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The Pope and his team almost found nuclear fission in 1934 in the course of experiments in which, looking for radioactive transformations, they systematically bombarded one element after another with the newly discovered neutron. They missed by the thickness of the sheet of foil in which they wrapped their uranium sample; the foil blocked the fission fragments that their instruments would otherwise have recorded. It was a blessing in disguise. If fission had come to light in the mid-1930s, while the democracies still slept, Nazi Germany would have won a long lead toward building an atom bomb. In compensation, Fermi made the most important discovery of his life, that slowing neutrons by passing them through a light-element "moderator" such as paraffin increased their effectiveness, a finding that would allow releasing nuclear energy in a reactor.
If Hitler had not hounded Jewish scientists out of Europe, the Anglo-American atom bomb program sparked by the discovery of fission late in 1938 would have found itself shorthanded. Most Allied physicists had already been put to work developing radar and the proximity fuse, inventions of more immediate value. Fermi and his fellow emigres--Hungarians Leo Szilard, Eugene Wigner, John von Neumann and Edward Teller, German Hans Bethe--formed the heart of the bomb squad. In 1939, still officially enemy aliens, Fermi and Szilard co-invented the nuclear reactor at Columbia University, sketching out a three-dimensional lattice of uranium slugs dropped into holes in black, greasy blocks of graphite moderator, with sliding neutron-absorbing cadmium control rods to regulate the chain reaction. Fermi, still mastering English, dubbed this elegantly simple machine a "pile."
The work moved to the University of Chicago when the Manhattan Project consolidated its operations there, culminating in the assembly of the first full-scale pile, CP-1, on a doubles squash court under the stands of the university football field in late 1942. Built up in layers inside wooden framing, it took the shape of a doorknob the size of a two-car garage--a flattened graphite ellipsoid 25 ft. wide and 20 ft. high, weighing nearly 100 tons. Dec. 2 dawned to below-zero cold. That morning the State Department announced that 2 million Jews had perished in Europe and 5 million more were in danger; American boys and Japanese were dying at Guadalcanal. It was cold inside the squash court, and the crowd of scientists who assembled on the balcony kept on their overcoats.
Fermi proceeded imperturbably through the experiment, confident of the estimates he had charted with his pocket slide rule. At 11:30 a.m., as was his custom, he stopped for lunch. The pile went critical in midafternoon with the full withdrawal of the control rods, and Fermi allowed himself a grin. He had proved the science of a chain reaction in uranium; from then on, building a bomb was mere engineering. He shut the pile down after 28 minutes of operation. Wigner had thought to buy a celebratory fiasco of Chianti, which supplied a toast. "For some time we had known that we were about to unlock a giant," Wigner would write. "Still, we could not escape an eerie feeling when we knew we had actually done it."