CHAPTER 2 EFFECTS OF SMALL CHANGES IN BURST HEIGHT ON THE AMOUNT OF LOCAL FALLOUT Data from weapons tests show that radiation-intensity levels of local fallout decrease as the height of burst increases. An estimate of the magnitude of this decrease is of interest to DCPA since detonations in a nuclear attack may occur on contact with urban structures or even in trees, Such heights of burst will, however, be small when scaled from megaton-yield weapons. Taking a one-megaton burst as an example, the scaled height of burst ranges from \ = 12 (£t/kel/3yx* for a tall tree or a ten-story building to \ = 50 for a 40-story building. It is well known that the amount of local fallout is very low, almost insignificant, when the height of burst is greater than about a fireball radius, i.e., A = 180°, (except when the burst takes place on a tower.“9"). In such detonations, the soil or dust swept up by rising fireball either does not reach the fireball or enters it only after most of the radionuclides have condensed. In this case the small amount of local fallout depends primarily on the mass of the warhead assembly. The bulk of the radionuclides is carried by very small particles which do not fall to the ground fast enough to contribute to local fallout. For detonations at small heights of burst, the coupling of f 2° energy to and the interaction of the fireball with the ground apparently decreases rapidly as the height increases, as manifested in the rapid decrease in crater volume. The particle-size distribution is also shifted to smaller sizes. In the absence of a theoretical foundation for describing these effects quantitatively, one must take recourse in observation. Table 1 (Chapter 1) lists some K-factors (K)) derived from intensity-area integrals by a number of investigators, with data from bursts beyond the altitude range of immediate concern included for perspective. The averages have been plotted in Figures 1 and 2, along with the spread in various investigators' interpretations. The mode of support of each burst is indicated as a basis for interpretation. Most of these bursts were over dry desert soil at the Nevada Test Site (NTS) and in the low kiloton range. Thus these data are intercomparable. Also. shown are K-factors from a number of megaton bursts; although these fallout patterns are much less well known due to the difficulties of obtaining and interpreting data over water, it is comforting to see that their resultant K-factors arep consistent with those from NTS shots. In Figure 2, which extends the field of view to greater heights of burst, the horizontal part of line B represents the mean K-factor *See first footnote on Table 1. 7