collected In a very few cases, where the fallout particulates have a sufficiently high plutonium content, special methods have been employed that take advantage of the special properties of plutonium nuclides (Nathans et al., 1976). By autoradiographic registration of alpha particles ("hollow-star technique"), the plutonium content of each particle is registered as distinct tracks on a photographic emulsion. These tracks form a star and the number of tracks is converted to a “PuOj-equivalent" particle diameter. This method has been applied to soils from the fallout field of a safety shot (Schulz et al., 1975) and to a few resuspension samples. The lower-size limit depends on the isotopic jalf-life and on the exposure time, but with reasonable exposure times extends down to below 0.1 um Pu0:-equivalent diameter for plutonium-239. Particles above 1 or 2 um may be identified, measured, and isolated for additional measurements. These special methods also make it possible, in many cases, to obtain information from old debris for which normal autoradiographic techniques are no longer applicable. Density-gradient separations have also been applied in a few cases (Tamura, 1976; Soinski and Nathans, 1976), This method fs particularly useful for the gross separation of undiluted debris particles from particles that consist primarily of soil components and is based on the observation that such particles have specific gravities well in excess of 3.0 or thereabout. T a t uy uy us 4 a3 3 Bas _ ” 7 ° a a 4 oOo EZ 4 r Ww 8Be | Zavn = al 2 xXodq 4o 12 5 - 1 ropa} Most of the size distributions reported before the mid-60s show lognormality. These results are probably correct for the fallout samples, Lognormal size distributions of cloud samples, particularly early samples, are probably biased against the larger-particle end of the distribution, Later work has shown that many cloud samples have a particle size distribution that may be described as lognormal below a few micrometers, but obey approximately a r -law, with m having a value between 3 and 4, above the range of lognormality (Russell, 1965; Nathans et aZ., 1970) (Figure 2), a 492 r a 4 OTHER PHYSICAL PROPERTIES 1. Shape. Particle shape was measured to determine if the radioactive debris possesses any special characteristics that distinguish these particles from background, entrained inactive material, and contamination. The radioactive particles were generally identified by autoradiography, occasionally by micromanipulation, and the use of a beta-gamma meter or phosphors. Usually a sufficient number of particles were viewed to encompass the various shapes (and colors) that were present. 4 L t 4 ! a 1 ° 1 P) ! © AUN3SNOENI JAILV 134 907 234 i “ ° o> samples as derived from the particle Particle size distributions of three Johnie Boy size distributions of size fractions the measurements were made on fallout samples Figure 2. In quite a few cases, throughout the close-in fallout field. This may make it possible to derive estimates of the size distributions of debris on the soil anywhere in the fallout field. Such work would be quite complicated, however, in areas where fallout from more than one shot is present.