Residual radiation, however, from such a getonation may be expected to affect very large areas for @ considdrable period of time. Fallout is the phenomenon responsible forlthe major part of the residual radiation hazard, A. Description of Fallout The term "fallout" is used to describe thal radioactive particles produced by a nuclear detonation when they fall back upon the earth from the upper air. It is compose—t of fission troducts, particles of the bomb itself, substance made radioactive by neutrons, and material from the surface of thefearth carried aloft by the explosion. In the case of an air bufst, where the fireball does not come into contact with the earth, the radioactive products of the detonation are carried high into fhe atmosphere as very smell particles and are scattered widely the winds. The great bulk of this material will undergo radidective decay before the particles have fallen to the earth. Whe, however, the detonation is such that the fireball rests upon t ground, great amounts of earth are drawn into the rapidly rising fireball resulting in coarse, highly radioactive particles PWhich tend to fall rapidly while being carried along by the wind The cloud of a thermonuclear explosion risek highest levels of the atmosphere and spreads over miles in the firet hours. rapidly to the fundreds of square During this time the pafticles are being acted upon by the winds, including those up to 60,800 or 80,000 feet, which may vary greatly in direction and velogity at different heights. Particle size will affect the te of fall, and as the material descends through the rain cloud bedring levels the fallout may be slightly accelerated by rain or sno For a considerable distance around the point of detonation, radioactive particles will be distributed up-wind d cross-wind, as well as down-wind. this close-in The actual distance to which] OEARCHIVES -5- RIOT § Users. TN be. bo