The most important fact about the ranges of charged particles is that they are small. An alpha particle, for instance, with a typical radioactive energy of a few million electron-volts, has a range in water (or living tissue) of a few thousandths of an inch. Such a particle could not penetrate a sheet of paper. A fission fragment, despite its great energy, is even less penetrating than the alpha particle. The proton has a somewhat greater range than the alpha particle. But the beta ray, because of its low weight, has by far the greatest range of any of the charged particles. Even it, however, goes only a fraction of an inch in solid or liquid materials. The following table shows the ranges (in inches) of some of the charged particles in air and water as a function of energy (in millions of electron-volts): _Range_ _Air_ _Water_ (Same as living tissue) _Energy_ _5_ _1_ _2_ _5_ _5_ _1_ _2_ _5_ alpha 0.1 0.2 0.4 1.4 0.0001 0.0002 0.0004 0.0014 proton 0.3 0.9 2.8 13.4 0.0005 0.001 0.003 0.014 beta 49. 130. 300. 770. 0.063 0.16 0.38 1.0 The table shows that charged particles travel only short distances in matter. For this reason these particles are not a serious external radiation hazard. The protons and the alpha rays are usually stopped by less than a foot of air. Ordinary clothing or even the outer layer of our skin (which is composed of nonliving cells) will stop them completely. Beta rays are stopped by less than seventy feet of air or an inch or less of solid material. (Actually most of the beta rays produced in the fission process have energies less than a million electron-volts or so, and hence their ranges are even smaller.) Radioactive contamination of beta emitters directly on one’s clothes or body could cause trouble; but a good scrubbing soon after exposure will eliminate this problem. The interior of a house or building should be quite safe from any outside source of charged particles emitted by radioactive substances except possibly the most energetic beta rays. Only if the source of charged particles is inside the body so that in spite of their limited ranges the particles can find their way to sensitive tissues, is there any danger. In this case, as we shall see in a later chapter, the danger may be considerable.