The Pope of Physics

by Gino Segrè

If all other safety measures failed, there were three buckets of cadmium sulfate, held by a safety squad, with orders to splash the pile if it went out of control. The cadmium sulfate would absorb neutrons, aborting the experiment. It would also effectively destroy the costly black egg.

  Midmorning, after checking out the equipment with other team members, Fermi was ready to commence the trial. Respective roles had been carefully delineated. Fermi, the director, was giving orders from the balcony above. George Weil, another Columbia physicist who had come to Chicago with Fermi, was entrusted to be the executor of orders, manipulating the rods on the court below. Woods, the only female and the youngest person there, was in charge of calling out the counts. The role of the spectators was easy: keep quiet and continue breathing.

  All the control rods, other than the one guided by Weil, were pulled out. After the safety measures were verified as functioning properly, Fermi told Weil to pull the last one halfway out. The neutron count, read off by Woods, rose quickly and then leveled off precisely as Fermi had predicted. Her voice resounded strongly over the male assemblage. Volney Wilson, with Fermi on the balcony, checked the counters once again and set them to record a higher rate.

  Confident that everything was working as anticipated, Fermi directed Weil to continue pulling the last rod out, six inches at a time. Fermi checked the neutron count after each extraction to ensure it matched his calculations. Tension was mounting among the spectators after the third six-inch outward slide of the rod because they knew the pile was close to the point where a critical reaction would set in.

  At 11:30 a.m. everybody jumped as a sudden boom shook the room. The Zip control rod had fallen in, effectively turning the pile off. It turned out they had simply been too cautious in setting the radiation count value at which Zip would drop. Nerves were on edge. Fermi then said, “I’m hungry. Let’s go to lunch.” He sensed his team needed a break. Food was always good for nerves. All the rods were put back into place and locked in by Zinn and Anderson.

  After lunch they gathered again at the squash court, now joined by Compton, who had not been there in the morning. He was meeting with DuPont personnel and an external review committee appointed by General Groves. The committee had been formed to assuage DuPont’s understandable doubts about agreeing to build a plutonium production enterprise based on a process not yet achieved in a laboratory. It was indicative of Groves’s faith in Fermi that he proceeded with that plan before knowing that CP-1 would reach criticality.

  When Compton was informed that Fermi would be conducting the momentous trial in the afternoon, he decided the review committee would have to do without him. Although by then the balcony had limited space, Compton brought one member of the DuPont staff, Crawford Greenewalt, a dynamic forty-year-old chemical engineer who led the DuPont contingent that had come to Chicago.

  The experiment resumed at two in the afternoon. Fifty people would later lay claim to having been at the CP-1 experiment but there were only around forty at the moment when the pile went critical. Each of those present was aware of the dangers of the trial but each wished to bear witness. No one was running for the hills as Fermi had previously jested.

  Fermi again instructed that all the control rods but one be removed from the pile. After checking that the neutron radiation count was exactly the same as it had been earlier, he directed Weil to gradually slide out the last one, first to the halfway point, and then more slowly beyond that.

  Anderson, by Fermi’s side as the order was given to continue removing the rod, remembered what it had been like: “Again and again, the scale of the recorder had to be changed to accommodate the neutron intensity increasing more and more rapidly. Suddenly Fermi raised his hand and then announced, ‘The pile has gone critical.’”

  Anxiety grew as Fermi continued to let the pile run, the neutron count growing steadily. In his usual manner, Fermi remained calm. One minute passed, then a second and a third. After little more than a fourth minute, with the tension in the room becoming almost unbearable, Fermi ordered, “Zip in!” It was 3:53 in the afternoon. The control rod was immediately lowered. The intensity of the neutrons dropped as quickly as the stress in the room. Everyone resumed breathing.

  Compton described Fermi at the crucial moment as “alert, in as full control of his experimental crew as is the captain of a ship engaged in critical action. At this moment of great achievement his face showed no signs of elation. The experiment had worked precisely as expected. Cool, collected, Fermi’s mind was not dwelling on the significance of what had just been done. He was laying his plans for the next urgent stage of the work.”

  Bigger piles would have to be built. They would run for days or months, not minutes. Their products would need to be extracted. What had been accomplished today was only a beginning. It was a historic turning point that marked the birth of the atomic age. But there could be no letup.

  Compton and Greenewalt were among the first to leave the squash court after the pile had gone critical. Greenewalt hurried back to the review committee to give them the news. Compton remembered the expression on his face: “His eyes were aglow. He had seen a miracle … his mind was swarming with ideas of how atomic energy could mean great things in the practical life of men and women. As an industrial engineer, war at this moment was far from his mind. Here was a source of endless power that could warm people’s homes, light their lamps and turn the wheels of industry.”

  Other spectators who had witnessed what had just taken place had different thoughts. After the triumphant demonstration of a first chain reaction, wild cheers did not break out. The event was exciting but also sobering. It did not take long for those present to start asking what this might mean for the war effort. If they had succeeded in obtaining a self-sustaining critical reaction, wasn’t it probable that the Third Reich had been able to do so as well? And what might the Germans do next—or what had they already done? There would be no time to pause. Work toward a bomb would have to proceed with even greater resolve. Fermi knew this, and so did everybody else who was there in the squash court.

  It was up to Compton to relay the news to Washington. He stopped by his office after leaving the squash court and placed a phone call to James Conant. He remembered their exchange, couched cryptically in case it was being overheard. Compton began:

  “Jim,” I said, “you’ll be interested to know that the Italian navigator has just landed in the new world.”

  Then half apologetically, because I had led the S-1 Committee to believe that it would be another week or more before the pile could be completed, I added, “the earth was not as large as he had estimated and he arrived at the new world sooner than he had expected.”

  Conant’s excited response was “Is that so, were the natives friendly?” I answered, “Everyone landed safe and happy.”

  As darkness began to descend on Chicago that December afternoon, those in the squash court slowly drifted out. Finally, just as the last twenty or so were beginning to take off their gray lab coats, Wigner produced a straw-covered fiasco of Chianti and some paper cups that he had stored in a bag on the squash court floor. Fermi uncorked it and poured a sip for each of those present. Someone asked Fermi to sign the bottle’s covering and it was then passed around for the others to sign.

  Zinn, who had been the first to greet Fermi at the pile in the morning, was the last physicist to leave in the evening. He had wanted to double-check all the apparatus and make sure once again that all the rods were firmly locked in place. When he finally filed out, one of the guards stationed outside asked him, “What’s going on, Doctor, something happen in there?”

  Something had indeed happened. None of those in the squash court that afternoon forgot being there when the pile went critical. It had only generated a maximum power of half a watt, scarcely enough to light a flashlight battery. However, if that rate had been allowed to grow unchecked, it would have killed everyone in the squash court and wreaked havoc on the city of Chicago.

  PART 4


  THE ATOMIC CITY

  28

  THE MANHATTAN PROJECT: A THREE-LEGGED STOOL

  The first official, albeit secret, description of what happened on the Chicago squash court was straightforward. Written by Fermi with no rhetorical flourishes, it appeared in the monthly Met Lab report, an internal document circulated only among those who already knew they were part of history. Fermi stated: “The chain reacting structure has been completed on December 2 and has been in operation since then in a satisfactory way.”

  Twelve years later, on the day of Fermi’s death, a famous American broadcaster adopted a profoundly different tone in describing the stupendous event. On the CBS evening news, Edward R. Murrow reported: “The story of the lighting of the first atomic furnace will be told as long as stories can be listened to, for it was certainly one of the most dramatic moments in the unfolding of human knowledge.” Murrow then commented, “It was the good fortune of this country that Dr. Fermi found asylum in 1938. Under the present immigration laws, he might not be admissible … His exclusion would be shared by other immigrant founding fathers of the atomic age.” With trademark astuteness, Murrow summarized Fermi’s brilliance and the conflicts of the times.

  For Laura Fermi, the day of December 2, 1942, was much like other days except that Enrico returned from work a little earlier than usual. She thought he had simply done so to help prepare for a large party for his co-workers. Laura’s schedule had been thrown off by six-year-old Giulio, caught in a mischievous infraction. While his father was bombarding uranium with neutrons on a squash court, his son had been bombarding the neighbor’s sparkling windows with dirty snowballs. Enrico walked in to the tail end of a prolonged scolding of Giulio by Laura and the child’s ensuing remorse. Probably still engrossed with the criticality of the pile, he shrugged off his son’s misbehavior. After dinner, Laura and Enrico quickly organized things by moving chairs, choosing records for dancing, and lighting some candles. With the winter solstice fast approaching, the Fermis wanted the festivities to be cozy and welcoming. A glass of wine or red vermouth would help.

  The timing of the long-scheduled party was completely coincidental with the day’s discovery. Shortly after eight the first guests, Walter Zinn and his wife, Jean, arrived. Laura was surprised to see Zinn, with a big smile on his face, shake Enrico’s hand and say, “Congratulations.” Laura did not have a whiff of anything exceptional happening that day. Secrecy shrouded Enrico’s current pursuits and therefore conversations about it were prohibited. But still, her husband had given no indication of anything of note having taken place, had not seemed the least bit nervous the night before, and acted as though tomorrow would offer nothing out of the ordinary.

  Laura’s curiosity grew as the pattern of congratulations repeated itself time after time with other guests. Thinking that Leona Woods was the most likely person to tell her, Laura asked her what Enrico had done to deserve so many warm kudos. Because of security rules, Woods was at a loss for words. Flustered, and perhaps subconsciously thinking the pile might soon be used in retaliation for Pearl Harbor, she blurted out, “He sank a Japanese admiral.” Laura looked askance but Herbert Anderson, Fermi’s right-hand man since Columbia days, came to the rescue, saying Laura must know that “anything was possible for Enrico.” Laura dismissed it as some sort of private joke and went back to entertaining guests. The mood among the partygoers was celebratory, but tempered by the magnitude and meaning of what had happened on the squash court earlier that day.

  The next morning, Fermi slept a little longer than usual. Normally he was up at 4 a.m., which Fermi regarded as his form of insomnia, working at his desk at home for a few hours before heading to the lab. His routines seldom varied and colleagues would joke that they could set their watches according to Fermi’s comings and goings. He was in a very good mood that Thursday. No longer having to focus on whether or not the pile would work, he could apply its results to exciting new physics experiments. As Anderson wrote, “What thrilled Fermi most about the chain reacting pile was not so much its obvious promise for atomic energy and atomic bombs which many others were now prepared to pursue, but an entirely new and unsuspected feature. It was a marvelous experimental tool.” Obtaining greater intensity in neutron sources had been a continual challenge for Fermi since 1934 and now all he had to do was move a control rod in or out a little bit. With a simple gesture he could increase the neutron flux by a factor of a million. It was like waving a magic wand. Years later Fermi commented, “To operate a pile is just as easy as to keep a car running on a straight road.”

  By mid-February, there was an improved version of the experimental tool. CP-1, the first pile, was taken apart and rebuilt in the Argonne Forest, the site originally designated for assembling the pile. Erecting it in the squash court had been a matter of expediency but now that it had proven itself, a more permanent pile was warranted. The foremost change in design was the erection of a concrete shield surrounding it, allowing the pile to be operated at a much higher power without exposing observers to radiation. CP-2 typically ran at 100 kilowatts, more than 100,000 times as great a power as CP-1. This time it was not physicists building it, frantically hauling graphite blocks, but the firm of Stone & Webster, which had come back into government’s good graces after their initial debacle.

  Testing graphite and uranium samples for purity, once such an effort, was now made easy. Finding the best pile configuration went smoothly because one wall of the shield was movable, allowing access to the pile’s interior. Fermi delighted in going back to basic research, “the work he enjoyed most,” according to Anderson. Conditions at Argonne were pleasant: a quiet forest to walk in and enthusiastic young colleagues surrounding him. But the hiatus was brief for Fermi. The unrelenting pressure of the war effort could not be ignored, no matter how fascinating were the results of his research.

  Even before the success of the pile, General Groves had been moving at lightning speed to prepare for the further development of atomic weaponry. His track record of taking on giant projects and moving them aggressively reached new heights. With the implicit backing of President Roosevelt, who had signed a secret enabling document on May 12, 1942, Groves led what would first be called the Development of Substitute Materials Program. But the title was considered too revealing, so it became the Manhattan Engineer District, with headquarters located at 270 Broadway, eighteenth floor, in New York City. In turn, it became known as the Manhattan Project, evolving into a vast research and development empire of more than thirty sites spread across the United States. Groves left no one in doubt as to who was in charge.

  Within a few months’ time, the Manhattan Project had bought a total of more than 140,000 acres for three atomic communities devoted to conducting secret research. Site X was near Clinton, Tennessee. Groves had approved it within days of his appointment and it was purchased on September 29, 1942. Site Y, Los Alamos, New Mexico, was next, purchased on November 25. And Site W, near Hanford, Washington, was last, on February 9, 1943.

  Each site, X, Y, and W, had a distinct purpose but a shared mission: to develop a nuclear bomb. And Fermi would leave his fingerprints on all of them, to varying degrees. He was least involved in Site X, Oak Ridge, although the design of its large nuclear reactor resulted directly from Fermi’s successful nuclear pile. At Site W, Hanford, Fermi’s involvement was key in solving how an adequate amount of plutonium could be produced to make a bomb. At Site Y, Los Alamos, Fermi was a valued member of the team that put all the pieces together. He was the person to consult about almost any physics question. He tended to either answer it or to frame it so a route could be found to a solution.

  In his scientific career, Fermi had stood out—had in fact, shone—with his unique contributions. When he became involved with the Manhattan Project, he was one in a community of fellow geniuses with whom he worked side by side. Before, Fermi had made history with his discoveries. Now, with the Manhattan Project, he was caught in the sweep of history.

  Groves made certai
n the Manhattan Project was given the highest priority rating, AAA, reserved by the War Production Board for “emergencies.” Arguing that time, not money was crucial, he bulldozed his way through the bureaucracy. When the civilian head of the board hesitated in granting the request, Groves threatened to bring the issue directly to the president. The AAA rating was promptly bestowed.

  Both Sites X and W needed to be proximal to abundant water to generate electric power for running their respective plants and for the cooling reactors. Oak Ridge was aided by the presence of the Tennessee Valley Authority (TVA) and the nearby Clinch River. Groves immediately set to building a reactor bigger than Argonne’s CP-2. Fermi and his group guided the process.

  The most formidable task at Site X was separating U-235 from the much more plentiful U-238 ore. Its reactor was scheduled to be up and running by the end of 1943 and was expected to produce enough plutonium for further studies and applications. But the large amounts of plutonium needed for a bomb would have to be produced at a separate location. Site X was uncomfortably close to Knoxville, Tennessee. If there was a disaster, lives would be endangered and it would compromise Oak Ridge’s uranium ore plants. The two avenues toward building a bomb, U-235 and plutonium, had to be kept clear of one another.

  The remoteness of Site W, Hanford, allowed space for three nuclear reactors, ten miles apart, and three chemical separation plants for extracting plutonium from the irradiated uranium. It was an enormous industrial enterprise. While CP-1 had run at half a watt and CP-2 at 100,000, the Hanford reactors began functioning at 250,000 watts and ramped up from there to 100 million watts. Each one of them was cooled by 30,000 gallons of cold Columbia River water pumped through the reactor every minute. Mesh on water intake pipes ensured that none of the precious Columbia River salmon were sucked up. A professional fish scientist was hired to monitor the health of the fish. The water was then stored in large holding basins nicknamed Queen Marys, after the ocean liner. Once radioactivity had diminished to levels the scientists regarded as safe, the water was returned to the river.