The University of Columbia
Italian physicist Enrico Fermi fled Italy in late 1938, immediately after he received the Nobel Prize in physics. His wife Laura was Jewish, and the Italian government had begun passing racial laws similar to Germany's. With their two children, they moved to New York. Enrico took a job teaching standard physics courses and a geophysics course at the University of Columbia, and this was where I first met him.
At Columbia, I was a part of a team led by Fermi. His ability and scientific judgement never ceased to astound me. Often he would predict the outcome of our experiments before we completed them. As part of the government project to develop an atomic bomb, our focus was getting quantitative information on the secondary neutrons formed in fission. Not all of the neutrons emitted during fission will go on to split more uranium atoms. Many are absorbed before they have a chance to hit uranium nuclei. The neutrons are too fast, and not effective, unless they are slowed down. We managed to solve this problem by using graphite as a moderator to slow down the neutrons.
In December of 1941, Arthur H. Compton was put in charge of all the scientific work of the uranium program. He decided to bring all of the teams involved to the University of Chicago, where we would become a part of the Metallurgical Laboratory. Coincidentally, we had begun working with Leo Szilard to build an atomic reactor earlier that year. We called it "the pile." It consisted of alternating layers of graphite and graphite containing uranium. We constructed a pile that reached the ceiling, but it was not large enough for a chain reaction. There were not enough neutrons available to produce fission. We began looking for a larger room when we heard that we would be moving.
Italian physicist Enrico Fermi fled Italy in late 1938, immediately after he received the Nobel Prize in physics. His wife Laura was Jewish, and the Italian government had begun passing racial laws similar to Germany's. With their two children, they moved to New York. Enrico took a job teaching standard physics courses and a geophysics course at the University of Columbia, and this was where I first met him.
At Columbia, I was a part of a team led by Fermi. His ability and scientific judgement never ceased to astound me. Often he would predict the outcome of our experiments before we completed them. As part of the government project to develop an atomic bomb, our focus was getting quantitative information on the secondary neutrons formed in fission. Not all of the neutrons emitted during fission will go on to split more uranium atoms. Many are absorbed before they have a chance to hit uranium nuclei. The neutrons are too fast, and not effective, unless they are slowed down. We managed to solve this problem by using graphite as a moderator to slow down the neutrons.
In December of 1941, Arthur H. Compton was put in charge of all the scientific work of the uranium program. He decided to bring all of the teams involved to the University of Chicago, where we would become a part of the Metallurgical Laboratory. Coincidentally, we had begun working with Leo Szilard to build an atomic reactor earlier that year. We called it "the pile." It consisted of alternating layers of graphite and graphite containing uranium. We constructed a pile that reached the ceiling, but it was not large enough for a chain reaction. There were not enough neutrons available to produce fission. We began looking for a larger room when we heard that we would be moving.
In Chicago
Fermi was not happy about moving. He didn't want our smaller team to be swallowed up by the large organization of the Metallurgical Laboratory. Nevertheless, our group began relocating in stages. I moved to Chicago in March 1942.
I hardly ever saw Fermi at first. He was always in some meeting. We continued performing the experiments that he requested, but in Chicago the focus was mostly on determining which method was best to prepare material for fission. General Groves bought land at Argonne, near Chicago. There we planned to build an atomic reactor similar to the ones we had started at the University of Columbia. Construction began, but was soon brought to halt by a labor union strike. Then Fermi proposed that the reactor be built under Stagg Field, the stadium of the University of Chicago. The location in the middle of the city was worrisome to some, but Fermi's proposal was accepted nonetheless. The squash court beneath the west stands was thirty feet wide, sixty feet long, and more than twenty-six feet high. It was less room than we had all hoped for, but it would have to suffice.
Construction commenced on November 16, 1942. Our goal was to use the reactor (or the "pile") to prove that a self-sustaining nuclear chain reaction could be created. Success would mean that the released energy could possibly be harnessed to create a bomb.
Fermi believed that removing the air from the reactor might increase its performance. He had experimented with this idea in Columbia, but it hadn't caused any significant improvements. Despite the past, Fermi thought it might work on a larger pile like the one in Chicago. We began building the pile inside a giant balloon we had ordered from the Goodyear Tire and Rubber Company. The reactor had to be assembled through an open flap in the balloon, and the pile of graphite bricks swathed in a giant balloon made for a strange sight. Later we discovered that a vacuum was not necessary, so we never sealed the balloon.
The pile was to be a sphere about 26 feet in diameter. To control the reaction, we inserted cadmium sheets nailed to wooden rods into the pile. Cadmium is an element known for absorbing neutrons, so it would prevent the reaction from beginning before we were ready for it. I became part of the team of scientists who removed the cadmium rods each day to measure the reactivity of the pile. Our job was to measure the neutron level and report back to Fermi. He would pull out his slide rule- the predecessor to today's electronic calculators- and figure out how close we were to a chain reaction. Fermi's calculations showed that we would need to build 57 layers to make the pile self-sustaining.
Fermi was not happy about moving. He didn't want our smaller team to be swallowed up by the large organization of the Metallurgical Laboratory. Nevertheless, our group began relocating in stages. I moved to Chicago in March 1942.
I hardly ever saw Fermi at first. He was always in some meeting. We continued performing the experiments that he requested, but in Chicago the focus was mostly on determining which method was best to prepare material for fission. General Groves bought land at Argonne, near Chicago. There we planned to build an atomic reactor similar to the ones we had started at the University of Columbia. Construction began, but was soon brought to halt by a labor union strike. Then Fermi proposed that the reactor be built under Stagg Field, the stadium of the University of Chicago. The location in the middle of the city was worrisome to some, but Fermi's proposal was accepted nonetheless. The squash court beneath the west stands was thirty feet wide, sixty feet long, and more than twenty-six feet high. It was less room than we had all hoped for, but it would have to suffice.
Construction commenced on November 16, 1942. Our goal was to use the reactor (or the "pile") to prove that a self-sustaining nuclear chain reaction could be created. Success would mean that the released energy could possibly be harnessed to create a bomb.
Fermi believed that removing the air from the reactor might increase its performance. He had experimented with this idea in Columbia, but it hadn't caused any significant improvements. Despite the past, Fermi thought it might work on a larger pile like the one in Chicago. We began building the pile inside a giant balloon we had ordered from the Goodyear Tire and Rubber Company. The reactor had to be assembled through an open flap in the balloon, and the pile of graphite bricks swathed in a giant balloon made for a strange sight. Later we discovered that a vacuum was not necessary, so we never sealed the balloon.
The pile was to be a sphere about 26 feet in diameter. To control the reaction, we inserted cadmium sheets nailed to wooden rods into the pile. Cadmium is an element known for absorbing neutrons, so it would prevent the reaction from beginning before we were ready for it. I became part of the team of scientists who removed the cadmium rods each day to measure the reactivity of the pile. Our job was to measure the neutron level and report back to Fermi. He would pull out his slide rule- the predecessor to today's electronic calculators- and figure out how close we were to a chain reaction. Fermi's calculations showed that we would need to build 57 layers to make the pile self-sustaining.
The Experiment
Safety was a primary concern. A set of control rods was set up to automatically fall on the pile and shut down the reaction if the neutron intensity rose too high. We also had a backup set of control rods which hung over the reactor by a rope. This set could be cut to fall on the reactor if the automatic control rods failed. A group of three young physicists called the "suicide brigade" stood on the scaffolding above the nuclear pile with buckets of cadmium solution. Their job was to dump the cadmium to douse any dangerous runaway reactions.
On December 2, 1942, we began work at 9:45 am. The 57th layer of the pile was added, and Fermi kept extremely calm. Along with about forty others, I stood in silence. Fermi gave instructions for us to move this rod there and that rod over here, remaining expressionless the entire time. Finally there was just one rod left. A friend of mine, George Weil, stood on the floor next to it. His job was to pull it out, bit by bit, as Fermi instructed.
Another team of scientists had set up a mechanism that would display the behavior of the pile. Fermi explained it to his audience. "This pen will trace a line indicating the intensity of the radiation. When the pile chain-reacts, the pen will trace a line that will go up and up and that will not tend to level off... Weil will first set the rod at thirteen feet. This means that thirteen feet of the rod will still be inside the pile. The counters will click faster and the pen will move up to this point, and then its trace will level off. Go ahead, George!" It was exactly as Fermi had predicted- the pen rose and then leveled off. The rest of the morning proceeded in a like manner, with George pulling out the cadmium rod a little bit at a time. We moved ahead cautiously, for we were dealing with the unknown.
At 11:35 am, the automatic control rods slammed down on the pile. We had set the safety point too low. At that point, we all had lunch. After the most suspenseful meal of my life, we reassembled. At 2:30 pm, Weil pulled out the last cadmium rod in a series of measured adjustments. Shortly afterward, the counters clicked rapidly, and the pen traced a line. But this time, it did not level off. The chain reaction continued for 28 minutes. Enrico Fermi smiled, shut his slide rule, and said, "The reaction is self-sustaining." Eugene Wigner produced a bottle of wine to celebrate the occasion, and it was passed around in paper cups. Later, we all signed the bottle.
While I was excited about our success, I was also slightly uneasy about where this technology would lead- the science we had unleashed had the potential to become a monster. What consequences would our discovery unleash on the world? Only a few years later, I would find out.
Safety was a primary concern. A set of control rods was set up to automatically fall on the pile and shut down the reaction if the neutron intensity rose too high. We also had a backup set of control rods which hung over the reactor by a rope. This set could be cut to fall on the reactor if the automatic control rods failed. A group of three young physicists called the "suicide brigade" stood on the scaffolding above the nuclear pile with buckets of cadmium solution. Their job was to dump the cadmium to douse any dangerous runaway reactions.
On December 2, 1942, we began work at 9:45 am. The 57th layer of the pile was added, and Fermi kept extremely calm. Along with about forty others, I stood in silence. Fermi gave instructions for us to move this rod there and that rod over here, remaining expressionless the entire time. Finally there was just one rod left. A friend of mine, George Weil, stood on the floor next to it. His job was to pull it out, bit by bit, as Fermi instructed.
Another team of scientists had set up a mechanism that would display the behavior of the pile. Fermi explained it to his audience. "This pen will trace a line indicating the intensity of the radiation. When the pile chain-reacts, the pen will trace a line that will go up and up and that will not tend to level off... Weil will first set the rod at thirteen feet. This means that thirteen feet of the rod will still be inside the pile. The counters will click faster and the pen will move up to this point, and then its trace will level off. Go ahead, George!" It was exactly as Fermi had predicted- the pen rose and then leveled off. The rest of the morning proceeded in a like manner, with George pulling out the cadmium rod a little bit at a time. We moved ahead cautiously, for we were dealing with the unknown.
At 11:35 am, the automatic control rods slammed down on the pile. We had set the safety point too low. At that point, we all had lunch. After the most suspenseful meal of my life, we reassembled. At 2:30 pm, Weil pulled out the last cadmium rod in a series of measured adjustments. Shortly afterward, the counters clicked rapidly, and the pen traced a line. But this time, it did not level off. The chain reaction continued for 28 minutes. Enrico Fermi smiled, shut his slide rule, and said, "The reaction is self-sustaining." Eugene Wigner produced a bottle of wine to celebrate the occasion, and it was passed around in paper cups. Later, we all signed the bottle.
While I was excited about our success, I was also slightly uneasy about where this technology would lead- the science we had unleashed had the potential to become a monster. What consequences would our discovery unleash on the world? Only a few years later, I would find out.