mixed with the bomb debris it: the cooling mushroom cloud, they absorb activity. This radioactivity is not uniformly distributed throughout the particle but is absorbed primarily on the surface. It is also possible for two or more particles to aggregate and in this way more heterogeneous systems are formed. These particles are usually above 100 microns in size, and tend to fall out locally. The fall-out particles from the water surface (barge) shots of the CASTLE series were, for the most part, carried in water droplets. Sam- ples were collected, but the data give very little information on the transport or deposition mechanisms. An underground burst is one in which the center of detonation lies below the surface of the earth. Particle formation takes place as the earth particles and hot gases rise above the ground in an inverted cone or column. The particles are comparatively large and do not get as high into the air as for a surface burst, so that the major portion of the activity returns quickly from the column and cloud to the surface. Deep sub-surface shots deposit almost all of the activity in the ruptured earth surrounding the point of detonation. Particle size is one of the controlling factors in determining the amount of radioactivity deposited as fall-out, the degree of localization, and the time scale of deposition. The close-in fall-out at an early time after detonation of a surface or underground burst consists almost entirely of the larger particles (above 20 microns in diameter). Small particles, down to and including molecular size, do not tend to fail out early but tend to remain suspended in the upper atmosphere and fall out very slowly. The amount of activity carried per particle is thus an important consideration. One study showed, however, that less . than 1% of the fall-out from the JANGLE surface shot was radioactive. It is not known to what extent surface redistribution by the wind affected this measurement. 4 Various investigations have been made by several laboratories to collect data on the amount of radioactivity which is carried per 15