tions of the larger particles. Because of background dust and unavoidable debris on the trays, correlation of the concentrations of smaller particles with radiological measurements was more difficult. The concentrations of the smallest sizes remained almost constant withtime. Par. ticle diameters gradually decreased with time at each station during the slurry-particle shots, though remaining remarkably constant at ~ 100 to 200 microns on the ships during the entire fallout period. 4. Inthe vicinity of the ships, the gross body of fallout activity for the slurry-particle shots penetrated to the thermocline from a depth of 10 to 20 meters at the rate of 3 to 4 m/hr. A con. siderable fraction of the activity for the solid-particle shots penetrated to the thermoclineat about the same rate. This activity remained more or less uniformly distributed above the ther. mocline up to at least 2 days after the shot, and is presumed to have been in solution or assoc- iated with fine particles present either at deposition or produced by the breakupof solid aggre. gates in sea water. An unknown amountof activity, perhaps as much as 50 percentof the total, . penetrated at a higher rate and may have disappeared below the thermocline during the solidparticle shots. It is unlikely that any significant amount of activity was lost in this way during the slurry-particle shots. 5. Fractionation of Mo*®, Np***, and I’ occurred in the surface water layer following solid- particle deposition; a continuous variation in composition with depth is indicated. Only slight tendencies in this direction were noted for slurry fallout. Physical, Chemical, and Radiological Characteristics. 1. The fallout from Shots Zuni and Tewa consisted almost entirely of solid particles similar to those observed after the land-surface shots during Operations Ivy and Castle, consisting of irregular, spheroidal, and agglomerated types varying in color from white to yellow and ranging in size from < 20 microns to several millimeters in diameter. Most of the irregular particles consisted primarily of calcium hydroxide with a thin surface layer of calcium carbonate, although a few unchanged coral particles were present; while the spheroidal particles consisted of calcium oxide and hydroxide, often with the same surface layer of calcium carbonate. The agglomerates were composed of calcium hydroxide with an outer layer of calcium carbonate. The particles almost certainly were formed by decarbonation of the original coral to calcium oxide in the fireball, followed by complete hydration in the case of the irregular particles, and incomplete hydration in the case of the other particles; the surface layer, which may not have been formed by deposition time, resulted from reaction with CO, inthe atmosphere. sities of the particles were grouped around 2.3 and 2.7 gm/cm’. The den- 2. Radioactive black spherical particles, usually less than 1 micron in diameter, were observed in the fallout from Shot Zuni, but not in the fallout from Shot Tewa. Nearly all such particles were attached to the surfaces of irregular particles. They consisted partially of calcium iron oxide and could have been formed by direct condensation in the fireball. 3. The radionuclide composition of the irregular particles varied from that of the spheroidal and agglomerated particles. The irregular particles tended to typify the cloud-sample and distantfallout radiochemistry, while the spheroidal and agglomerated particles were more characteristic of the gross fallout near ground zero. The irregular particles tended to be enriched in Ba'‘*_ La"?and slightly depleted in Sr®*; the spheroidal and agglomerated particles were depleted in these nuclides but were much higher in specific activity. It should be recognized that this classification by types may be an oversimplification, and that a large sample of individual particles of all types might show a continuous variation of the properties described. The inference is strong, nevertheless, that the fractionation observed from point to point in the fallout field at Shot Zuni was due to the relative abundance and activity contribution of some such particle types at each location. 4. The activities of the irregular particles varied roughly as their surface area or diameter squared, while those of the spheroidal particles varied as some power higher than the third. Indications are that the latter were formed in a region of higher activity concentration in the cloud, with the activity diffusing into the interior while they were still ina molten state. Activity was not related to particle density but varied with the weight of irregular particles in a manner consistent with a surface-area function. 152