ve ceed dbs at ree [Vou. 91 | ‘|| 5 Us iH if HEE STATUTE MILES * lb ed] f— weoziuy vO wavaan 1964] WARTIN : RADIOECOLOGY AND STUDY OF ENVIRONMENTAL RADIATION 293 Needless to say, there are many possible variations on the themes outlined above, and fallout formation is truly a complex process which involves the interaction of a great many variable factors. Factors such as kiloton or megaton yield, kind of support, height of detonation above the surface, and kind of surface are all important in determining the kinds and amounts of material incorporated in the fireball. The kinds and amounts of material which take part in the process are important in determining the physical and chemical properties of the particles formed. And finally, properties such as the size, shape, density, chemical composition, solubility, and radionuclide content of the particles formed are important in determining their initial distribution and subsequent ecological behavior in the biosphere. To determine which of these parameters are biologieally significant is one of the major objectives of the study of fallout phenomenology. FALLOUT PATTERNS. Fallout ean be classified as ‘‘local,’’ ‘‘tropospherie,’’ or ‘‘stratospheric.’’ Local fallout is composed primarily of larger particles which fall, usually in a matter of hours, within a few hundred miles of ground zero. The fine debris produced by most detonations in the kiloton range may remain suspended in the troposphere for a period of weeks or months before it is removed by some scavenging mechanism such as rain or snow. The fine debris produced by detonations in the megaton range is carried into the stratosphere where, in the absence of weather, it may remain for a period of months or years before reentering the troposphere. Both tropospheric and stratospheric fallout contribute to ‘‘world-wide’’ fallout. 1. Local fallout. Local fallout can be defined as the fallout occurring in areas corresponding to a fallout time of H+12 hours (H +12 hrs. = the 12 hrs. immediately after detonation). Most of the local fallout patterns produced by detonations at the Nevada Test Site (Fig. 2) have been roughly cigar-shaped. In more or less ‘‘typical’’ patterns, the isodose contour for 1.0 mr/hr (milliroentgens per hour), as determined by ground and aerial surveys, has extended downwind from ground zeros for distances of 200-300 miles. The lateral distances from the midline of fallout (the line of maximum radiation in the fallout path) to the 1.0 mr/hr contour have been on the order of 15-30 miles. In most cases, the isodose contours have been approximately concentric. Occasional hot-spots have been found (Larson et al. 1960), but most of these were small in area. Isodose contours of more than 1000 mr/hr at +12 hours have been measured near ground zeros, but the areas so contaminated have been small, and these levels of contamination persist for only a few warson et al, * days or a few hours. As indicated by isodose contours and by analyses of fallout collections (Baurmashet al. 1958, Larson et al. 196?, and Rainey et al. 1954), the unit area activity of local fallout decreases (Table 6) with increasing distance