Various units have been used to express exposure to radiation such as the roentgen, rep, rem, and rad. All are intended to express some relationship between the radiation ‘energy absorbed and biological effects. Since it is not critical for the following discussions to understand the technical differences among the units, only the “roentgen” will be used. To provide some perspective as to the magnitude of the “roent- gen” table I is included. 2. Sources and Nature of Fallout The major source of radioactive materials in fallout is the fissioning or splitting of atoms of uranium and plutonium that gives rise to a large numberof unstable radioisotopes. Inthe © fusion process hydrogen nuclei are joined together. Induced radioactive products result when inert materials capture neutrons that are released during either the fission or fusion process. Generally, these induced radioactive materials are relatively short-lived and contribute only in a minor way to radiation exposures to man. Theprincipal exception is carbon 14 described in section I F (page 16). Some of these radioactive materials escape as gases and are dispersed and diluted in the atmosphere. Most of the fission products, however, become incorporated into or attached onto minute inert particles of dust and debris from the immediate environment of the bomb. The dust particles, together with the associated radioactive nuclides, are swept high into the air by the heat and force of the nuclear explosion. The larger particles and those in the lowerlevels of the cloud fall nearby. Smaller particles in the upper levels are carried away to be spread worldwide. The worldwide distribution of these radioactive particles follows the same pattern as would occur with any other small particles injected into the same regions of the atmosphere —radioactivity has essentially no effect on the pattern of distribution. Roughly, a nuclear detonation of one-half million tons or less, fired at a low altitude—but high enough so the fireball does not intersect the ground—results in most of the fission products remaining in the lower atmosphere, the troposphere. They are deposited on the earth’s surface at a rate such that one-half of the amount remaining in the atmosphereat any one time falls in 2-4 weeks (called tropospheric residence halftime). As the energy yields of the nuclear detonations increase, more and moreof the fission products are swept higher and higher into the stratosphere—the layer above the tropo- sphere(fig. 2). The residence half-time here is more like onehalf a year for injection into the lower stratosphere in the polar 2 U.S. WEATHER BUREAU PHOTO. FIGURE 2.—Generalized drawing of the earth’s atmosphere. regions and one year or somewhatless at the equator. Radio- active debris from nuclear detonations occurring at very high altitudes (about 30 miles and higher) may have a residence half-time of five years or more. Roughly two-thirds of the radioactive particulate debris injected into the lower stratosphere at the north polar regions has been observed to fall in the 30°-60° North latitude zone, where about 80 percent of the world’s population live. Injection at the equatorial regions has been observed to result in a more even distribution between the two hemispheres. For surface bursts of high (million ton range) yield about 50-80 percent of the radioactive debris is deposited as “early fallout,” i.e., within 24 hours. Air bursts—wherethe fireball does not approach the surface — result in little, if any, local fallout. Table 2 tabulates some of the key data on estimated nuclear energyyields from all past nuclear weaponstests. Of the total energy released of 511 million tons equivalent of TNT about 70 TABLE 2.— Estimates of Yields from All Nuclear Weapons Tests USSR US and UK 161 Total million tons*.........0.....2. cece cece cece eee ec eee nee een eene 350 82 Fission million tons............00...:ccccccceceeeeeeeeeeeeeneeanees 111 51 Fission million tons scattered globally.................... 110 Totalt 611 193 161 *TNT equivalent. tThe French tests have contributed only smal! amounts. 145-413 O-64—2 . 3