5. The fallout from Shots Flathead and Navajo collected at the ship stations was made up entirely of slurry particles consisting of about 80 percent sodium chloride, 18 percent water, and 2 percent insoluble solids composed primarily of oxides of calcium and iron. The individual insoluble solid particles were generally spherical and less than 1 micron in diameter, appearing to be the result of direct condensation in the fireball. 6. The radionuclide composition of individual slurry drops could not be assessed because of insufficient activity, but the results of combining a number of droplets were similar to those obtained from gross fallout collections. In general, much less fractionation of radionuclides was evident in the slurry-particle shots than in the solid-particle shots. The amount of chloride in a slurry drop appearedto be proportional to the drop activity for the ship stations at Shot Flat- head; however, variability was experienced for Shot Navajo, and the relation failed for both shots at close-in locations. the total drop activity. Conflicting data was obtained on the contribution of the insoluble solids to While the slurry nature of the fallout and certain properties such as drop diameters, densities, and concentrations have been adequately described, further experimentation is required to establish the composition of the insoluble solids, and the partition of activity among the components of the drop. Radionuclide Composition and Radiation Characteristics. 1. The activities of products resulting from slow-neutron fission of U?™ are sufficiently similar to those resulting from device fission to be quantitatively useful. It should also be noted that the absolute calibration of gamma counters is feasible, permitting calculation of the countper-disintegration ratio of any nuclide whose photon-decay scheme is known. For establishing the quantity of a given nuclide in a complex mixture, radiochemistry is the method of choice; at the present time, gamma-ray spectrometry appears less reliable, even for nuclides readily identifiable. In addition, gross spectra obtained with a calibrated spectrometer led to computed counting rates for a laboratory gamma counter which were generally low. 2. Fractionation of radionuclides occurred in the fallout of all surface shots considered. By several criteria, such as R-values and capture ratios, Shot Navajo was the least fractionated, with fractionation increasing in Shots Flathead, Tewa, and Zuni. For Shot Zuni, the fractionation was so severe that the ionization per fission of the standard cloud sample was ~5 to 6 times greater than for close-in fallout samples. Important nuclides usually deficient in the fallout were members of the decay chains of antimony, xenon, and krypton, indicating that the latter products, becauseof their volatilities or rare-gas state, do not combine well with condensing or unaltered Carrier particles. Although empirical methods have been employed to correct for fractionation in a given sample, and to relate the fractionation observed from sample to sample at Shot Zuni, the process is not well understood. As yet, no method is knownfor predicting the extent of fractionation to be expected for arbitrary yield and detonation conditions. 3. Tables of values are given for computing the infinite-field ionization rate for any point in the fallout field where the composition and fission density are known. The sametables permit easy calculation of the contribution of any induced nuclide to the total ionization rate. Based on How Island experience, rates so obtained are approximately twice as high as a survey meter would indicate. It is evident that unless fractionation effects, terrain factors, and instrument- Tesponse characteristics are quantitatively determined, accurate estimates of the fraction of the device in the local fallout cannot be obtained by summing observed dose-rate contours. Correlations. 1. The maximum fission densities observed during the various shots were, in fissions per Square foot, approximately 4 x 10'5 for Shot Tewa, 8 x 10'4 for Shot Zuni, 6 x 10" for Shot Flathead, 9 x 10'3 for Shot Navajo, and 9 x 10!° for Shot Cherokee. The fallout which was deposited during Shot Cherokee arrived as slurry particles similar to those produced by Shots Flathead and Navajo and appeared to be relatively unfractionated with regard to radionuclide composition; the total amount deposited was small, however, and of no military significance. 2. Reasonable agreement between the predicted and observed perimeters and central axes of the preliminary fallout patterns for Shots Zuni and Tewa was achieved by assuming the radioactive material to be concentrated largely in the lower third of the cloud and upper third of the Stem, restricting particles larger than 1,000 and 500 microns in diameter to the inner 10 per153