Anderson went out of his way to explain why this was the last time that Fermi and Szilard collaborated directly on an experiment:
This was the first, and also the last, experiment in which Szilard and Fermi collaborated together. Szilard’s way of working on an experiment did not appeal to Fermi. Szilard was not willing to do his share of the experimental work, neither in the preparation nor in the conduct of the measurements. He hired an assistant to do what we would have required of him. The assistant, S. E. Krewer, was quite competent, so we could not complain on this score, but the scheme did not conform with Fermi’s idea of how a joint experiment should be carried out, with all the work distributed more or less equally and each willing and able to do whatever fell to his lot. Fermi’s vigor and energy made it possible for him to contribute somewhat more than his share, so that any dragging of feet on the part of the others stood out more sharply in contrast.
Fermi was never so explicit. During his January 1954 lecture to the American Physical Society about this period at Columbia, all he would say about his brilliant but frustrating collaborator was that he was “a very peculiar man, extremely intelligent,” a description that brought hearty laughter from the audience. Whatever reservations Fermi had about Szilard’s willingness to get his hands dirty in the lab—and uranium oxide is very dirty—he retained enormous respect for Szilard’s ability to think creatively about difficult scientific problems.
Fermi, Szilard, and Anderson submitted their experimental results for publication in Physical Review in early July 1939. Soon Fermi and the family were off to Ann Arbor, Michigan, for another summer school session, where he lectured on the absorption of cosmic rays in the atmosphere and in solids. He also met up with an old acquaintance from his days in Germany, and they had a conversation that lingered in Fermi’s mind for the next six years.
IN LATE JULY 1939, WERNER HEISENBERG ARRIVED IN ANN ARBOR to spend a week participating in the Goudsmit-Uhlenbeck summer session, seeing old friends and discussing the state of the world.
The state of the world was grim. To anyone reading the daily reports coming out of Berlin, Moscow, Paris, and London, it was clear that European powers were preparing for an outbreak of hostilities in the very near future. The betting was that Germany would invade its ideological enemy, the Soviet Union, within the month.
At that moment, Heisenberg arrived on the scene in Ann Arbor.
Max Dresden, a young student tending bar at a party hosted by refugee physicist Otto Laporte, witnessed an encounter between Fermi and Heisenberg. Dresden, who would go on to a distinguished career at Stanford, describes the evening:
There was actually not much to do, so we could pay close attention to the conversations. There was really only one central topic. Fermi had just left fascist Italy to come to the US; Heisenberg had decided to return to Nazi Germany. The crucial part of their argument was whether a decent, honest scientist could function and maintain his scientific integrity and personal self respect in a country where all standards of decency and humanity had been suspended. Heisenberg believed that with his prestige, reputation and known loyalty to Germany, he could influence and perhaps even guide the government in more rational channels. Fermi believed no such thing. He kept on saying: “These people [the Fascists] have no principles; they will kill anybody who might be a threat—and they won’t think twice about it. You have only the influence they grant you.” Heisenberg didn’t believe the situation was that bad. I believe it was Laporte who asked what Heisenberg would do in case of a Nazi-Soviet pact. Heisenberg was totally unwilling to entertain that possibility: “No patriotic German would ever consider that option.” The discussion continued for a long time without resolution. Heisenberg felt Germany needed him, that it was his obligation to go back. Fermi did not think there was anything anyone could do in Italy (or Europe); he was afraid for the life of his wife (her father was later killed); and so he felt it was better to make a fresh start in the US. But none of the decisions had come easy. The role of physics and physicists was mentioned off and on. After the party was over everybody left in a state of apprehension and depression.
Some three decades later Heisenberg recalled another one-on-one conversation with Fermi, at Fermi’s Ann Arbor apartment. Fermi started out on a positive note, describing how his move to the United States was liberating, how the United States had been home to European refugees for generations, and how stimulating it was to start all over again in his new homeland. “Here, in a larger and freer country, [Europeans] could live without being weighed down by the heavy ballast of their historical past. In Italy I was a great man; here I am once again a young physicist, and that is incomparably more exciting. Why don’t you cast off all that ballast, too, and start anew?” Heisenberg replied that he understood the attraction, but that abandoning Germany now, he would feel himself a traitor, particularly to younger physicists who did not have the ability to emigrate and to find work wherever they wanted. For Fermi, however, any responsibility he felt for the students he left behind was outweighed by the many compelling reasons he had for leaving Italy. In later years, Fermi’s notably generous treatment of his American students may have been an effort to compensate for lingering feelings of guilt he had over abandoning his Italian students.
Fermi pressed on, explicitly referring to the possibility of using the discovery of atomic fission to create a bomb. He warned that Heisenberg would be expected to work on such a project. Heisenberg expressed doubt that such a weapon could be built, at least not quickly. Fermi then asked, “Don’t you think it possible that Hitler may win the war?” Heisenberg expressed doubt, given the balance of technological resources available on each side. Fermi was incredulous that under the circumstances Heisenberg still wanted to return to Germany. Heisenberg explained that patriotism was a stronger factor for him than doubt about the war’s outcome.
Fermi ended the conversation, noting, “That’s a great pity. Let’s just hope we will meet again after the war.”
Fermi may have had reservations about the possibility and wisdom of pursuing an atomic weapon. He had soft-pedaled the idea in his meeting with the Navy in March 1939 and continued to have doubts about the technological feasibility of the weapon. His encounter with Heisenberg dramatically altered his perspective. He had known Heisenberg since 1922 and, though Fermi might not have particularly liked the man, he had followed his career and his contributions with great interest. He had even nominated Heisenberg for a Nobel Prize. Heisenberg would now return to his home country and when the inevitable war broke out would be tasked by Hitler with developing a weapon based on nuclear fission. Fermi had enough respect for Heisenberg to know the serious threat he posed. Whatever reservations Fermi had about the project, he would now have to pursue it with vigor. He had no real choice in the matter.
THE RESULTS OF THE SPRING EXPERIMENT WITH SZILARD AND Anderson were never far from Fermi’s mind. Over the summer he corresponded with Szilard on a central problem: if water was not a suitable moderator for the chain reaction, was there another substance that would be suitable?
Their thoughts turned to a form of carbon called graphite.
CHAPTER FIFTEEN
PILES OF GRAPHITE
IN VIA PANISPERNA, CORBINO’S BOYS DISCOVERED THAT LIGHT nuclei could slow down neutrons. The paraffin block experiment was a perfect demonstration of this phenomenon. At the time the Rome team did not know they were splitting atoms and thus had no interest in the neutrons that might be emitted from fission reactions. Now, as he worked to create a controlled chain reaction, the behavior of neutrons themselves mattered greatly to Fermi.
If hydrogen was not suitable for sustaining the chain reaction, what light nucleus would work? The periodic table of elements is organized from lightest to heaviest—hydrogen, with an atomic number of 1, is first and, in 1939, uranium, the heaviest naturally occurring element, ended the table with an atomic number of 92. It was quite natural to examine elements sequentially along the periodic table from hydrogen onward
for the next best neutron moderator, especially for someone as methodical as Enrico Fermi.
Hydrogen has two heavy isotopes, deuterium and tritium, both of which would be less likely to capture neutrons, but these isotopes are rare in nature and difficult to manufacture. Moving up the periodic table, helium is naturally found as a gas, and liquid helium is so cold that it needs special handling. Lithium, beryllium, and boron are the next in line, but the first two are relatively dangerous to work with and boron was not readily available—the major source of all boron is Turkey. Fermi later discovered that boron absorbs neutrons and would not make a suitable moderator. Next after boron is carbon.
Carbon is safe, plentiful, and comes in a variety of solid forms. Coal, of course, is one such source, but it is too soft to be machined with precision. Diamond is another form, but it is far rarer, and as the hardest substance on the planet it is virtually impossible to machine. Graphite, a crystalline form of carbon, is not quite as plentiful as coal but still easily obtained in nature in large quantities and quite easy to machine with precision. Every common pencil contains a piece of graphite that has been machined down to a thin rod. Making graphite bricks is relatively easy.
Fermi and Szilard came to the idea of substituting graphite for water and spent much of the summer corresponding about the possibility of using graphite as a moderator. Carbon atoms are about twelve times heavier than hydrogen atoms, but both scientists believed that carbon might absorb a sufficient amount of a neutron’s kinetic energy to slow it down for the purpose of a fission chain reaction. Perhaps it would not grab neutrons out of the chain reaction, the way hydrogen did. The scheme that Fermi had in mind would require a lot of graphite, and Szilard was a man who knew how to get it.
WHILE HE WAS CORRESPONDING WITH FERMI, SZILARD WAS ALSO scheming with his old friends Edward Teller and Eugene Wigner to kick-start the US government’s interest in uranium chain reaction research. The story of how Szilard and his fellow Hungarians persuaded the most famous scientist in the world, Albert Einstein, to sign a letter to President Roosevelt on August 2, 1939, urging the president to initiate large-scale research into the possibility of a nuclear weapon is well known. The image of a carload of Hungarian geniuses, chauffeured by a New York investment banker friend of Szilard, hunting for Einstein’s house in the wilds of Long Island’s North Fork is one of the more vivid of this entire period. The letter galvanized American research in fission weapons. Fermi was neither directly involved in writing the letter nor in getting it signed by the great man, but he was mentioned in the famous first paragraph, drafted by Szilard himself:
Some recent work by E. Fermi and L. Szilard, which has been communicated to me in manuscript, leads me to expect that the element uranium may be turned into a new and important source of energy in the immediate future. Certain aspects of the situation which has arisen seem to call for watchfulness and if necessary, quick action on the part of the Administration. I believe therefore that it is my duty to bring to your attention the following facts and recommendations.
Events that summer in Europe only served to heighten the sense of urgency. In diplomatic, military, and intelligence circles, rumors swirled that Germany was poised to invade Poland in a lightning attack. The negotiations between Germany and the Soviet Union that led to the Molotov-Ribbentrop Pact remained secret for much of August—they were underway when Heisenberg dismissed out of hand Laporte’s speculative question at the Ann Arbor party about a Nazi-Soviet alliance—but the two governments sprang their surprise alliance on an incredulous world on August 23, 1939. Hostilities began a week later. The United States was not yet involved, and many influential politicians and public figures remained opposed to US involvement, but Roosevelt was already engaged in quiet efforts to bring American public opinion and industrial might in line behind eventual engagement in a European war on the side of the Allies.
Roosevelt finally received the letter in October 1939 and authorized work on fission as an immediate priority. The national effort launched by Roosevelt eventually evolved into the largest, most complex military-scientific program ever conceived. At that moment, however, the Manhattan Project was limited to Fermi’s work at Columbia’s Pupin Labs.
BEGINNING IN THE FALL OF 1939, FERMI WROTE FORTY-SEVEN papers describing the experimental work leading to the creation of the world’s first controlled self-sustaining chain reaction on December 2, 1942. The work was methodical, demanding, and sometimes dangerous and involved a growing group of physicists. Fermi continued relying on Szilard’s skill in obtaining increasingly pure batches of graphite. Szilard also served as a sounding board and a sometimes irritating cheerleader for the project. Pegram, as head of the physics department at Columbia and a dean of the college, threw his considerable weight and sound judgment behind the project. Anderson and Zinn lent their experimental knowhow and attention to detail, and Fermi added a bright young Columbia graduate student, George Weil, to his team. Other students were to follow, including Albert Wattenberg and Bernard Feld.
Given the importance of the chain reaction experiments, one might assume that they completely preoccupied Fermi, but he continued to pursue other research interests along the way. For example, he lectured on the geophysics of iron in the core of the earth at the 1940 Washington Conference, the one subsequent to the conference at which Bohr and Fermi sprang fission on an unsuspecting world. In the spring of 1940, Fermi went to Berkeley to give the annual, highly prestigious Hitchcock Lecture on “High Energies and Small Distances in Modern Physics.” His archival notebook demonstrates the effort he made to prepare for these lectures. His Ann Arbor work on cosmic rays passing through gases and solids continued into the fall at Columbia, resulting in laborious and frustrating calculations on the relationship between the density of a medium and the speed with which an ionized particle slows down. Apropos of this, Fermi quipped to Anderson that “he could calculate almost anything to an accuracy of ten percent in less than a day, but to improve the accuracy by a factor of three might take him six months.” Colleagues sometimes noted his tendency to get frustrated if he could not immediately solve a problem. This was a good example.
He also continued with a full teaching load, including courses on geophysics and quantum theory.
Like all Americans, he followed the war in Europe. On the first of every month, he and a group of faculty colleagues—a “Society of Prophets” Laura called them—would meet at the faculty club to predict developments over the coming month, writing down answers to ten yes-or-no questions. Laura reports that by the time the society dissolved, when the Fermis departed for Chicago in the summer of 1942, Enrico had established himself as the Prophet, having predicted successfully 97 percent of the time. He did this, she writes, using the most conservative algorithm imaginable: the next month would look almost exactly like the previous month. He did, however, miss one prediction—the surprise German invasion of the Soviet Union. The game was ideal for someone of Fermi’s temperament, invariably conservative and skeptical of any predictions of quick or revolutionary change.
In the lab, the bulk of his efforts focused on fission and chain reactions and, within a few months of returning from Ann Arbor, he and his team were making progress.
THE CONCEPT OF A PILE DIFFERS LITTLE CONCEPTUALLY FROM THE configuration of the water tank experiments that Fermi, Szilard, and Anderson carried out during the winter and spring of 1939. Effectively, Fermi decided to substitute graphite bricks for the water and to build up, as well as out.
The water tank experiments were not designed to analyze the way neutrons diffused within the moderating medium. Realizing how crucial it would be to understand this diffusion process with graphite, Fermi devised a series of experiments to do just that. In rooms at Pupin and later in the basement of nearby Schermerhorn Hall, he and his colleagues—with the occasional help of burly members of the Columbia football team who were press-ganged by Pegram—stacked graphite bricks into square columns several feet thick, placed rhodium foil at key location
s throughout the stacks, set a neutron source at the top of the stacks, and studied how neutrons made their way through the pile. Once the foils were exposed, they were quickly extricated from the stacks and run down a corridor so the radioactivity could be measured by Geiger counters. Fermi and Anderson raced down the corridors to take advantage of rhodium’s short, forty-four-second half-life, re-creating scenes from Via Panisperna in 1934, most likely with Fermi in the lead. They built one stack after another, getting covered in fine black graphite dust that made them look more like coal miners than experimental physicists. As the stacks grew in height, to over ten feet, the physicists required ladders to get to the top of the stacks and place the neutron source. Many years later, when Fermi recalled these experiments at a public lecture, he drew a laugh from the audience as he described it as “the first time when I started climbing on top of my equipment because it was just too tall—I’m not a tall man.” These diffusion experiments were critical in establishing how neutrons slowed down during their voyage through the graphite and gave some sense of how often neutrons might be absorbed by the graphite.
These experiments began in the spring of 1940 and continued throughout much of the rest of the year. Anderson, chasing after Fermi running down the corridor with rhodium foil, played Amaldi’s role from six years earlier at Via Panisperna. Szilard kept up with the increasing demand for ever larger quantities of graphite, playing the same procurement role as Segrè did in Rome. Szilard had help from the “Committee on Uranium,” a group of senior scientists and military officers established by Roosevelt to provide guidance and coordination for the new effort authorized by the president. It was an “all-American” group. As foreign nationals, Fermi, Szilard, Teller, and Wigner were formally excluded but met frequently with the committee to provide input into their decisions. One of their first decisions was to allocate $6,000 to buy what Fermi described as “a huge amount” of graphite.
The Last Man Who Knew Everything Page 21