ORIGIN AND NATURE OF RADIOACTIVE DEBRIS WORLDWIDE EFFECTS OF ATOMIC WEAPONS 17 It must be emphasized, however, that these considerations assume spe- cific activity independent of size. Since some of the activity comes from gaseousprecursors and is presumably deposited on the surface of particles, the specific activity may vary with size. It is not practical to attempt an exact analysis of all the effects, but some general conclusions may be drawn. 1. The activity presentin all particles having a diameter less than about one-fifth of the median diameter is entirely negligible compared with that in particles having diameters four times larger than the median diameter. In the RANGER and GREENHOUSE shots, from which the data seem to be fairly reliable, the median diameter in airborne material is 1.2 to 2.2 p; thus the particle sizes of most interest from high air bursts (as far as overall ground contamination from fallout or rainout is concerned) are those in the diameter range of 0.8 to 15 p. 2. If the exponential number distribution of particle sizes is correct, the small fraction of the activity in extremely small particles maystill be very important for some consideration. For example, inhalation of these small particles may be a serious hazard. A consideration of the form of the fission products at the time of condensation is important, since this is the controlling factor in determining whether a given fission product will be distributed more or less uniformly in particles or will form a surface layer on existing patticles at a later time. Even though some uncertainties exist in the half-lives of the short-lived members of the fission-product decay chains, the fraction of the total activity present in the various elements in the first 5 sec after detonation can be estimated. At 10 sec and longer, the calculation can be made with some certainty; such calculations have been presented by Hunter and Ballou,” the results of which are shownin Table 1 on page 18. During the interval of 1 to 5 sec, it appears that the percentages given in Table 1 would not be vastly different from those at 10 sec. The striking 2 4 6 o/b 8 10 12 Fig. 2—Numberfraction and mass fraction with diameters less than a given diameter D’ 14 eee 16 fact about these considerations is that during the time when the tempera- Tires fpmtor condensation of sold materials, a very large fraction (approximately 80 per cent) of the radioactivity resides in the more vola- tile elements, such as noble gases, halogens, and alkali metals, which do not condense until low temperatures are reached or until decay to another less volatile element has occurred. An important example of this type is