The debris particles generated by free airbursts were usually spherical or spheroidal in shape (Mamuro et ai., 1962; Bjornerstedt and Edvarson, 1963; Benson and Leventhal, 1964). As a result of fusion at high temperatures, occasionally an irregular particle was seen. More common was the occurrence of accretions, small particles attached to larger ones. The attachment obviously took place when at least one of the partners had solidified, or had become sufficiently viscous to prevent coalescence. When the scaled height of burst becomes small enough to allow entrainment of ground surface material (between 100 and 125 m), the round radioactive particle population is augmented by radioactive particles that are irregular in shape. These irregular particles may or may not have a smooth surface. The fraction of spherical particles tends to decrease with decreasing scaled height of burst, but does remain significant in the size range below a few microns, as was inferred from microscopic examination of cloud samples from Johnie Boy, a low-yteld shot buried just under the surface. Sometimes spherical particles were found to be attached to irregular particles originating from the soil or rock at and around ground zero (GZ). Large (>1 om), smooth, glassy particles have also been found in fallout from ground-surface shots. It has been speculated that these particles were formed by throw-out of molten material from the crater at early times. Several classifications of particles have been reported is well as color. There is some subjectivity involved, ing descriptions and details. In addition, populations types usually vary from test to test. This observation reported classifications. with regard to shape particularly in providof particles of different also affects the A general review of shapes of particles from tests through the 1956 Redwing series has been written by Adams et al. (1958). His review covers most of the work that has been done on balioon shots, tower shots, and ground- and watersurface bursts. Some additional information has been obtained from tests of these types from the post-Redwing period (Plumbbob, Hardtack, Sunbeam series, cratering shots), but detailed morphological examination of particles had become deemphasized. It may generally be concluded that fallout particles present on and in soil are not all recognizable. Glassy particles, that have the appearance of having been fused at one time or another, are the most likely candidates for being radioactive fallout particles. Any other particle types cannot be so assigned on the basis of shape alone. 2. Color. For lack of universally used color standards, the colors as reported are somewhat subjective. Generally, however, the colors vary from very dark (black) to very light and transparent, with many gradations within a shot. The predominant color range appears to depend on the soil, and on the presence or absence of a steel tower. Some observers report color in more detail than others (Schuert, 1958). In tower shots, dark colors usually dominate some of the particles from NTS surface shots. 236 Color is not a good criterion for the identification of debris, alone or Correlation between color and activity in combination with particle shape. is tenuous at best for airburst debris and is expected to be nonexistent for debris from surface and near-surface shots. Surface appearance and surface characteristics Surface Characteristics. also are nonspecific and correlate only partially with radioactivity, depending on the history of the individual particles. Material that has entered the fireball very early and that has become vaporized or that has become liquified into a thin liquid usually formed spherical particles with a smooth surface. Smooth surfaces have also been noted on some large particles from surface shots that appear to have been formed from molten soil or rock ejected from an incipient crater. Smooth surfaces are further found on particles that became only partially fused, but that maintained their approximate dimensional ratios. Other particles never became hot enough for even partial fusion and are unaltered except for an occasional accretion of fused particles. More recently, scanning electron microscopy has been applied to the study of surface characteristics. Thus, several types of particles have been identified, for example, in debris recently collected from Area 13 (Schultz (1) exfoliating, but generally et al., 1975; Soinski and Nathans, 1975): spherical; (2) irregular in both shape and morphology; (3) rough interior partially covered by a smooth shell; (4) spheroidal with smooth surfaces. The possibilities of using the scanning electron microscope, particularly for the study of weathering, have not been sufficiently investigated, however. Internal Characteristics. Spherical or other-shaped particles have been ground to hemispheres for physical observation or in preparation for electron microprobe analysis. The internal structure of these particles range from fused solid homogeneous to particles with voids and instertices and inclusions and onion skin. These effects are obviously based on the cooling conditions and the environment in which the cooling occurred. Density. The density of spherical particles may be determined by measuring their rate of all in a liquid and applying Stokes’ law with or without Cunningham's slip correction. A microcolumn with observation by means of a microscope is used for particles between 3 and 10 ym (Benson et al., 1967.). Smaller particles become too subject to convection currents, larger particles fall too fast. This method requires careful manipulation of the particles, and losses run between 10 and 30%, dependent on the operator. The density of larger spherical particles may also be determined by a Stokes’ fall method, but in a longer column having a larger diameter (Wrigley and Gleit, 1964). For nonspherical particles, the aforementioned method fails because the shape factors are generally not sufficiently known. Density gradient columns are used for these particles, but with particle assemblies rather than with individual particles. As many density cuts as desired can be made. 237