no ready vehicle for local faii-out"/ - The fireball engulfs enough air to insure an adequate supply of oxygen to enable the atoms to form oxides when the proper temperature 1s reached. Although many of the elements combine to form oxides, some wmite with oxygen to form negative radicals while the halogens form halides which combine with the strong- ly electropositive elements to form compounds. The noble gases, kryp- ton and xenon, remain in the atomic state awaiting radioactive decay to change them to elements which can form an oxide or halide. These two noble gases are precursors for strontium and barium, respectively. The cloud rising from the point of detonation carries all these materials as separate molecules, but with the rapid cooling of the fireball, from 7000°%K to approximately 2000°K in one to five seconds, oxides with similar condensation temperatures become available to form small mixed crystals for later incorporation into fall-out particles. It is reasonable to assume that this condensation may be aided by the intense ionization that accompanies the nuclear explosion, although there is no definitive data to support this statement. The tiny | crystal nuclei, containing the mixture of oxides, halides and noble gases, are carried throughout the cloud, where they can be absorbed on dust particles or grow by self-nucleation as a result of many collisions. As a result of aggregation, they attain sufficient bulk to be carried to earth by free fall or mass air transport. Eddy currents tend to diffuse and scatter the particles, in some cases hastening and in others retarding the return to the earth. The sizes of fall-out particles from an air burst are not well documented. Investigators agree that the size distribution during the early life of the cloud is logarithmic, with the numbers in the aggregation decreasing as the diameter of the individual particle increases 4/ Greenfield, S.M., et al., Transport and Early Deposition of Radioactive Debris from Atomic Explosions. Project AUREOLE of the RAND Corporation, July 1954, SECRET Restricted Data. 12