other in case of war. It seemed a foregone conclusion that Italy would enter the conflict on Hitler’s side, but Mussolini seemed hesitant. Many still harbored a hope that Italy, like Spain, would remain on the sidelines.
Amid this uncertainty, Fermi was focused on becoming a typical American. As with almost every other task in life he undertook, Fermi worked incredibly hard and diligently at this one. As Segrè wrote of him, “Among adult immigrants, I have never seen a comparably earnest effort toward Americanization.” Fermi’s “earnest effort” meant attempting assiduously to get rid of his Italian accent, making frequent use of colloquialisms in his day-to-day speech, and reading regularly what he thought were the most typical American publications: Reader’s Digest and comic strips. Fermi drew the line at following baseball, but he had not been much of a soccer fan in Italy either.
Laura’s Americanization was more gradual. Her acquisition of English began largely with the age-old rituals of shopping and cooking, done in tandem with a nursemaid who had become a general housekeeper. Other than that she learned from Nella, who caught on easily to English while attending a progressive school. Giulio was more intent on having others speak like him than on his speaking like them. But both children were catching on to the meaning of America as the Land of the Free. Nella was always asking for “more freedom,” and Giulio would proclaim, “You can’t make me wash my hands. This is a free country.”
The family’s rented apartment, even though conveniently near the Columbia campus, did not quite fit the Fermis’ image of middle-class society in their new world. They wanted to live the American dream. Laura recalls Enrico feeling this goal particularly strongly. To be true Americans, the Fermis needed a suburban house with a garden and good public schools for their children. Two cars would round it out nicely.
Harold Urey, a Columbia colleague and fellow Nobel Prize winner, urged them to meet all those conditions by moving to where he lived: Leonia, a New Jersey community directly across the George Washington Bridge from New York City. The Ureys were persuasive. By September 1939, back from Michigan, the Fermis were settled into a comfortable house “on the Palisades, with a large lawn, a small pond and a lot of dampness in the basement.” There were flowerbeds and a rock garden. Urey encouraged Enrico to become even more American by gardening and by mowing his lawn, but as Laura said of her husband, “his peasant blood was not aroused.” His assimilation had limits. Fermi preferred going for a walk or playing tennis in his free time rather than pulling up crabgrass. His utilitarian streak reasoned that it was green and covered the lawn just as well as any other grass.
By this time, it was well past the Fermis’ six-month temporary leave granted from Italy. The American press had it right after all: Fermi was settling in America. No longer pretending otherwise, Laura and Enrico retrieved their furniture, still in Rome. The continuing presence of the furniture in their apartment had been noted in police surveillance reports, providing a bit of assurance that the family intended to come back to Italy. However, the Fermis had not escaped the suspicions of the police, who reported in March 1939: “We have noticed that Professor Fermi has always shown a deferential attitude toward the regime but not a great deal of enthusiasm. We don’t know if and when he’ll return.” There was finality to their move to America when the Fermis were reunited with their furniture in Leonia. Things had been packed so hastily that a bag full of garbage was included.
Despite their efforts to fit in, within little more than a year the Fermis and the other recent Italian immigrants would be classified by the American government as enemy aliens. Italy had entered the war. When Mussolini witnessed the German invasion of Belgium, Holland, and France, he feared that postponing his decision any longer would mean missing the spoils of victory. Nothing seemed to stop Hitler’s aggressions. In early June, French and English troops had been evacuated at Dunkirk in the face of German conquest. On June 10, with the Germans rushing toward Paris, Mussolini announced that Italy would fight at their side. As was often the case, both Il Duce’s timing and his judgment were terrible.
Italy’s entry into the war on Hitler’s side added greatly to the Fermis’ worries about friends and relatives, especially Jewish ones. Laura’s father was a widower, her mother having died in 1935. Partially paralyzed by a stroke, he showed no interest in leaving Italy. Despite its passage of racial laws, the retired Jewish admiral remained a strong supporter of the monarchy he had served for so many years. He also believed his own safety was assured.
Fermi had not taken a public stand against the Mussolini regime, but the Italian embassy had followed his activities with suspicion ever since his departure in December 1938. With a war on, Rome wanted an indication of where Italy’s most prominent scientist stood. In September 1
940, the embassy sent a report on him to the Ministry of Foreign Affairs, “Prof. Fermi, though not actively engaging in an antifascist campaign, must be considered as having passed to the camp of the opponents. He does not frequent any Italian milieus and all his relations are with the intellectuals who are the most persistent enemies of our Regime.” The embassy, unenlightened about Fermi’s research on nuclear fission, had no idea how advantageous it would be for America that Fermi had, as they put it, “passed to the camp of the opponents.”
Although Fermi’s experiments on fission had ground to a halt in the summer of 1939 for lack of funds, they resumed in the early spring of 1940 when the promised six thousand dollars from the Advisory Committee on Uranium finally reached Columbia. Fermi and Szilard agreed that Szilard should lead the search for needed supplies but that Fermi would be responsible for conducting experiments. This was an understanding that suited both of them. Neither knew what the research might lead to. It seemed unlikely that it would aid weapon development, but Szilard was keeping a sharp eye on such a prospect.
The experiments’ official kickoff was marked by the arrival of one and a half tons of graphite blocks. Fermi and Anderson worked side by side, assembling them into an eight-foot-high column with a nine-square-foot base. Slots were cut into fifteen of the bricks strategically located at different heights, and foils that registered neutron counts were placed in the slots. The neutron source was put at the structure’s bottom and operated for one minute. The foils were then quickly removed and carried down the hall to Fermi’s office, where Geiger counters measured their induced radioactivity. It was reminiscent of the process at Via Panisperna: rushing scientists, repeatedly careening down a hall. This was the kind of experiment Fermi enjoyed. He was in charge, understood all the details, and was involved at every step. Things ran like clockwork.
The purity of the graphite Anderson and Fermi used turned out to be crucial. Getting what they wanted required circumventing the Bureau of Standards, the intermediary between Columbia and the graphite’s provider, the National Carbon Company. Szilard remembered a lunch he and Fermi had with two representatives of the company in February 1941 during which an embarrassed silence followed his asking about its purity, “You wouldn’t put boron in your graphite, or would you?” Since the probability of boron absorbing a thermal neutron is more than a hundred thousand times as great as that of graphite, even a minute amount of this contaminant made a huge difference. The flaw was corrected.
A policy of silence on fission research had fortunately been instituted within the physics community. The editors of the Physical Review asked authors submitting papers related to uranium research to withhold them from publication. Szilard’s plea for secrecy had finally been heard. The secrecy was judicious and timely because Walter Bothe, a distinguished German experimentalist, was in the process of measuring the absorption in graphite of thermal neutrons. The result he obtained, more than twice Anderson and Fermi’s value, led him to conclude that graphite was unsuitable for slowing neutrons in uranium experiments. Had Bothe known that impurities were almost certainly present, he would have continued his explorations. Because Bothe didn’t, Germany turned its attention to using heavy water for slowing down neutrons. Its progress was seriously hampered because heavy water—water in which ordinary hydrogen is replaced by its isotope deuterium—is far more arduous to obtain.
Meanwhile, Fermi and Anderson, cautiously optimistic that graphite would be effective, introduced uranium into their cagelike structure. All the experiments the two of them conducted over the next eighteen months were aimed toward obtaining a self-sustaining chain reaction induced by fission. The technical problems were innumerable, but Fermi proceeded to solve them one by one.
This was where Fermi’s extraordinary ability to bridge experimental and theoretical physics came to bear. To describe in accessible terms a self-sustaining chain reaction, Fermi introduced a coefficient labeled k, his famous neutron reproduction factor. The probabilities of a neutron being slowed down, being absorbed by either uranium or by carbon and eventually producing fission, were all taken into account and multiplied together to reach a convenient single term he called k. It explicated what happened in each generation of neutron production. If k was greater than one, the number of neutrons would keep increasing, while if it was less than one, that number would decrease. A self-sustaining chain reaction would only be possible if k was greater than one.
There was one more consideration. As Fermi wrote in a postwar article, having k greater than one meant the desired reaction will “always take place provided the leakage loss of neutrons is sufficiently small. This, of course, can always be achieved if the size of the pile is large enough.”
The term “pile” was coined by Fermi because the assemblage of carbon and uranium could in principle take any shape: it might be a cube, a sphere, a column, or whatever was most opportune. It was just a pile. The bigger the pile, the smaller the leakage. Fermi and Anderson realized that a self-sustaining chain reaction would require their building a much bigger pile. They would need more graphite, more uranium, and a bigger space to work in. Obtaining all of that was going to be expensive. They had no idea where the money would come from.
The government seemed to be the only possible source. The Uranium Committee founded in late 1939 had proved to be powerless and its leader, Lyman Briggs, ineffective. However, in mid-1940, the time when Fermi was appealing for more funds, a far more authoritative organization had been created within the government. The new super-organization was the National Defense Research Council (NDRC). Its creation was an American acknowledgment that scientific research would play a major role in the war effort.
The NDRC was the brainchild of Vannevar Bush, a talented engineer who left the vice presidency of MIT in 1938 to become president of Washington’s Carnegie Institution. His motivation was precipitated by the desire to be closer to the seat of power and government decision making. With war in Europe and the United States’ imminent entry into it, he surmised the country needed a dedicated government agency led by scientists capable of discerning what should be supported and what funding levels were appropriate. The organization was to be independent from the military and report directly to the executive branch of government.
Approaching key contacts with both scientific and administrative credentials to help found his brainchild, Bush selected three with particular care. They were Karl Compton, the president of MIT and the older brother of the Nobel Prize–winning Chicago physicist Arthur Compton; James Conant, a distinguished chemist who was president of Harvard; and finally, the physicist Frank Jewett, president of both Bell Telephone Laboratories and the National Academy of Sciences. All three accepted the invitation to join him.
It was a formidable assembly. After lobbying the army, the navy, and Congress, Bush proposed the idea to the president on June 12, 1940. The meeting was brief: the NDRC was approved and Bush was appointed its director. The group was charged with coordinating resources and advising on scientific research related to warfare. Nuclear physics fell under its purview, and accordingly Briggs’s committee was subsumed as a subcommittee.
Inexplicably, Briggs did not alert the NDRC to the warnings he had been receiving from the likes of Szilard and Wigner. The upshot was that the NDRC was not even thinking about nuclear weapons. These simply were not on its radar. Conant, Bush’s right-hand man and the chair of the NDRC’s chemistry and explosives division, admits to not having heard of the possibility of using fission in a bomb until March 1941. He only learned of it then because Churchill’s scientific adviser happened to tell him during a visit Conant made to Britain. So much for military preparedness.
The situation changed within a short time, but developing a self-sustaining chain reaction was not an NDRC priority. On the other hand, a large pile might be a useful and novel way to generate power and Fermi’s stellar research was widely respected. Columbia’s application for a large-scale graphite-uranium experiment was funded by the NDRC but
granted only $40,000 of the $140,000 requested. The grant wasn’t as large as Fermi had wanted, but it was enough for him to get started on a bigger pile.
Fermi worked under primitive conditions. He had explored the university in 1940 together with Pegram in search of the bigger space the new pile would need. The search led them “to dark corridors and under heating pipes and so on to visit possible sites for this experiment.” They found what they were looking for in the basement of Columbia’s Schermerhorn Hall: a large, gloomy, cavernous room with a very strong floor.
Pegram, the physics department chairman, suggested he hire some undergraduate football team members to carry fifty tons of graphite into the space for building a bigger pile. Anderson, Fermi, and a few other physicists who joined the bulky athletes began building a new column. The old column had been eight feet high and three by three feet at the base. This one was eleven feet high and eight by eight feet at the base.
Naturally, Fermi again did not flinch from taxing manual work, pushing blocks of graphite through a bench saw and disappearing into a cloud of black dust. The eight tons of uranium oxide presented other problems. It arrived as powder, the only form available for purchase. To remove impurities, it needed to be heated to several hundred degrees and, while still hot, placed in cubical metal boxes. There were in all 288 of these boxes. They were soldered shut to ensure no moisture entered and arranged in a lattice structure inside the graphite. Slots were cut for foils to measure the neutron flux throughout the pile.
