[Illustration: _Cosmic ray and trapped Van Allen Belt energetic particles produced the dark tracks in this photo of a nuclear emulsion that had been carried aloft on an Air Force satellite. The energetic particles cause ionization of the silver bromide molecules in the emulsion._] [Illustration: _Alpha particles emitted by the source at right leave tracks in a cloud chamber. Some tracks are bent near the end as a result of collisions with atomic nuclei. Such collisions are more likely at the end of a track when the alpha particle has been slowed down._] [Illustration: _Beta particles originating at left leave these tracks in a cloud chamber. Note that the tracks are much farther apart than those of alpha particles. As the particle slows down, its path becomes more erratic and the ions are formed closer together. At the very end of an electron track the proximity of the ions approximates that in an alpha-particle track._] Ionizing radiation is capable of imparting so much energy to molecules as to cause them to vibrate themselves apart, producing not only ions but also high-energy uncharged molecular fragments called _free radicals_. The direct effect of ionizing radiation on chromosomes can be serious. Enough chemical bonds may be disrupted so that a chromosome struck by a high-energy wave or particle may break into fragments. Even if the chromosome manages to remain intact, an individual gene along its length may be badly damaged and a mutation may be produced. [Illustration: _Effects of ionizing radiation on chromosomes: Left, a normal plant cell showing chromosomes divided into two groups; right, the same type of cell after X-ray exposure, showing broken fragments and bridges between groups, typical abnormalities induced by radiation._] If only direct hits mattered, radiation effects would be less dangerous than they are, since such direct hits are comparatively few. However, near-misses may also be deadly. A streaking bit of radiation may strike a water molecule near a gene and may break up the molecule to form a free radical. The free radical will be sufficiently energetic to bring about a chemical reaction with almost any molecule it strikes. If it happens to strike the neighboring gene before it has disposed of that energy, it will produce the mutation as surely as the original radiation might have. Furthermore, ionizing radiations (particularly of the electromagnetic variety) tend to be penetrating, so that the interior of the body is as exposed as is the surface. The gonads cannot hide from X rays, gamma rays, or cosmic particles. All these radiations can bring about somatic mutations—all can cause cancer, for instance. What is worse, all of them increase the rate of genetic mutations so that their presence threatens generations unborn as well as the individuals actually exposed. Background Radiation