Charged particles of one type stand pretty much by themselves. These are the mesons found in cosmic rays. These particles move as fast as energetic beta rays and, like the beta rays, carry unit charge. Their biological effects are therefore the same as the biological effects of beta radiation, with one important difference. The cosmic ray mesons carry much more energy and therefore have a much greater range. Whereas the beta rays are stopped in the skin, the mesons can cause damage throughout the entire body. The mesons produce the same effects as a substance which emits beta radiation uniformly in the whole body. This fact is important. It puts us in the position to compare effects of man-made radioactivity with effects of the cosmic rays to which we are constantly exposed. Not all the energy in cosmic rays is carried by mesons. We also find showers of electrons. These are almost the same as beta rays except that they have more energy and arrive frequently in fairly sizeable numbers traveling along nearly parallel tracks. Their effects, however, are the same as the effects of the mesons. We have been talking now about the interactions between charged particles and the atomic electrons. No mention has been made of interactions between the charged particles and nuclei. Nuclear interactions do occur sometimes, but by and large they have only a negligible influence in slowing down the charged particle. They do affect, however, beta rays. When a beta ray collides with a highly charged nucleus, the beta particle is violently deflected. The violence of this process is due to the heavy charge of the nucleus and the small mass of the beta particle. In the sudden change of velocity which occurs, part of the electric force field which surrounds the electron breaks loose; the result is high-frequency radiation called X-rays. The importance of such electromagnetic radiation is that it can penetrate more deeply into matter. In our bodies, for typical beta-ray energies, only a small part of the beta-ray energy is converted into X-rays. But in many radioactive processes gamma rays (which are physically the same as X-rays) are produced quite abundantly. These rays may carry as much or more energy than the beta rays. Unlike charged particles, which constantly interact as they move through matter, gamma rays can go for long distances without having a single encounter. The actual distance depends on the energy of the gamma ray, the medium in which it moves, and pure chance. On the average, a one-million-volt gamma ray goes about six inches in water before anything at all happens to it. A four-million-volt gamma ray goes about a foot. In living matter the distances are approximately the same. Thus gamma rays from an external source can find their way deep inside the body.