Thus even before fission was discovered, Szilard laid the basis for constructing the atomic bomb and the nuclear chain reactor. As materials in which a chain reaction might conceivably be made to occur he named thorium, uranium and beryllium. On beryllium he was wrong because the mass of this atom was incorrectly known. On thorium, his guess was good. On uranium, he hit the bull’s eye.
Finally in December 1938 the secret broke. Hahn and Strassmann in Germany made a chemical analysis of a uranium target that had been exposed to neutrons. They were far more thorough than previous investigators had been, and they found barium, charge 56, which had not been present in the target material before the experiment. The only possible explanation was the fission process. Within a few weeks the violent kicks caused by the fission products in counters were found, and in the following days this experiment was repeated around the world.
There was no doubt that neutrons could induce fission in uranium nuclei. A few more weeks, and it was ascertained that the fission process released neutrons which might lead to more fissions.
The chain reaction, however, was still far from a reality. Niels Bohr and John Wheeler proved that a neutron could not cause fission in U²³⁸ unless its energy were greater than about one million electron-volts. When the neutrons are first made in the fission process, many of them do have energies greater than one million electron-volts. But before they can cause a fission, they usually make a few nonfission collisions with uranium nuclei, giving part of their energy to the nuclei and escaping with the remainder. The nuclei are then left with too little energy to undergo fission and the neutrons with too little energy to cause fissions in their next encounters. Thus too few neutrons reproduce themselves and no chain is possible.
Bohr and Wheeler suggested, however, that the rare isotope of uranium, U²³⁵, can undergo fission when any neutron, even a slow neutron, hits it. Thus a chain reaction is possible in U²³⁵. This was confirmed experimentally shortly afterwards by John Dunning and Alfred Drier and their co-workers at Columbia University.
Why the isotopes 235 and 238 behave so differently, is not difficult to understand. The 235 is more explosive and more prone to undergo fission than 238 because it is smaller and therefore its protons repel each other more strongly. More important still, when a neutron is captured by 235, it acquires a greater kinetic energy by virtue of the short-range nuclear attraction than a neutron acquires when it is captured by 238. This happens for the simple reason that nuclei tend to be more stable when they have an even number of neutrons (or protons) than when they have an odd number. U²³⁵, having an odd number of neutrons, is more eager to receive an additional neutron than 238, which already has an even number of neutrons. Consequently, the capture of a slow neutron by 235 almost always eventuates in the fission process; while in 238, the excess energy, introduced by the neutron, is merely ejected from the nucleus in the form of a gamma ray, and U²³⁸ becomes U²³⁹.
A chain reaction is possible in U²³⁵ , but it is necessary to separate this rare isotope from the abundant U²³⁸. The separation process is anything but simple since isotopes of the same element are chemically indistinguishable. Even the weight difference in this case, is little more than one per cent. Bohr rejected the idea of a large-scale separation with the remark: “You would have to turn the whole country into a factory.” Of course it is now a matter of history that the job was actually done under the Manhattan project during World War II. During the war Bohr (alias Nicholas Baker) again visited the United States and was shown the separation plants. He said: “You see I was right. You did turn the country into a factory.”
Natural uranium contains U²³⁵ in the ratio of 1 part to 139 of U²³⁸. It was hoped at first that this concentration would be sufficient to make a chain reaction, and that the expensive enrichment processes could be avoided. This seemed possible because at energies of a fraction of an electron-volt the neutrons are much more easily caught by U²³⁵ than by U²³⁸, which compensates for the low concentration. Actually neutrons are slowed down until their energy is as low as the energy of all other particles participating in the general agitation caused by the temperature. This energy is low enough for the purpose.
However, the neutrons are made in the fission process with an energy of about a million electron-volts. Before they slow down sufficiently, they must pass through a stage in which their energy is about 7 electron-volts. In the neighborhood of this energy, it happens that the U²³⁸ has an extremely high probability for capturing a neutron and changing into U²³⁹. Near some other energies, similar though smaller absorption hurdles must be passed. Therefore natural uranium by itself cannot be used to make a chain reaction. In 1940, Fermi and Szilard, working now in the United States, found a way around this difficulty.
Their trick was to mix the natural uranium with a material whose nuclei are so lightweight that they suffer a big recoil when struck by a neutron and thus absorb a large fraction of the neutron energy. The neutron is thus moderated down to a low energy, rapidly and in big energy jumps, so that either it does not spend much time at the unfavorable energies where it can be caught by U²³⁸ or else it misses these energies altogether. By imbedding the uranium in lumps in the moderating material instead of making a homogeneous mixture of the two, the absorption can be circumvented even better.