The Pope of Physics

by Gino Segrè

  The talks even had a certain amount of wry humor. Leona Woods, who was taking notes for later use, edited out some of the American slang as well as some of the analogies Fermi used. But she did leave in Fermi’s comment on the key neutron reproduction factor k. If it was less than one, the reaction would not be self-sustaining. If it became much, much greater than one, Fermi’s advice, spoken with a broad smile on his face, was to “run quick-like behind a big hill many miles away.”

  The CP-1 members began their secret work over the summer and into the early fall. When the members broke for lunch, they often headed to the university’s beautiful Main Quadrangle to discuss technical issues of the project. It gave them a chance to breathe fresh air. The Quad, framed by handsome Gothic buildings, was a large open area, crisscrossed by paths among majestic honey locust trees. With students busily rushing to and from classes, the Met team was sure that no one took notice of them and that security measures were not being breached. They spoke to one another softly, but intensely.

  The task was sufficiently outrageous that the joke among participating scientists reportedly was “If people could see what we’re doing with a million and a half of their dollars, they’d think we are crazy. If they knew why we are doing it, they’d know we are.” Those involved were aware of the project’s promise as well as its dangers, particularly the hazards of radiation tragically experienced in the past by other scientists, most famously by Marie Curie. The Met Lab included biophysicists and radiologists who focused on the physiological effects of ionizing radiations, there for research and for safety advice.

  The inevitable first step in building a pile, whether small or large, was to prepare the graphite bricks that would serve as neutron moderators for the uranium pellets embedded in them. The key to the success of the effort was the quantity as well as the quality of uranium and graphite, making both the accumulation and the purity of the materials major factors. As Anderson and Walter Zinn—another physicist who had transferred with Fermi from Columbia—proceeded with their experiments under Fermi’s supervision, they were constantly coping with these issues. In regard to uranium, they got a major boost from another Met team member, the chemist Frank Spedding, who enlisted his colleagues at Iowa State University to produce pure samples.

  The process of building the bricks was laborious and slow. Zinn took charge of the incoming graphite, cutting the bars to the standard size needed for CP-1, then smoothing them and finally drilling two rounded holes in the 25 percent of them that would hold five-pound uranium lumps. Each brick weighed close to twenty pounds. Zinn and his crew cut and smoothed 45,000 graphite bricks and drilled 19,000 holes with shaped bottoms, each having a 3¼" diameter. It was an amazing feat of ingenuity and tenacity.

  The crew that worked on the 45,000 bricks was a motley one. In addition to half a dozen young physicists and a carpenter, there were approximately thirty local boys hired from a Chicago neighborhood known as Back of the Yards, situated behind the legendary stock and meat packing yards. The impoverished area had been immortalized in Upton Sinclair’s book The Jungle (1906); at the time of CP-1, the neighborhood was predominantly Slavic, with a large population of Poles. It became a fertile CP-1 recruiting ground for finding strong kids ready for heavy-duty lifting and drilling. One of the young physicists remembered, “These tough kids used for the production work had quit school to earn money while waiting to be drafted. These kids created a real set of challenges as some of them had a negative commitment to work.” Despite this “negative commitment,” the work progressed.

  Many of the scientists working on the pile had already experienced being covered with graphite dust while at Columbia. They had gone home, as Fermi put it, “looking like coal miners,” but that was nothing compared to the situation they experienced in Chicago. Al Wattenberg, a Columbia graduate student of Fermi’s, described the omnipresent dust as a great equalizer:

  The graphite machining produced graphite dust all over the place. We breathed it, slipped on it, and it oozed out of our pores even after we washed and showered. Everyone dressed for this work in coveralls and a young professor could not be distinguished from the Back of the Yards kids.

  Worried about what the work force was breathing in, Met Lab’s medically trained scientists urged them to wear gas masks. The suggestion was declined with particular vehemence by the prevalence of habitual smokers. While the medical experts were defeated on this front, there were still concerns about pervasive uranium oxide dust. As a precautionary gesture, a cage of mice was placed in the area where the pile was being assembled. In this novel version of a canary in a coal mine, researchers were reassured to see that the mice seemed to be surviving nicely.

  Since the end of the summer of 1942, Fermi had been sure the pile would work. It was just a question of ensuring the materials were pure enough and the engineering was done right. That confidence spread to those around him. As Anderson said, describing his relation to Fermi at the time, “It was a privilege and a thrilling experience to be associated with him in those days.” By the beginning of November, Fermi was convinced they would very soon have enough high-quality graphite and uranium for the grand experiment to take place. The final pile was ready to be built. But in the meantime workers at Stone & Webster had gone on strike. The building designated to hold the final pile was not ready.

  This was not the first time there had been a conflict between the physicists and Stone & Webster. A briefing earlier in the fall by the company’s engineers left the assembled Met Lab staff shocked by the ignorance they displayed. Afterward, Volney Wilson, a young physicist in charge of pile instrumentation, had called a lab meeting to protest the company’s involvement in the venture. What followed bordered on the surreal. Compton was in charge of the meeting, and this deeply religious son of a minister began by reading a passage from the King James Bible, Judges 7:5–7, in which the Lord directs Gideon to select good men to fight the Midianites by observing how they drink water. Then Compton sat down. A long silence followed, during which the gathering pondered how this could possibly be relevant. No one could figure it out. After a while, Wilson and others again spoke about Stone & Webster’s perceived incompetence. Their discomfort was warranted.

  But what could the Met Lab do now? There was no telling when the strike would be over, and who else could construct housing for the pile in the Argonne Forest? Forty-five thousand graphite bricks could be readied and significant amounts of uranium collected by the time to build the pile. The scientists could complete their tasks in this regard. But the pile would still need a place where it could be housed. With the lack of a building, the pile was homeless.

  Fermi approached Compton with an unusual proposal: the physicists would build the pile themselves. They would do so right on the campus and they would have it completed no later than early December. Preparations for the experiment had been carried out in the squash courts under the stands of Stagg Field, the University of Chicago’s football field. Fermi suggested the big pile be placed inside the largest of those courts, one used for doubles matches. Having redone his calculations and taking into account the purity of the materials they were receiving, he had concluded there would be enough room there. The site met the demands for space and easy access and was relatively secluded, the university having abandoned college football and the stadium a few years earlier.

  But building the pile in the heart of the campus, only a few miles from downtown Chicago, posed special risks. What if it couldn’t be turned off after it went critical? The ensuing meltdown would spew radioactive material into a major metropolitan area, not to speak of a campus population and the surrounding Hyde Park neighborhood. Fermi assured Compton and Groves this doomsday scenario wouldn’t happen. Was there a chance he was wrong? There was great faith in Fermi, but Compton—known as a cautious man—had to ask himself if there was even a remote possibility of disaster.

  Compton was also forced to anguish over a difficult question: should he or should he not notify Robert Hutchi
ns, the charismatic and innovative forty-three-year-old president of the University of Chicago? After all, CP-1’s existence at the university had been decided in good faith. Compton and Fermi had met with Hutchins to ask permission for this project vital to the war effort. Hutchins had readily understood and assured them of complete cooperation.

  Despite their mutual respect, Compton finally decided that asking Hutchins would be tantamount to killing the project. Hutchins, a former professor and then dean of the Yale Law School, would almost certainly deny permission on legal as well as safety grounds. Compton concluded that the experiment was crucial. He could not risk the prospect of having the university’s president forbid it. But if something went askew, Compton was aware of who would be held responsible. His name would go down in ignominy.

  The decision to build the pile in the squash court was made on Saturday, November 14; construction began the following Monday. Work proceeded continuously in twelve-hour shifts, nothing new for the physicists since they had already been working ninety-hour weeks for more than a month.

  Zinn headed the day shift and Anderson the night one. The first thing they did was to set down in the squash court a giant cloth balloon designed to enfold the pile. Because nitrogen absorbs neutrons, the cloth had been ordered in case it became necessary to evacuate air from the structure. This extra precaution turned out not to be needed, but the group was trying to anticipate all contingencies. Ordering it had been Anderson’s idea. He admitted to receiving some curious stares from the Goodyear Company when he had done so. Experienced in building balloons, they had wondered why somebody wanted one shaped like a cube. But during a war one didn’t ask too many questions about something the army was ordering.

  They started to build. The squash court’s interior quickly became a beehive of nonstop activity. The Back of the Yards boys who had helped shape the graphite bricks now were asked to move them. After a little over two weeks, wood scaffolding to hold the pile was in place and a portable elevator had been installed to lift the bricks and place them at designated levels.

  There were no blueprints for assembling a pile or, as Woods jokingly remarked, no blackprints, a reference to the pervasive graphite dust. Fermi’s technical ingenuity and improvisational skills were fully employed. The construction depended on the radiation counts they measured as they proceeded and these in turn depended on the purity of the materials. Some of the graphite was higher quality and some lower. Some of the uranium was metal, the preferred form, and some was pressed uranium oxide powder. A record was kept of each brick’s location and its fabrication in case later adjustments were needed. The shape of the pile was not predetermined. Fermi had originally thought a sphere would be best. Ultimately it was egg-shaped, lying on its side, twenty-five feet across at its widest and twenty feet high, all firmly ensconced in scaffolding.

  Anderson and Zinn met every day with Fermi, at which time the three of them would examine what was available and decide the optimal location for the next layers of bricks. Slots were carefully lined up so that the all-important control rods could be inserted. These thirteen-foot rods were made by nailing a sheet of cadmium to a strip of wood. Cadmium was the most potent neutron absorber available.

  As material was being delivered to build the pile, negotiations were going on with the giant DuPont chemical company for the production of plutonium on an industrial scale. The company would be in charge of operating a vast enterprise at a site yet to be selected. The project, eventually built in Hanford, Washington, was so crucial that one couldn’t wait for the results of CP-1; the experiment’s success was assumed. Time was of the essence. On the eighteenth of November, DuPont expressed their willingness to take on the job and selected a team of their scientists and managers to visit the Met Lab for a detailed briefing. A date of December 2 was set for the meeting.

  In the meantime, DuPont asked for a series of reports to explain the nature and scope of the project: these had to be written in language their managers could understand. Fermi wrote one about the feasibility of a chain reaction. In it he addressed three main questions: “Will the reaction be self-sustaining? Will the reaction be thermally stable? Will the reaction be controllable?” His answers were “yes,” “probably yes,” and “yes.” In each case he explained them in simple terms. This was enough for DuPont.

  By the twentieth of November, fifteen layers of the pile had been laid. From now on its neutron activity was measured each day at a central point of the pile with all the control rods pulled out: appropriate precautions for quickly reinserting them were always taken. The rods were held in place at other times with a simple hasp and padlock mechanism. Only Anderson and Zinn had keys to the locks.

  Fermi had originally thought more than seventy layers might be needed, but the pile was working better than expected; it seemed that fewer than sixty would be sufficient. At the end of November, Fermi predicted that the pile would go critical once the fifty-seventh layer had been placed and the control rods taken out. One control rod was operated automatically, another one would be regulated manually from the squash court floor at Fermi’s command, and a third was connected to the balcony at one end of the squash court.

  That fifty-seventh layer was placed during the night of December 1. After taking a neutron count with all the rods but one removed, Anderson saw that Fermi had been right. He ordered all the rods reinserted, put the locks on, and went off to get some sleep.

  The pile had a crude appearance, consisting of a stack of black bricks and wooden timbers. This was a primitive precursor to sleek modern-day nuclear reactors built far from urban centers and having extensive radiation shielding, elaborate cooling systems, and internal control rooms.

  It had been an astonishing effort. Because the squash courts were unheated, it had meant working around the clock in freezing temperatures. A small crew had moved almost a million pounds of graphite bricks. The bricks, with holes drilled into them, held almost a hundred thousand pounds of uranium. Running through the pile, the rods were its only moving parts.

  The most unbelievable feature was that all this had been accomplished in fifteen days, a miracle of planning and cooperation. It was a sterling example of how academia, government, and industry could work effectively together and even keep things secret in the process. There were no leaks of any kind, neither of information nor of radiation. It was inspiring how Americans and recent emigrants from fascist countries cooperated. The top secret project in the United States owed much to these refugees. And notably, Fermi’s classification as an enemy alien had been lifted less than two months earlier than December 2, 1942.

  27

  THE DAY THE ATOMIC AGE WAS BORN

  Fermi’s steady hand did not waver throughout the construction process of the pile, his colleagues agreeing that he seemed completely self-confident. As leader of the team, he threw himself into every phase of the preparations, never pulling rank or displaying a modicum of conceit. His precision, down to forecasting exactly when the last brick had to be placed, was a source of wonder.

  On the night of December 1, Fermi slept well, unperturbed by doubts: he was certain that the following day would be successful. Early the next morning Fermi made his way to the court, stopping to pick up Leona Woods, who lived nearby. They trudged together slowly through the snow in the subzero weather. Chicago, the Windy City, lived up to its reputation as raw winds blew off Lake Michigan. They walked in silence, the freezing air stilling their voices. Thoughts of the day ahead preoccupied them.

  At the squash court they met Walter Zinn; they checked with him that all the equipment was in place. And then a still sleepy Herbert Anderson appeared. He had had only a few hours of rest after coming off the night shift. The pile, carefully minded around the clock by either Zinn or Anderson, was never left alone. Anderson was hungry, so Fermi and he went with Woods for a quick breakfast at the apartment she shared with her sister. She hurriedly made pancakes, and then the three returned for final preparations.

  The squash cou
rt had a balcony at one end, originally intended for spectators to look down at an ongoing game. The December 2 event was no game. The view from the balcony that morning was not of players moving nimbly about but of a large immobile black egg-shaped object encased in a nest of wooden timbers. Scientists, Fermi foremost among them, would replace the spectators on the balcony. On the court, a trusted lone physicist tended the egg.

  The balcony held various recording devices for monitoring the neutron count, their scales adjusted to enable continuous recordings of the count rates. Underlying the setup was the assumption that the pile would go critical. In anticipation, the safety mechanisms had been installed. The thirteen-foot control rods that absorbed neutrons were all in place, ready to be pushed in or pulled out of the pile. Zinn had devised the special weighted one poised above the pile. Given the name of Zip, it was held there by an electromagnet connected to an ionization counter measuring neutron activity. If the activity level passed a certain preset safety value, Zip would be automatically released and descend into the pile.

  The control rod tied with a rope to the balcony had an extra and unusual safety feature. There stood Compton’s associate director, Norman Hilbery, with an ax in hand. If all else failed, he would chop the rope. CP-1 technology ran the range from the most sophisticated to the most primitive.