Our Nuclear Future: Facts, Dangers and Opportunities

by Edward Teller; Albert L. Latter

A second way in which the gamma ray may interact with matter is by scattering. In this case the gamma ray does not disappear but merely loses a part of its energy to the atomic electron. Again the electron is free to cause biological damage, while the gamma ray goes on to its next encounter.

The third way requires that the gamma ray be near a nucleus and have an energy greater than a million electron-volts. (Ordinary X-rays such as are used in medical practice are not energetic enough for this process to occur.) Under these conditions the gamma ray may disappear, with the simultaneous appearance of an electron and a positron. This is an example of the creation of matter out of pure energy. In accordance with the formula E = mc², a part of the gamma-ray energy is consumed in producing particles with definite masses. This amounts to about one million electron-volts. The remainder of the gamma-ray energy goes into kinetic motion of the two particles. Again biological damage results from the subsequent ionization due to the charged particles. After the positron has expended its kinetic energy in the ionization process, it will join with an electron in a disappearing act. The energy reappears in the form of two or three gamma rays (each having less energy than the original gamma ray).

In no case is the gamma ray directly responsible for any biological damage. The damage is always made by electrons (or positrons) to which the gamma ray has transferred some or all of its energy. But this only makes gamma rays the more dangerous. They can first penetrate to the sensitive tissues of the body, and then cause ionization.

We have already mentioned that X-rays are the same as gamma rays. The latter are produced by an excited nucleus, the former in the collision of an electron (or a beta ray) with a nucleus. The man-made X-rays are obtained by first accelerating a stream of electrons and then letting them impinge on a target containing highly charged nuclei.

The usefulness of X-rays is, of course, due to their power of penetration; that is the same property which renders X-rays dangerous. One can use X-rays to find out what happens to be inside the human body. But this cannot be done without producing some disruption and rearrangement in the tissues which lie in the path of the X-rays. The damage is of the same kind as that caused by radioactivity or cosmic rays.

The effects of neutrons on matter are rather similar to the effects of gamma rays. Like gamma rays, neutrons can travel long distances in matter without interacting. On the average, a million-volt neutron goes a few inches in water before having a collision of any kind. Also like the gamma rays, the neutrons are not themselves directly responsible for any biological damage. Being neutral, they interact only with the atomic nuclei to which they are strongly attracted. By far the most important of these interactions is with the nuclei of hydrogen. There are a great number of these in living tissue in the form of protein and water molecules.

The collisions with hydrogen nuclei (i.e., protons) are important because a large fraction of the neutron energy is transferred in the process. This happens because the neutron and the proton have very nearly the same weight. If the neutron hits a heavy nucleus, it loses only a small fraction of its energy in the impact.[9] After colliding with hydrogen or a heavier nucleus, the neutron continues on to other such collisions. The nucleus, however, being charged and energetic, now causes excitation and ionization of atomic electrons. Thus, like gamma rays, energetic neutrons are exceedingly dangerous, because they can first penetrate and then cause ionization.

Neutrons are dangerous even when they are not energetic. A nonenergetic neutron may react with nuclei of living matter in a number of ways of which two are particularly probable. Either the neutron may be captured by a proton to form a deuteron, in which case the excess energy will be emitted in the form of a two-million-volt gamma ray that will cause further damage. Or the neutron may react with a nucleus of nitrogen¹⁴ (abundantly present in living matter) to produce a nucleus of carbon¹⁴ and an energetic proton. Thus a nonenergetic neutron will have a biological effect equivalent to an energetic gamma ray, or to an energetic proton plus an energetic carbon¹⁴ ion.

In summary, all particles, charged or not, have a similar action on matter. Directly or indirectly, they produce excited atoms, molecules, and ion pairs. These processes always occur in practically the same proportions, and therefore the number of ion pairs formed can be used as a measure of the radiation effects. The more ion pairs produced in living matter, the greater the extent of biological damage. For this reason it is customary to describe radiation effects in terms of the number of ion pairs created per gram of living tissue in various parts of the body. Since each ion pair corresponds to an energy transfer of about 32 electron-volts, an alternative description may be given in terms of the amount of energy deposited. The unit in common usage for this purpose is the roentgen, which means specifically an energy equivalent to lifting the body (in which the radiation is deposited) by one twenty-fifth of an inch. This is equivalent to about 60 million million ion pairs in each ounce. It is less exact but more significant to say that one roentgen deposits in a cell of our body a few thousand ion pairs.

Of course the amount of ionization within individual cells is not a quantity that is easily measured. What one usually knows instead, is the roentgen dosage to a piece of tissue, which consists of many cells. If the charged particles inducing the ionization are electrons (as they are when the primary radiation is a beta ray or a gamma ray), the ionization will be distributed more or less uniformly among the cells in the affected neighborhood. If the charged particle is heavy—a proton or an alpha ray—the density of ionization which it produces is much greater, so that some cells receive a good many more ion pairs, while others nearby may receive none. For this reason it is sometimes important to specify not just how many roentgens the tissue has been exposed to, but also which kind of radiation has been responsible.