be weeks, months, or even years old. The SUNSHINE data, to be discussed later, show the presence of strontium-90 in o significant number of samples taken at many of the fer flung net of fall-out collecting stations. Strontium-90 is signif- icant in that it is not a naturally occurring isotope and thus must have originated in an atomic detonation and must have been distributed through the atmosphere by processes such as described above. E. Mechanisms of Fall-out. By definition, as used in this report, close-in fall-out is that process by which fission products are returned to earth from a bomb cloud within the isodose line bounding the area within which at least 50 roentgens of radiation would be accumulated if a person remained there in the open indefinitely. Such fall-out generally results from the incorporation of the molecular state radioactive bomb debris in or upon particles that have diameters in excess of 10 to 25 microns. It can be assumed that within a matter of hours only those particles remain in the air that are such as to respond at least equally well to both the gravitational influence of the earth and the vertical air movements that tend to keep them aloft. This aerosol of radioactive particles that is still in the air is not likely to return to earth in appreciable amounts if the particles involved are small enough to be supported by Brownian motion. Association of these light particles with rain droplets is one means by which they are brought to earth. However, rain does not occur above the tropopause so that some other mechanism would have to operate as a scavenging agent if such particles in the stratosphere are to be brought to earth. When the fireball starts rising after a land surface detonation, any dust or debris that moves with it is available for radioactive particle formation. However, there is other dust, first disturbed by the ground shock and then pulled into the stem by the low pressure region that follows the rapid rise of the cloud, and much of which is subsequently distributed throughout the stabilized cloud. 26 This dust,