CHAPTER 1 THE PORTION OF ACTIVITY DEPOSITED IN LOCAL FALLOUT A typical deterministic fallout-prediction system is based upon a forecast of climatological winds and a postulated (or calculated) initial distribution of radioactivity on particles of various sizes located at various positions within a radioactive cloud.” The amount and kinds of radioactivity postulated depend on the yield of the nuclear explosion, its fission-fusion ratio, the type of fissionable material used, and the kinds of induced activities produced. The height and other dimensions of the cloud depend on the yield and on ambient atmospheric conditions, particularly on the variation of temperature and relative humidity above the ground. During their fall the radioactive particles move laterally under the influence of the wind field. If the above factors are properly accounted for, one can predict levels of deposition of radioactivity on the ground, from which radiation exposure rates can be derived, DCPA and hence this paper is concerned primarily with surface and near~surface bursts. Possibly important perturbations, which will be taken up later, are small changes in the height of burst (Chapter 2), the chemical and physical properties of the soil or other substrate over which the explosion takes place (Chapter 3), and the influence of adjacent, nearly simultaneous bursts (Chapter 8). A central problem in fallout prediction is that of relating radia thon exposure rates at various locations to the yield of the detonation that produced the fallout. Sophisticated models can, at least in principle,rigorously compute this relation nuclide by nuclide for each point on the ground, subject to the accuracy of the fission-product data base, the assumed relation between radioactivity and particle size, and available wind and weather information. Simpler models, however, predict only the gross deposition of mixed-fission products. Any model implies, and one of the models used by DCPA explicitly uses, an empirical factor called the K-factor* to relate deposition to radiation intensity. In the literature, this term has referred to at least two different but related things: (1) the ratio of exposure rate measured at a particular place in the fallout field to the density of deposition of radioactivity there; and (2) an integrated, weighted average of this ratio over the "local" fallout field. The confusion caused by the various uses of the concept has been well reviewed by Rapp? and Cane.3 The customary unit for K-factors is R/hr per kt/mi* at H + 1 hour. Since this is a rather unwieldy unit, we shall not repeat it hereafter. An idealized limit of the K-factor corresponds to unfractionated fission products uniformly spread ovér a smooth ideal plane, and measured with an ideal detector 3 feet above the plane. This limit, here called Ko, varies depending on what particular fission process is being considered, According to Tompkins ,* Ko = 3067 for U-235 fissioned by *Also called the Normalization Factor, the Magic Number, and the Exposure Rate Conversion Factor. 2