(from Richard Rhodes,
The Making of the Atomic Bomb)
HITLER’S RISE to power forced many important scientists to flee to American shores during the 1930s. Among them were Albert Einstein; John von Neumann, who conceived of using zeroes and ones as the language for computers; and physicists Niels Bohr, Leo Szilard, Edward Teller, and Eugene Winger.
Italian physicist Enrico Fermi won the Nobel Prize in 1938; after receiving the prize in Sweden, he never returned to Italy, which was then under Mussolini’s fascist dictatorship. Concerned for the safety of his Jewish wife, he fled to the United States, where he took a faculty position at Columbia University and later at the University of Chicago. Fermi possessed an extraordinarily creative analytical mind. He told his physics students that if you conducted an experiment, and the results match an existing theory, then you have made a measurement. On the other hand, if the results do not conform to an existing theory, then you have made a discovery. And discover he did. Oscillating between theoretical andexperimental physics, Fermi proved man can control nuclear fission for power plants and release it in the atomic bomb.
At Princeton, Szilard and Winger conferred with fellow émigré Albert Einstein about the import of current experiments at Columbia. In August 1939 the three prepared a letter addressed to President Franklin D. Roosevelt. The letter read, in part, “Some recent work by E. Fermi and L. Szilard, which has been communicated to me by manuscript, leads me to
expect that the element uranium may be turned into a new and important source of energy in the immediate future. This new phenomenon would also lead to the construction of bombs, and it is conceivable—though much less certain—that extremely powerful bombs of a new type may thus be constructed.” From this letter sprang the Manhattan Project to develop the atomic bomb, which culminated in the destruction of Hiroshima and Nagasaki, the surrender of Japan, and the end of the war in the Pacific.
Fermi explained his concept, which built on the German work of Otto Hahn, Fritz Strassman, and Lise Meitner, this way: “It takes one neutron to split one atom of uranium. We must first produce then use up that one neutron. Let’s assume however that my hypothesis is correct, and that an atom of uranium undergoing fission emits two neutrons. There would now be two neutrons available without the need of producing them. It is conceivable that they might hit two more atoms of uranium, split them, and make them emit two neutrons each. At the end of this second process of fission we would have four neutrons, which would split four atoms. After one more step, eight neutrons would be available and could split eight more atoms of uranium. In other words starting with only a few man-produced neutrons to bombard a certain amount of uranium, we would be able to produce a set of reactions that would continue spontaneously until all uranium atoms were split.”
This was the basic theoretical idea of a self-sustained chain reaction. Fermi next moved to prove it in a University of Chicago squash court, acutely aware that the outcome was unknown and potentially catastrophic. From Richard Rhodes’s The Making of the Atomic Bomb, the following passage tells the story of the first sustained nuclear reaction.
ANDERSON’S CREW assembled this final configuration on the night of December 1:
That night the construction proceeded as usual, with all cadmium covered wood in place. When the 57th layer was completed, I called a halt to the work, in accordance with the agreement we had reached in the meeting with Fermi that afternoon. All the cadmium rods but one were removed and the neutron count taken following the standard procedure which had been followed on the previous days. It was clear from the count that once the only remaining cadmium rod was removed, the pile would go critical. I resisted great temptation to pull the final cadmium strip and be the first to make a pile chain react. However, Fermi had foreseen this temptation and extracted a promise from me to make the measurement, record the result, insert all cadmium rods, and lock them all in place.
Which Anderson dutifully did, and closed up the squash court and went home to bed.
The pile as it waited in the dark cold of Chicago winter to be released to the breeding of neutrons and plutonium contained 771,000 pounds of graphite, 80,590 pounds of uranium oxide and 12,400 pounds of uranium metal. It cost about $1 million to produce and build. Its only visible moving parts were its various control rods. If Fermi had planned it for power production he would have shielded it behind concrete or steel and pumped away the heat of fission with helium or water or bismuth to drive turbines to generate electricity. But CP-1 was simply and entirely a physics experiment designed to prove the chain reaction, unshielded and uncooled, and Fermi intended, assuming he could control it, to run it no hotter than half a watt, hardly enough energy to light a flashlight bulb. He had controlled it day by day for the seventeen days of its building as its k approached 1.0, matching its responses with his estimates, and he was confident he could control it when its chain reaction finally diverged. What would he do if he was wrong? one of his young colleagues asked him. He thought of the damping effect of delayed neutrons. “I will walk away—leisurely,” he answered.
“The next morning,” Leona Woods remembers—the beginning of the fateful day, December 2, 1942—“it was terribly cold—below zero. Fermi and I crunched over to the stands in creaking, blue-shadowed snow and repeated Herb’s flux measurement with the standard boron trifluoride counter.” Fermi had plotted a graph of his countdown numbers; the new data point fell exactly on the line he had extrapolated from previous measurements, a little shy of layer 57.
Fermi discussed a schedule for the day with Zinn and Volney Wilson, Woods continues; “then a sleepy Herb Anderson showed up…Herb, Fermi and I went over to the apartment I shared with my sister (it was close to the stands) for something to eat. I made pancakes, mixing the batter so fast that there were bubbles of dry flour in it. When fried, these were somewhat crunchy between the teeth, and Herb thought I had put nuts in the batter.”
Outside was raw wind. On the second day of gasoline rationing Chicagoans jammed streetcars and elevated trains, leaving almost half their usual traffic of automobiles at home. The State Department had announced that morning that two million Jews had perished in Europe and five million more were in danger. The Germans were preparing a counterattack in North Africa; American marines and Japanese soldiers struggled in the hell of Guadalcanal.
Back we mushed through the cold, creaking snow…Fifty-seventh Street was strangely empty. Inside the hall of the west stands, it was as cold as outside. We put on the usual gray (now black with graphite) laboratory coats and entered the doubles squash court containing the looming pile enclosed in the dirty, grayish-black balloon cloth and then went up on the spectators’ balcony. The balcony was originally meant for people to watch squash players, but now it was filled with control equipment and read-out circuits glowing and winking and radiating some gratefully received heat.
The instrumentation included redundant boron trifluoride counters for lower neutron intensities and ionization chambers for higher. A wooden pier extending out from the face of the pile supported automatic control rods operated by small electric motors that would stand idle that day. ZIP, a weighted safety rod Zinn had designed, rode the same scaffolding. A solenoid-actuated catch controlled by an ionization chamber held ZIP in position withdrawn from the pile; if neutron intensity exceeded the chamber setting the solenoid would trip and gravity would pull the rod into position to stop the chain reaction. Another ZIP-like rod had been tied to the balcony railing with a length of rope; one of the physicists, feeling foolish, would stand by to chop the rope with an ax if all else failed. Allison had even insisted on a suicide squad, three young physicists installed with jugs of cadmiumsulfate solution near the ceiling on the elevator they had used to lift graphite bricks; “several of us,” Wattenberg complained, “were very upset with this since an accidental breakage of the jugs near the pile could have destroyed the usefulness of the material.” George Weil, a young veteran of the Columbia days, took up position on the floor of the squash court to operate one of the cadmium control rods by hand at Fermi’s order. Fermi had scalers that counted off boron trifluoride readings with loud clicks and a cylindrical pen recorder that performed a similar function silently, graphing pile intensities in ink on a roll of slowly rotating graph paper. For calculations he relied on his own trusted six-inch slide rule, the pocket calculator of its day.
Around midmorning Fermi began the crucial experiment. First he ordered all but the last cadmium rod removed and checked to see if the neutron intensity matched the measurement Anderson had made the night before. With that first comparison Volney Wilson’s team working on the balcony took time to adjust its monitors. Fermi had calculated in advance the intensity he expected the pile to reach at each step of the way as George Weil withdrew the last thirteen-foot cadmium rod by measured increments.
When Wilson’s team was ready, writes Wattenberg, “Fermi instructed Weil to move the cadmium rod to a position which was about half-way out. [The adjustment brought the pile to] well below critical condition. The intensity rose, the scalers increased their rates of clicking for a short while, and then the rate became steady, as it was supposed to.” Fermi busied himself at his slide rule, calculating the rate of increase, and noted the numbers on the back. He called to Weil to move the rod out another six inches. “Again the neutron intensity increased and leveled off. The pile was still subcritical. Fermi had again been busy with his little slide rule and seemed very pleased with the results of his calculations. Every time the intensity leveled off, it was at the values he had anticipated for the position of the control rod.”
The slow, careful checking continued through the morning. A crowd began to gather on the balcony. Szilard arrived, Wigner, Allison, Spedding whose metal eggs had flattened the pile. Twenty-five or thirty people accumulated on the balcony watching, most of them the young physicists who had done the work. No one photographed the scene but most of the spectators probably wore suits and ties in the genteel tradition of prewar physics and since it was cold in the squash court, near zero, they would have kept warm in coats and hats, scarves and gloves. The room was dingy with graphite dust. Fermi was calm. The pile rising before them, faced with raw 4-by-6-inch pine timbers up to its equator, domed bare graphite above, looked like an ominous black beehive in a bright box. Neutrons were its bees, dancing and hot.
Fermi called for another six-inch withdrawal. Weil reached up to comply. The neutron intensity leveled off at a rate outside the range of some of the instruments. Time passed, says Wattenberg, the watchers abiding in the cold, while Wilson’s team again adjusted the electronics:
After the instrumentation was reset, Fermi told Weil to remove the rod another six inches. The pile was still subcritical. The intensity was increasing slowly—when suddenly there was a very loud crash! The safety rod, ZIP, had been automatically released. Its relay had been activated by an ionization chamber because the intensity had exceeded the arbitrary level at which it had been set. It was 11:30 a.m., and Fermi said, “I’m hungry. Let’s go to lunch.” The other rods were put into the pile and locked.
At two in the afternoon they prepared to continue the experiment. Compton [MIT] joined them. He brought along Crawford Greenewalt, the tall handsome engineer who was the leader of the Du Pont contingent in Chicago. Forty-two people now occupied the squash court, most of them crowded onto the balcony.
Fermi ordered all but one of the cadmium rods again unlocked and removed. He asked Weil to set the last rod at one of the earlier morning settings and compared pile intensity to the earlier reading. When the measurements checked he directed Weil to remove the rod to the last setting before lunch, about seven feet out.
The closer k approached 1.0, the slower the rate of change of pile intensity. Fermi made another calculation. The pile was nearly critical. He asked that ZIP be slid in. That adjustment brought the neutron count down. “This time,” he told Weil, “take the control rod out twelve inches.” Weil withdrew the cadmium rod. Fermi nodded and ZIP was winched out as well. “This is going to do it,” Fermi told Compton. The director of the plutonium project had found a place for himself at Fermi’s side. “Now it will become self-sustaining. The trace [on the recorder] will climb and continue to climb; it will not level off.”
Herbert Anderson was an eyewitness:
At first you could hear the sound of the neutron counter, clickety-clack, clickety-clack. Then the clicks came more and more rapidly, and after a while they began to merge into a roar; the counter couldn’t follow anymore. That was the moment to switch to the chart recorder. But when the switch was made, everyone watched in the sudden silence the mounting deflection of the recorder’s pen. It was an awesome silence. Everyone realized the significance of that switch; we were in the high intensity regime and the counters were unable to cope with the situation anymore. Again and again, the scale of the recorder had to be changed to accommodate the neutron intensity which was increasing more and more rapidly. Suddenly Fermi raised his hand. “The pile has gone critical,” he announced. No one present had any doubt about it.
Fermi allowed himself a grin. He would tell the technical council the next day that the pile achieved a k of 1.0006. Its neutron intensity was then doubling every two minutes. Left uncontrolled for an hour and a half, that rate of increase would have carried it to a million kilowatts. Long before so extreme a runaway it would have killed anyone left in the room and melted down.
“Then everyone began to wonder why he didn’t shut the pile off,” Anderson continues. “But Fermi was completely calm. He waited another minute, then another, and then when it seemed that the anxiety was too much to bear, he ordered ‘ZIP in!’ ” It was 3:53 p.m. Fermi had run the pile for 4.5 minutes at one-half watt and brought to fruition all the years of discovery and experiment. Men had controlled the release of energy from the atomic nucleus.