4 RADIATION STANDARDS, INCLUDING FALLOUT tissue before losing all its energy and coming to rest as an ordinary atom of helium. These characteristics lead to the conclusion that any radioactive materials such as plutonium 239 and uranium 238 which emit only alpha particles are of no biological consequence as long as they remain outside the body. The inert outer surface of the skin, which protects us from many environmental agents, is thick enough to stop completely the most energetic alpha particles you will encounter in your present deliberations. Alpha particles emitted inside the body are another matter and appear to produce more biological injury per unit energy absorbed than do the other radiations. This is a matter we shall con- sider when the remunit is discussed. (5) Beta particles: Beta particles are simply high-speed electrons. They are familiar as the agent which traces the picture on thetelevision tube. As a result of their relatively small mass and of their single charge, beta particles lose energy much less rapidly in passing through matter than do alpha particles and as a consequence have much greater ranges, as the table shows. ; To illustrate: a 1-million electron volt beta particle has a velocity nearly that of light and is stopped completely by about 10 feet of air, or by about three-sixteenth inch of tissue before coming to rest as an ordinary electron. These characteristics mean that beta particles with energies greater than about 0.1 million electron volts can penetrate the inert layer of the skin and can therefore reach living tissue even when the beta source is outside the body. Beta particles emitted by radioactive materials inside the body will, like alpha particles, deliver all their energy to livingtissue. (c) Gamma rays and X-rays: Except for the fact that y-rays (gamma) are produced inside the atomic nucleus, with one or a few discrete energies, while X-rays are produced outside the nucleus usually with a broad spectrum of energies, these two radiations are much the same. Like visible light, they have no mass or charge and travel with the speed of light. Unlike alpha and beta particles, X- and yrays do not have definite ranges in matter. A thickness of material, knownas the half-value layer, will reduce the intensity of y-rays to half their initial value; two half-value layers will reduce the intensity to one-fourth of the initial intensity; three such thicknesses to oneeighth, and so forth. One can reduce the intensity to any value he wishes with sufficient shielding, but in principle can never reduce it to zero, reminiscent of Zeno’s paradoxical arrow which never quite reaches the target. Because of their high penetrating power, X- and y-radiation ofall but the lowest energies can reach living tissue even whenthe sourceis a considerable distance away. 38. UNITS OF MEASUREMENT The units of measurement pertinent to your present deliberations are the curie, the roentgen, the rad, and the rem. I shall discuss each of these briefly. (a) Curie: The curie is defined as that amount of any radioactive material in which nuclear disintegrations occur at the vate of 37 bil- bsspabinidesARRNIEI aliasDisaalsciieaitiaCRGAaHRN,