Los Alamos. Manhattan Project. Niels Bohr and his son, Aage, were tasked with designing the chain reaction initiator. The soon-to-be-developed solution would be called the “urchin,” a spherical version of the neutron sources Fermi had used in Rome. The initiator would be located inside a plutonium sphere and would be ignited under the enormous pressure of the implosion of TNT, releasing between 10 and 100 neutrons before being destroyed. These neutrons would be enough to initiate explosive fission throughout the plutonium sphere. The Urchin was a hollow beryllium sphere, just two centimeters in diameter. The inside of the sphere was machined with grooves facing inward. At the center of these grooves was another beryllium sphere, centered by pins embedded in the outer shell. Both the internal grooves of the outer shell and the outer surface of the inner sphere were coated with nickel and gold. The nickel of the inner sphere was coated with a thin film of virulently radioactive polonium. Polonium emits alpha particles; in the unexploded state of the “Urchin,” these would be absorbed harmlessly by the gold and nickel. But when the bomb imploded around it, the beryllium and polonium mixed violently, producing a well-known reaction (beryllium + an alpha particle = carbon + neutron) that produced the necessary neutrons.
