For the purpose of making a controlled chain reaction, one may use the method of enrichment, or the method of moderation, or both. But to produce a violent chain reaction, an atomic bomb, only the enrichment method will work. The reason is that all the energy of the bomb must be generated in a time that is as short as the time it takes the bomb to fly apart, which is a fraction of a microsecond. If natural uranium were used, the reaction would be slow and sluggish and would be extinguished before a substantial fraction of the nuclei could have reacted.
It is interesting to consider that chain-reacting substances could have been obtained easily six billion years ago, before the U²³⁸ had time to decay and become a rare isotope. (The U²³⁵ was then about as abundant as U²³⁸.) A chemical separation would still have been necessary and so we do not need to imagine that chain-reacting mixtures accumulated spontaneously on the young earth.
On the other hand, six billion years from now U²³⁵ will have become so rare that it will be impossible to get a reactor going by moderation. At the same time the isotope separation will have become most expensive since the isotope to be separated will be present in an abundance of less than 100 parts in a million. For those who like to worry about the distant future we should hasten to add that other methods of obtaining atomic energy will remain possible. And in any case there is good reason to believe that some stellar explosions produce fresh supplies of U²³⁵ which space merchants could undoubtedly make available.
As to our present terrestrial supplies: uranium, like other heavy elements, is quite rare. But the earth is divided into layers of which the topmost 10 miles, forming something of a slag or scum, contain quite a few rare compounds. In particular almost all of the uranium in our planet is conveniently collected right under our feet, for us to use as we see fit.
CHAPTER VIII
Action of Radiation on Matter
When an energetic particle moves through matter (living or nonliving), what happens is a question of chemistry. Chemistry is the subject that deals with the arrangement and rearrangement of electrons in atoms and molecules. A chemical rearrangement generally requires an energy in the neighborhood of a few electron-volts. (As we have seen, an electron-volt is the energy released when an electron moves through a potential of one volt, i.e., a little less than one per cent of the driving force in a standard electric outlet.) An energetic particle, such as might be emitted in a radioactive decay, typically has an energy of a few million electron-volts. Thus a single such particle has the potentiality of about a million chemical rearrangements.
Energetic particles may be charged or neutral, light or heavy, or electromagnetic in nature. Because of this diversity one might think there would be no common grounds for comparing the action on matter of different particles. Each particle might conceivably make its own inimitable variety of chemical rearrangements. Actually this is not the case.
Unlike some chemical poisons, which seek out specific molecules in our body, the energetic particles strike at whatever atoms or molecules happen to get in their way. They act, in this sense, like a sledge hammer. Their effects can be measured directly from the strength (or energy) of the blow. Which particle delivers the blow is of little consequence provided the same amount of energy is delivered and provided the same tissues are affected (in the case of living matter). After the blow, however, some specific chemical effects may occur. When water or some other molecule in the body is broken up by radiation, the fragments produced may themselves be chemical poisons and attack the biologically important large molecules in a secondary way. In fact, it seems probable that a considerable part of the radiation damage caused in living systems, both healthwise and genetically, occurs in this manner.
Although the energetic particles are all similar in their ultimate action on matter, namely in producing wholesale destruction of atoms and molecules, they differ somewhat in the way in which they bring about this destruction. Charged particles act in one way, gamma rays in another, and neutrons in still another. It is simplest to begin our discussion with the charged particles.
The most important charged particles are those connected with the natural background of radioactivity and cosmic rays, and the fission process. These include alpha rays, beta rays, mesons, and fission fragments. For review, a table of the weights and charges of these particles, as well as a few others, is shown. As usual, we have used the weight and charge of the proton as units.