The Atom Bomb Part 1: Making It Possible
Before an atomic bomb could be designed, there had to be controlled, sustained nuclear fission. Enrico Fermi and his crew made it work.
“I‘M HUNGRY. LET’S GO TO LUNCH!”
Had Enrico Fermi not paused for a bite, the atomic age would have begun that much sooner. But Fermi was accustomed to having lunch at noon, and he was completely confident that Chicago Pile One would go critical with further withdrawal of a control rod. So, deliberate experimenter that he was, Fermi called for lunch; mankind’s first nuclear chain reaction could wait. And wait it did—until 3:42 P.M. on December 2, 1942. At that moment, Fermi’s team had accomplished a feat deemed impossible a scant decade before by the eminent British physicist Ernest Rutherford.
Back in 1911 Rutherford had been the first to demonstrate that a tiny nucleus lies at the heart of every atom. Great stores of energy might lie within that nucleus, he later said, but they were locked away. Nuclear energy was accessible only in dribs and drabs, in the transmutations of radioactivity. Controlled production of nuclear energy? Impossible! “Moonshine” was the term Rutherford used, though he stressed that he meant “with the means at present at our disposal and with our present knowledge.” Other physicists who agreed, using less colorful language, included 1.1. Rabi, Niels Bohr, Robert Millikan, and Albert Einstein.
With the knowledge we have today, Rutherford’s dismissal looks naive, for the process behind a nuclear chain reaction is quite simple to understand. A neutron collides with an atomic nucleus having suitable characteristics, causing it to undergo fission (that is, split) into lighter elements. In the process it releases energy and more neutrons. These neutrons in turn collide with other nuclei and make them split. Let the fission build up exponentially, and you’ve got a bomb; hold it steady, and you can collect the energy that’s released. The concept is easily grasped by any modern high school physics student, but today’s high school students know about things that Rutherford could not have anticipated in 1933, when he made his forecast.
Rutherford himself had speculated about the existence of the neutron as early as 1920; it was confirmed experimentally by James Chadwick in 1932. During the 1930s physicists were familiar with simple radioactive decay, in which an atom throws off small particles, sometimes changing into a different element in the process. But fission—in which an atom produces two sizable fragments, instead of one big one and one tiny one—was still in the realm of speculation. Even as Rutherford made his prediction, however, research was going on that would eventually make atomic energy—and atomic weapons—a reality.
The story of nuclear physics begins, logically enough, with Rutherford’s discovery of the nucleus. He did it by shooting alpha particles (helium atoms without their electrons) at a thin layer of gold foil. As they passed through the foil, most of the particles were scattered through only a small angle, continuing pretty much along their original path. But a few of the alpha particles were deflected back in the direction from which they had come, showing up as flashes of light on a zinc sulfide detector screen. The astonished Rutherford said later, “It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.”
Those brief, barely visible flashes provided evidence for a new view of the atom. Instead of its mass being more or less uniformly distributed, Rutherford’s experiments showed that the atom was mostly empty space. The largest part of its mass, and all of its positive charge, were concentrated at its center in a core 10,000 times smaller—the nucleus.
Enrico Fermi was not yet ten, a schoolboy in Rome, when Rutherford first shared these startling results. While it’s unlikely that he knew of Rutherford’s work, from an early age Fermi showed an amazing aptitude for the sciences and an exceptional memory. He was largely self-taught, using books he bought in the stalls at Rome’s Campo del Fiori. Adolfo Amidei, a colleague of Fermi’s father at the Italian Ministry of Railroads, served as a mentor for young Fermi, giving him yet more books to absorb and encouraging him to learn German so that he could keep up with the latest scientific advances.
In 1918, after completing high school, Fermi applied for admission to the Scuola Normale Superiore in Pisa. As part of the entrance requirements he submitted an essay on sound, in the course of which he used Fourier analysis to solve the partial differential equation of a vibrating rod. It so transcended the work usually submitted that the examining professor sought out Fermi and told him he had never seen its like, that Fermi was destined to become an important scientist.
That promise was fulfilled, as Fermi moved to a position of leadership at Pisa, not only among his fellow students but even with the faculty. Throughout his life Fermi was adept at both theory and experiment, an uncommon talent in physicists, who usually specialize in one or the other. While in college Fermi continued his self-education, using some of his sum- mer vacation in 1919 to organize his knowledge of selected fields of physics in a remarkable notebook that included Rutherford’s work on radioactivity and Niels Bohr’s papers on the hydrogen atom. Fermi’s friend and biographer Emilio Segrè said the notebook “would be very creditable even for a teacher with a long educational career.”
AFTER RECEIVING HIS doctorate at Pisa in 1922, Fermi did postgraduate work at G’f6ttingen, Germany, where his youth and down-to-earth approach did not fit in with such formidable, philosophical-minded colleagues as Max Born, Werner Heisenberg, and Wolfgang Pauli. After taking a temporary teaching post at the University of Rome, Fermi did further work at Ley den, the Netherlands. He returned to his native land in 1924 to accept a post at the University of Florence; in November 1926 he was named to a newly created chair of theoretical physics at the University of Rome, making him the youngest professor ever at the university. There, until his emigration a dozen years later, Fermi strengthened an already impressive worldwide reputation in physics by making key theoretical discoveries in quantum and statistical mechanics and nuclear beta decay.
Fermi’s interest in the neutron began shortly after the first production of artificial radioactivity in late 1933 by Frédéric Joliot and his wife, Irène Curie. Irene’s mother, Marie Curie, had discovered radium and polonium with her husband, Pierre, at the turn of the century. That research formed the basis for the study of natural radioactivity, in which an atom—or, more accurately, its nucleus—spontaneously emits some sort of radiation (three types were known, designated with the Greek letters alpha, beta, and gamma) and in the process is often transformed into an atom of a different element—one of lower mass, if alpha particles were emitted. After a time period that varies greatly from one substance to the next, the daughter nucleus can throw off a particle of its own and decay further. This chain, known as a disintegration series, continues until a stable substance—one not capable of further spontaneous decay—is reached.
The Joliot-Curies reversed this process. Instead of measuring the emissions from a sample as it decayed into lighter elements, they shot alpha particles at a sample in hopes of creating a heavier element. Sure enough, bombardment of boron and aluminum yielded new radioactive forms of nitrogen and phosphorus, respectively. The discovery opened up many exciting new possibilities for studying radioactivity, which could now be created on demand with common elements instead of painstakingly probed with tiny amounts of rare ones.
Fermi, then thirty-two, wanted to see if he could induce radioactivity with the neutrons newly discovered by Chadwick rather than with alpha particles. The chief disadvantage of alpha particles is that they are positively charged, just like the nuclei they are meant to penetrate. Since like charges repel, an alpha particle has to be extremely energetic to do its job; this problem increases with increasing atomic number. Neutrons, by contrast, have no charge, so Fermi figured they would be much easier to get into a nucleus; and fortunately, his laboratory had developed a reliable neutron source.
In her affectionate memoir, Atoms in the Family , Fermi’s wife, Laura, recounts his efforts: “Being a man of method, he did not start by bombarding substances at random, but proceeded in order, starting from the lightest element, hydrogen, and following the periodic table of elements. Hydrogen gave no results: when he bombarded water with neutrons, nothing happened. He tried lithium next, but again without luck. He went on to beryllium, then to boron, to carbon, to nitrogen. None were activated. Enrico wavered, discouraged, and was on the point of giving up his researches, but his stubbornness made him refuse to yield. He would try one more element. That oxygen would not become radioactive he knew already, for his first bombardment had been on water. So he irradiated fluorine. Hurrah! He was rewarded. Fluorine was strongly activated, and so were the other elements that came after fluorine in the periodic table.” The date of Fermi’s breakthrough with fluorine was March 21, 1934. In a letter written the following month, Rutherford congratulated Fermi on his “successful escape from theoretical physics.”
Fermi and his colleagues exposed each element in turn to the neutrons and measured the type, energy, and half-life (the time needed for radioactivity to decrease by half) of radiation emitted. The team eventually bombarded sixty-three stable elements and produced thirty-seven new radioactive versions. To avoid stray radiation from the neutron source itself, they would carry the irradiated sample down a long corridor to Geiger counters at the far end. Many of the radioactivities induced by this neutron bombardment were short-lived, and the sources that were being used gave off only a weak stream of neutrons to begin with, so Fermi and his colleagues would race down the corridor to get to their counters before too much of the activity had dissipated. Fermi was highly competitive, and he enjoyed winning these inducedradioactivity footraces.
IN DOING EXPERIMENTS, THE Fermi team assumed at first that faster, more energetic neutrons were more likely to enter the target nucleus and create a reaction. In many cases, to their surprise, the opposite turned out to be true: Slow neutrons produced a greater reaction rate, just as a gently stroked putt in golf may have a better chance of sinking than a hard-hit one. The researchers discovered this important fact by accident. They noticed that much more radioactivity was induced when the neutron source was placed on a wooden table than when it was placed on a marble table. Wood, composed of lighter elements, had slowed the neutrons more efficiently than marble, which is largely silicon. This phenomenon was puzzling at first, but it led Fermi to try putting a wedge of paraffin in front of his neutron source. As soon as he did so, the induced radiation increased dramatically.
Among the elements Fermi bombarded with slow neutrons was uranium, at that time the heaviest known. He got some puzzling data that would suggest fission to a modern physicist, but he did not know how to interpret them. The German chemist Ida Noddack sent Fermi a paper showing how his results might indicate fission, but he found her theories too speculative. Instead, Fermi thought he had created a new, transuranic element, but he couldn’t prove it. In fact, Fermi had come very close to discovering fission in early 1935—as close as the thickness of a layer of aluminum foil.
Fermi had covered his uranium targets with foil before bombarding them with neutrons, to screen out the alpha particles naturally emitted by uranium. Without realizing it, he was also screening out the fission fragments from neutron bombardment. Segrè later wrote, “It is impossible to say whether we would have correctly interpreted the phenomenon had we observed it.” Still, it’s sobering to think what might have happened if fission had been discovered in Fascist Italy in the mid-1930s.
Though he missed discovering fission (as did the Joliot-Curies), Fermi did illuminate an entire class of reactions—those induced by slow neutrons. That accomplishment was recognized in 1938, when he was awarded the Nobel Prize in Physics “for his demonstrations of the existence of new radioactive elements produced by neutron irradiation, and for his related discovery of nuclear reactions brought about by slow neutrons.”
When Fermi went to Stockholm that fall to accept the prize, he took his wife, their two children, and a secret: He was leaving his native country for good. He had made arrangements to spend the next seven months at Columbia University, in New York City, and since he had been to America four times before, Italian authorities had no doubt that he would return when the seven months were over. But Mussolini’s regime had begun to copy Hitler’s racist decrees, and with a Jewish wife and no taste for dictators, Fermi knew he had to get out while he could.
His timing was auspicious, for even as the Franconia was bringing the Permis to America in December 1938, physicists were struggling to grasp yet another startling experimental result: In Germany Otto Hahn and Fritz Strassmann, inspired by Fermi’s 1934 work, had irradiated uranium with neutrons and found barium among the products.
Transforming one element into another with radiation was nothing new in itself, of course, but the products had always been within a step or two of the starting material on the periodic table, and very close in mass as well. Yet barium is nowhere near uranium in mass or atomic number. Implausible as it seemed, Hahn and Strassmann had managed to split the uranium atom into two large fragments, instead of merely chipping off a small piece.
Lise Meitner, a colleague of Hahn and Strassmann who had fled Nazi Germany in late 1938, explained the results in theoretical terms in a January 1939 paper written with her nephew Otto Frisch. They added a further insight that would turn out to have enormous implications: The mass of the fission fragments was slightly less than that of the starting materials. The “missing” mass had been converted into energy (mostly as kinetic energy of the fragments) during the fission process, according to Albert Einstein’s famous formula E = mc 2. Before 1939 was out, Meitner’s explanation and the coming of war would change nuclear fission from an exciting scientific development to a political and military challenge of the utmost urgency.
WORD OF THE DISCOV ery that had eluded him five years earlier reached Fermi shortly after his arrival in New York on January 2, 1939. Also awaiting him at Columbia was Leo Szilard, the eccentric genius who appears and reappears throughout the realization of atomic energy. (By coincidence Szilard had arrived in New York on the same ship exactly a year before Fermi.) As early as 1934, in a secret British patent, Szilard had raised the possibility of a nuclear chain reaction triggered by the recently discovered neutron. The process he envisioned, which involved beryllium decaying to helium, did not turn out to be practical; but now, with uranium fission a reality, he and Fermi would have the chance to put his vision into practice. Szilard had moved from his native Hungary to Berlin in 1920; in 1933, days after the Reichstag fire, he fled the Nazis to Vienna, after which his travels took him almost immediately to London and finally, in 1938, to America. Szilard disliked hands-on experimentation, but he was an innovative theorist. Equally important, he was bold and ingenious at getting support and materials for experiments.
The talents of both Fermi and Szilard would be needed, because 1939 was one of those giddy times when news of a truly radical discovery races through the world of science. Researchers in laboratories around the globe hurried to conduct experiments that would give direct evidence of fission. Fermi decided to try confirming the Hahn-Strassmann results at Columbia. A pair of his colleagues, Herbert Anderson and John Dunning, used slow neutrons to bombard a thin layer of uranium on the inside of an ionization chamber, this time without the protective layer of aluminum foil.
Sure enough, on January 25 Anderson and Dunning were rewarded with unmistakably large pulses on their oscilloscope screen, pulses that could arise only from heavy, slow-moving, but highly energetic fission fragments. As Meitner had predicted, a lot of energy was released in fission. Not only that, but as Fermi, Szilard, and others at Columbia soon learned, so were additional neutrons. The ingredients for the chain reaction Szilard had envisioned five years before were now at hand: A neutron could trigger an energy-producing reaction in which still more neutrons were released. It did not require a great leap of understanding to see what that might lead to. The possibility of chain reactions and nuclear explosives was very much in the air in 1939.
Though physicists and the press were abuzz with talk of such explosives, the U.S. military was much less enthusiastic. In March 1939 George B. Pegram, head of the physics department at Columbia, wrote to Adm. Stanford C. Hooper, technical assistant to the chief of naval operations, about Fermi’s research. He outlined “the possibility that uranium might be used as an explosive that would liberate a million times as much energy per pound as any known explosive” and recommended a meeting with Fermi. That meeting took place on March 17, but Fermi did not press the case very hard (nor had Pegram, whose letter went on to say, “My own feeling is that the probabilities are against this”). The Navy showed some interest in the phenomenon of fission but decided against funding a research program.
It’s a measure of Fermi’s lack of urgency in pursuing nuclear energy that he spent the summer of 1939 at the University of Michigan working on cosmic ray physics. As a sign of the sluggish response of Washington, consider Einstein’s famous letter of August 2, 1939, instigated by Szilard to alert President Roosevelt to the potential for atomic bombs. That letter, which was hand-delivered to Roosevelt on October 11 by his friend Alexander Sachs, an economist, also had a limited result—the establishment of the federal Advisory Committee on Uranium and a modest appropriation of $6,000—even though Britain and France were by then at war with Germany. In accordance with the languid pace of the times, the money did not reach Columbia until early in 1940.
NOT UNTIL OCTOBER 1941, after several successive reviews and the impetus of British work on feasibility, would the United States commit to exploring full-scale development of an atomic bomb. Three months later, after Pearl Harbor, came the goahead to start actual construction. Yet if Washington was slow to appreciate the new world opening around it, many physicists—particularly those who had emigrated in the face of Hitler’s totalitarian advance—understood that there was the potential for weapons of great power, and they feared that the Germans would get there first.
One vital task in beating the Germans was to create a controlled chain reaction. That would allow researchers to investigate the phenomenon and extract important data about the fission process. One constraint on Fermi’s plans for a chain reaction was the quality of uranium available. Uranium is not particularly rare—it is more common in the earth’s crust than silver—but before 1939 no one had put much effort into mining and processing it on a large scale. Fermi naturally would have preferred to use pure metal in his pile, but it was not available in nearly the quantities he would need.
The lamp division of Westinghouse could prepare small quantities of reasonably pure metal by means of a photochemical reaction of uranium oxide with potassium fluoride. Westinghouse’s light source for this process was simple: It exposed the reactants to sunshine on the roof of its building in Bloomfield, New Jersey. Metal Hydrides, Inc., of Beverly, Massachusetts, had a process that involved reacting uranium oxide with calcium hydride. Again, quantities were small, and the product was a fine powder that ignited on exposure to air. It would be well into 1942 before uraniummetal purity and productivity problems began to be licked.
Chemically purifying uranium for use in piles—difficult as it was—pales in comparison with the challenge of physically separating uranium isotopes. Uranium 235, which makes up less than 1 percent of natural uranium, undergoes fission; the other main isotope, uranium 238, does not, but it can be converted into plutonium, which also undergoes fission and can be used in a bomb. Not surprisingly, separating the two turned out to be extremely difficult and caused numerous headaches for the Manhattan Project’s leaders.
For Enrico Fermi in the spring of 1940, though, such considerations were remote. With no time to spare, he had to make do with what was available: uranium oxide. His goal was to build a device in which at least one of the neutrons born in each fission, on average, would reach another U-235 nucleus and cause another fission to occur (the others would be captured by U-238 and impurities, or would escape out the sides of the pile and be absorbed by the shielding). In other words, the reproduction factor, k , of the pile had to be greater than or equal to 1 for a self-sustaining chain reaction to be possible. (Research had shown that on average each fission event gives off about 2.5 neutrons, depending on the products; this gives an upper bound on k of 2.5.)
If k was significantly greater than 1, the reaction would quickly multiply out of control, since at normal energies a fission chain will undergo thousands of generations per second. Even with k very close to 1 and the propagating neutrons cooled way down, you would expect this very rapid multiplication to make a chain reaction balloon out of control in a flash.
Fortunately it turns out that a bit less than 1 percent of the neutrons released during fission are delayed, by times on the order of ten seconds. These neutrons are emitted in radioactive decay of certain fission products, rather than being “boiled off” in the initial fission itself. Thus for k between 1 and about 1.01, delayed neutrons govern the propagation, and the rise in neutron flux, while exponential, will be slow enough to control. These delayed neutrons are what allowed Fermi’s team to build its atomic pile without leveling several square blocks of Chicago in the process.
Fermi’s first step in designing an atomic pile was finding a way to slow down the neutrons. Neutrons emitted in fission are quite energetic, which makes them unlikely to cause another fission but distressingly likely to be captured by non-fissionable U-238. For a successful chain reaction, you need to embed pieces of uranium in some medium that can absorb excess energy from the fast neutrons emitted in fission.
After considering and rejecting various substances, Fermi and Szilard decided that carbon would make the best moderator among materials that were available in bulk. Like most light elements, it slows neutrons well by simple collision, yet unlike, say, hydrogen, it does not absorb many of them in the process. The carbon would be in the form of graphite. It had to be very pure graphite because even trace amounts of a strongly neutron-absorbing impurity such as boron would spoil its effectiveness. What was then considered high-purity graphite was far from satisfactory, so Szilard began discussions about improving the situation with the National Carbon Company, Monsanto, U.S. Graphite, and others. He also managed to borrow five hundred pounds of uranium oxide from the Eldorado Radium Corporation.
FERMI SET ABOUT HIS task with characteristic deliberateness. Even in 1941 he didn’t have nearly enough material for a self-sustaining chain reaction, so he used what he had. He took measurements and did calculations to tell him how much uranium and carbon he would need and how they might best be arranged.
Fermi’s team started by building stacks of graphite bricks measuring about eight feet on a side by eleven feet tall, with cans of uranium oxide spaced evenly in a lattice arrangement. (The experimenters hired members of Columbia’s football team to help stack the heavy graphite bricks.) These test “piles” were not big enough, nor were the materials pure enough, to support a self-supporting chain reaction, but they could point the way.
A radium-beryllium source at the base of the stack provided a modest but steady stream of neutrons. With no uranium in the pile, you would expect the flux of neutrons to decrease as you proceeded up the stack, from leakage out the sides. With uranium present, the falloff is more gradual, because neutrons are added from fission. The decrease with distance follows an exponential curve, so such stacks were called exponential piles.
Slots machined into the graphite bricks allowed the experimenters to insert pieces of rhodium foil into the piles. The neutrons induced radioactivity in the rhodium that could be measured with a Geiger counter. The greater the stream of neutrons, the more intense the radiation from the rhodium would be. The exponential falloff in the population of neutrons yielded a value for what the allimportant reproduction factor, k , would be in a full-scale pile.
Albert Wattenberg, now an emeritus professor of physics at the University of Illinois, was a graduate student on Fermi’s team at Columbia. He has written of how systematic Fermi was in taking his readings: “We made two measurements, then we ran a radioactive standard to check that the counters were reliable, then we measured without a foil or a standard in the Geiger counter. Fermi was always running checks on everything he did.”
FERMI’S EMPHASIS ON CARE ful measurements enabled the group to identify the numerous improvements that were necessary to reach its goal of k = 1. Szilard and others continued the quest for graphite and uranium with fewer neutron-absorbing impurities. Fermi and his team modified the spacing of the uranium oxide slugs to slow emitted neutrons the optimum amount. As 1941 wore on, Fermi’s group continued its gradual improvements and slow progress to bring k closer and closer to 1.
Things picked up after the Japanese bombed Pearl Harbor on December 7, 1941. The programs to develop an atomic bomb suddenly became much better funded, with direct access to President Roosevelt, as part of the Office of Scientific Research and Development (OSRD), under Vannevar Bush. The old Advisory Committee on Uranium was shifted to the OSRD, with Arthur Compton, a University of Chicago physicist who had led earlier independent reviews of America’s nuclear research, in charge of the chainreaction path to a bomb.
Compton reasoned that progress would be speeded if he concentrated the effort at one site instead of maintaining scattered pile-related projects at Columbia, Princeton, Chicago, and elsewhere. Work on chain reactions was to be centered at the University of Chicago, in what was code-named the Metallurgical Laboratory. Fermi and his team, who had achieved k = 0.918, reluctantly moved their exponential piles to Chicago in March 1942 and resumed their progress toward k = 1.
Thanks to Szilard’s hectoring, purer and purer graphite started coming in from American producers, who had learned to use purer coke as input and to increase the baking time and temperature to drive off impurities. In July a batch of very pure uranium oxide arrived and gave a value of k = 1.004; even purer uranium dioxide in August brought results as high as 1.014. No fewer than sixteen complete exponential piles were built, and hundreds of tests were conducted, between September 15 and November 15,1942. By the end of this period the team was getting values for k of 1.04 with uranium oxide and 1.07 with uranium metal.
Fermi’s calculations and the team’s experimental results made it increasingly clear that a complete chain-react- ing pile could be achieved with the amount of graphite and uranium then on hand. On November 16, in a squash court under the west grandstand at the university’s football stadium, work began on a full-scale pile, dubbed Chicago Pile One. The pile had the shape of a flattened ball twenty-five feet wide at its midpoint and twenty feet high. Stacked within it would be some four hundred tons of graphite, forty tons of uranium oxide, and six tons of uranium metal, which had recently become available from a group at Iowa State College.
Stacking—and measuring as they went—meant ceaseless effort for the project workers. But there was a war on, and everyone still feared that the Germans might be even farther along. Fermi organized two 12-hour shifts, a day shift under Walter Zinn and a night shift under Herbert Anderson, both of whom had been with him since the first fission experiments back at Columbia. They toiled around the clock; ninety hours was a typical workweek, Wattenberg remembers. They used woodworking equipment to saw and plane the graphite logs to size (about four inches square and several feet long), then stacked them in layers that formed squaredoff circles—small in area at first, then larger, to create the flattened sphere that would best use the available material. Wooden scaffolding supported the successive layers and made the growing pile look like a cube from the outside.
To eliminate every possible source of neutron absorption, they even made provision for removing the air from the pile. Anderson got the Goodyear Tire and Rubber Company to produce a balloon in the shape of a cube, twenty-five feet on each side. The pile was erected on the inside of one face of the balloon. The balloon eventually proved unnecessary, but it shows how far Fermi’s team was prepared to go to achieve criticality.
The uranium oxide had to be pressed in a hand-powered die to form, one at a time, some 22,000 egg-shaped slugs. These were fitted into holes drilled into the graphite, layers containing uranium alternating with graphite-only layers. Care had to be taken to align the graphite bricks that had slots milled out, to create channels for the cadmium rods that would control the neutron population. (Cadmium is a strong absorber of neutrons.) It was a sooty, slippery business; machining the graphite produced a fine dust that made researchers resemble coal miners.
WITH HIS ADMINISTRA tive duties increasing, Fermi no longer had time to participate in the construction itself, but he continued to be in control of the process. After every shift the minute but detectable neutron flux from the occasional spontaneous fission of uranium 235 was measured. In addition, indium foil was inserted and irradiated overnight to get an independent measure of the neutron production.
Every morning Fermi would discuss the results with Anderson and Zinn. Early in the fall Fermi had given weekly lectures to help everyone in the project share his exquisite understanding of the dynamics of the new machine. Now he applied that theory, day by day, to track the approach to a structure that could yield a self-sustaining chain reaction with k = 1.
On November 30 the team members stacked the fifty-second layer. Fermi’s latest calculations showed that the pile would be at k = 1 when the fifty-sixth layer was in place. As insurance, Fermi called for a fifty-seventh layer to be added by the night shift on December 1—with the control rods locked in place.
Thus it was that December 2, 1942, became the day on which Fermi would bring to criticality this roughly spherical structure—fifty-seven layers of graphite, half of them seeded with uranium, resting on wooden scaffolding and kept in control with cadmium rods.
With characteristic thoroughness, Fermi first checked his instruments and verified that the neutron intensity with the cadmium control rod in place was the same as had been measured the evening before. Fermi completely understood the behavior to be expected from the world’s first chain-reacting pile, but there was some uncertainty about a constant in the formulas that governed the approach to criticality. He decided to approach criticality with measurements at every step, to confirm the expected behavior.
George Weil was assigned the task of positioning the one remaining control rod. As an extra safety precaution, three men crouched on top of the pile with buckets of a cadmium-salt solution, ready to douse the reaction should it start to go out of control. Another man stood by with an ax, ready to chop the rope that held up a weighted cadmium safety rod in case the automatic rod failed. Fermi started with Weil’s control rod about halfway out. As expected, the neutron intensity increased with its removal. Fermi had two separate measures of where things stood: the neutron intensity at which the pile came to equilibrium and the rate at which it got to that point. Three successive times Fermi had Weil withdraw the control rod yet another six inches. Each time Fermi tracked the intensity; each time the rate of rise and the equilibrium intensity agreed with his on-the-spot slide-rule calculations.
At this point the neutron intensity had risen to the point where some of the instruments had to be adjusted. After a check that the new readings agreed with the old ones, Weil was told to remove the rod six more inches. The intensity increased obligingly, then CRASH! An automatic safety rod banged into its slot. The intensity had exceeded the point for which it was set.
THEN FERMI CALLED FOR lunch. Everything was going well, as predicted. All the rods were inserted fully and locked into place.
After lunch Fermi continued to proceed carefully. He had all but the manually controlled rod removed, and he checked the rate of rise and the neutron intensity. By now the intensities were high enough that a chart recorder had been substituted for the electronic counters. Fermi could see the further progress toward criticality in the curves of neutron intensity versus time following each successive withdrawal of the remaining control rod.
His slide rule told him just where criticality would be achieved. He had the safety rod reinserted to bring the intensity back to low levels; that would give him a longer growth interval to track. Fermi then told Weil to pull his control rod out another twelve inches. The safety rod was removed, and the intensity again began its slow climb. This time there was no leveling off. The intensity grew and grew, increasing exponentially, but at a slow, controlled rate—a sure sign that the pile had gone critical. Fermi put away his slide rule and broke into a big smile. “The reaction is self-sustaining,” he announced.
Eleven minutes later Fermi ordered the safety rod reinserted. The neutron intensity fell away rapidly. Eugene Wigner, the theorist and pile designer, had brought a bottle of Chianti with which to celebrate. Paper cups from a water cooler let all forty-nine people who were present share in the toast. Most of them signed the straw-encased bottle, which came in handy a few years later as the only record of who had been present on the historic day. One of the team, Hugh Barton, has kept his paper cup ever since. Everyone knew they had made history.
The maximum power reached in that first pile was only half a watt. In the months that followed, the physicists and chemists of the Chicago Met Lab collaborated with engineers of the Du Pont Corporation to build giant reactors with a million times that power at Hanford, Washington. From the Hanford plant came plutonium 239 in sufficient quantity to make the very first atomic bomb tested in New Mexico and the one exploded over Nagasaki. (The Hiroshima bomb used U-235.)
WHAT MIGHT ADMIRAL Hooper have thought in the years that followed, as nuclear reactors became the prime source of power for the propulsion of submarines and large aircraft carriers? At Chicago and at the new Argonne Laboratory nearby, Fermi’s team members learned what an exquisite tool they had in their pile and its successors. It could be controlled with ease and, as Herbert Anderson put it, “its sensitivity for neutron absorption and production was beyond the wildest dreams of those who had struggled so hard to make such measurements before.” A neutron flux could now be amplified or reduced enormously at will. In the years since, the neutron has proved itself to be invaluable for understanding the properties of matter, and radioisotopes from nuclear reactors have been an irreplaceable research resource and a superb diagnostic tool for physicians.
With his work on the atomic pile and its subsequent evolution into the massive production reactors at Hanford behind him, Fermi became a consultant to the new bomb laboratory in Los Alamos, New Mexico. The Permis moved there in September 1944, and he was made head of a special group called the F (for Fermi) Division, suited to his manifold talents. It was nicknamed the Problem Division because it solved the problems that stumped other divisions.
Fermi was at Alamogordo for the July 16, 1945, test of the first atomic bomb. With characteristic directness, he measured its explosive force in a very simple way by dropping scraps of paper. When the shock wave hit, it propelled some of the scraps several feet ahead. Fermi had—as was his custom—calculated the range of results in advance. His result from this “scrap meter” was impressively close to the yield that came from the very extensive blast instrumentation.
Fermi returned to Chicago after the war and established a school of physics at the University of Chicago that has produced several Nobel Prize winners. He died of cancer in 1954. Nearly forty years later, at a symposium commemorating the fiftieth anniversary of Chicago Pile One, colleagues and students recollected the Enrico Fermi they had known: “a kind and brilliant friend,” “enjoyed so much the act of teaching,” “absolute integrity.” One co-worker may have summed up the Fermi approach best of all: “Having done his own work between 4 and 8 A.M. , he was fully prepared to spend the day solving everyone else’s problems.”