In these, it sometimes happened that a fast neutron might strike a nucleus with sufficient energy to cause it to emit 2 neutrons. In that way the nucleus, absorbing 1 neutron and emitting 2, would become a lighter isotope of the same element.
But what would happen if each of the 2 neutrons that emerged from the original target nucleus struck new nuclei and forced the emission of a pair of neutrons from each. There would now be a total of 4 neutrons flying about and if each struck new nuclei there would next be 8 neutrons and so on. From the initial investment of a single neutron there might soon be countless billions initiating nuclear reactions.
Szilard, fearing the inevitability of war and fearing further that the brutal leaders of Germany might seek and use such a nuclear chain reaction as a weapon in warfare, secretly applied for a patent on a device intending to make use of such a nuclear chain reaction. He hoped to turn it over to the British Government, which might then use its possession as a way of restraining the Nazis and keeping the peace.
However, it wouldn’t have worked. It took the impact of a very energetic neutron to bring about the emission of 2 neutrons. The neutrons that then emerged from the nucleus simply didn’t have enough energy to keep things going. (It was like trying to make wet wood catch fire.)
But what about uranium fission? Uranium fission was initiated by slow neutrons. What if uranium fission also produced neutrons as well as being initiated by a neutron? Would not the neutrons produced serve to initiate new fissions that would produce new neutrons and so on endlessly?
It seemed very likely that fission produced neutrons and indeed, Fermi, at the conference where fission was first discussed, suggested it at once. Massive nuclei possessed more neutrons per proton than less massive ones did. If a massive nucleus was broken up into 2 considerably less massive ones, there would be a surplus of neutrons. Suppose, for instance, uranium-238 broke down into barium-138 and krypton-86. Barium-138 contains 82 neutrons and krypton-86 50 neutrons for a total of 132. The uranium-238 nucleus, however, contains 146 neutrons.
The uranium fission process was studied at once to see if neutrons were actually given off and a number of different physicists, including Szilard, found that they were.
Now Szilard was faced with a nuclear chain reaction he was certain would work. Only slow neutrons were involved and the individual nuclear breakdowns were far more energetic than anything else that had yet been discovered. If a chain reaction could be started in a sizable piece of uranium, unimaginable quantities of energy would be produced. Just 1 gram of uranium, undergoing complete fission, would deliver the energy derived from the total burning of 3 tons of coal and would deliver that energy in a tiny fraction of a second.
Szilard, who had come to the United States in 1937, clearly visualized the tremendous explosive force of something that would have to be called a “nuclear bomb”. Szilard dreaded the possibility that Hitler might obtain the use of such a bomb through the agency of Germany’s nuclear scientists.
Partly through Szilard’s efforts, physicists in the United States and in other Western nations opposed to Hitler began a program of voluntary secrecy in 1940, to avoid passing along any hints to Germany. What’s more, Szilard enlisted the services of two other Hungarian refugees, the physicists Eugene Paul Wigner (1902- ) and Edward Teller (1908- ) and all approached Einstein, who had also fled Germany and come to America.