A lot of effort and imagination is being devoted to the problem of making a controlled thermonuclear reaction. The motivation for this project comes from the fact that good thermonuclear fuels, such as deuterium (H²), are abundant and cheap. There is enough deuterium in the oceans of the world to supply man’s energy needs for many millions of years. One difficulty, of course, is to find a container for the reaction.
Even under stellar conditions the rate of fusion reactions is not very great. It takes approximately a billion years for only one per cent of the nuclei to react. Consequently even higher temperatures than those found in stars are required to produce large amounts of energy in a short time. But no known materials can withstand temperatures of more than a few thousand degrees centigrade. One idea is to keep the “burning” fuel away from material walls by means of magnetic fields.
Is there a way to make nuclei react without the extreme temperatures needed in the thermonuclear reactions? What one is really trying to do is bring two nuclear particles into intimate enough contact so that the nuclear forces can act between them. There is no reason why one should not use a cold target material, which is bombarded from the outside by energetic nuclear projectiles, for example protons or alpha particles. The projectiles, if they are energetic enough, can overcome the electrical repulsion of the target nuclei, and they actually can penetrate. The resulting “compound” nuclei would either be unstable and disintegrate instantaneously, or else be almost stable (i.e., radioactive) and disintegrate after some period of time. In either case nuclei of new elements would probably be formed in the reaction. This procedure sounds simple, but it has its difficulties.
Interior of the sun. The thermonuclear reactions take place mainly in the very hot, very dense central region (shaded). This region is about 20,000 miles in radius and has a density approximately 80 times the density of water.
The main difficulty is that the nucleus is a very tiny target. Its area is about 100 million times smaller than the area of the atom as a whole. If a piece of matter is bombarded by an energetic particle, chance alone will determine whether the particle is directed toward a nucleus. To be sure, if the particle misses the nucleus of one atom, it still has the opportunity of hitting the nuclei of other atoms which may lie in its path. It does not have many such opportunities, however, because, being charged, it constantly interacts with the atomic electrons, which gradually absorb energy causing the particle to slow down.
As the particle slows down, its chance of hitting a nucleus decreases, even if it is heading directly toward one, because of the repulsion between its charge and that of the nucleus. Unless the particle has sufficient speed, it cannot overcome this repulsion.
Charged particles may be given the required speeds by accelerating them through large electric fields. If a unit charge is accelerated through a potential difference of one volt, it acquires an energy of one electron-volt. The energies required for nuclear bombardment are of the order of several million electron-volts, which can be provided by atom-smashing machines such as the cyclotron.
Even at such high energies very few of the nuclear projectiles actually find their way to a target nucleus. Most of them are slowed down by the electrons, wasting their energy in heating up the target material. Perhaps one particle out of a million will be lucky enough to induce a nuclear reaction.
If the purpose of the nuclear accelerating machines were to produce cheap energy, they would not be of much value. A nuclear reaction may typically release five to 20 million electron-volts of energy. But to obtain this reaction, a million particles had to be accelerated to energies of several million electron-volts. The recoverable and useable energy will be only a minute fraction of the total invested.