Free Bomb by Steve Sheinkin Page B

outsider,” Fermi later said. “He would have seen only what appeared to be a crude pile of black bricks.” Shaped like an oval, the black pile was about twenty-five feet wide in the middle and twenty feet high.
    Woods and Fermi climbed up to a balcony high above the court. “The balcony was originally meant for people to watch squash players,” said Woods, “but now it was filled with control equipment and read-out circuits glowing and winking.”
    A young physicist named Herb Anderson walked in, yawning, and helped do a few last-minute checks. Everything was set for one of the most important experiments in the history of science.
    But first, breakfast.
    â€œHerb, Fermi and I went over to the apartment I shared with my sister,” Woods said. “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.”
    After the quick meal, the three set out across campus to the football stadium. “Back we mushed,” said Woods, “through the cold, creaking snow.”
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    O PPENHEIMER WAS BUSY recruiting scientists for Los Alamos—but that didn’t mean he knew for sure an atomic bomb was technically possible. He and other physicists had spent a few years studying fission. They knew they could bombard a uranium atom with neutrons and cause its nucleus to split. They knew the splitting nucleus would release energy. But what happened next?
    Theoretically, as the uranium nucleus split in two, more neutrons would break free and fly off on their own. The speeding neutrons would collide with other uranium atoms, causing them to fission also. As these uranium atoms split, they would release more neutrons, which would hit more uranium atoms. These atoms would also split, releasing still more neutrons, which would hit more uranium atoms, causing more fission, more free-flying neutrons, more fission, more neutrons, and so on. Though they didn’t know if it would actually happen, physicists had a name ready for this process: chain reaction .
    Each splitting atom would release a small amount of energy. So scientists knew that if they could cause a fast enough chain reaction, they might be able to build atomic bombs. But first they had to prove a chain reaction was even possible.
    That’s what Enrico Fermi and his team were trying to do in the squash court under the football stands in Chicago. The black blocks were graphite, the mineral used to make pencil leads. Slid into holes in some of the blocks were small pieces of uranium. Fermi used graphite to slow down the speeding neutrons—he knew that neutrons would bounce off the carbon atoms that make up graphite and lose speed. Traveling a bit more slowly, they’d be more likely to hit the uranium atoms and cause fission.
    Stuck through the pile at various points were long wooden poles wrapped with a bluish-white metal called cadmium. Cadmium was chosen for its ability to absorb huge numbers of neutrons. As long as the cadmium poles were in place, they would absorb the neutrons shooting out of the uranium. This, Fermi told Leslie Groves, would prevent a chain reaction from starting.
    Still, the idea of attempting to release nuclear energy in the middle of a city of three million made Groves very nervous. “If the pile should explode, no one knew just how far the danger would extend,” Groves fretted. “Because of this I had serious misgivings about the wisdom of doing the experiment there.”
    Fermi assured Groves he knew exactly what he was doing.
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    A LITTLE BEFORE 10:00 A.M., Fermi and his team of about fifteen students and scientists assembled in the freezing squash court. Most climbed to the balcony, but three stood on an elevated platform near the ceiling, holding buckets full of cadmium. If the reaction got out of control,