[Vou. 91 The ‘‘prithe fireball und. Some AFTER AVE ASED 1964] MARTIN : RADIOECOLOGY AND STUDY OF ENVIRONMENTAL RADIATION 291 into the stem by updrafts may be sueked into or circulated through and around the toroidal fireball. At some distance from ground zero (the pomt on the ground directly below the detonation), the ‘‘primary’’ shock wave is augmented bya ‘‘reflected’’ shock wave. This combined front, the ‘‘Mach’’ front, travels away from ground zero at about the speed of sound. Movinglike a wall of highly compressed air or a wave of increased pressure, the Mach front may produce dense, turbulent clouds of soil and other debris at some distance from ground zero. After the passage of the Mach front, ‘‘afterwinds’’ of hurricane velocity converge on ground zero and may carry more materials into the stem beneath the rising fireball. The fireball continues to rise until its temperature matches that of the surrounding air. The height to which the fireball and “‘cap’’ of the mushroom cloud may rise is dependent primarily upon the amount of thermal energy released by the detonation and on the altitude i00 MPH .L COOLS IND PRIMARILY N AT ABATED OR SOME 3 detonation above the surface at which the detonation occurs. Initially, the fireball contains only the fission products and other materials vaporized by the explosion. If it intersects the ground, tons of soil may also be vaporized and incorporated directly into the fireball. Considerable amounts of the material carried into the stem by updrafts and afterwinds mayalso be incorporated into the fireball as vapor-or as molten or solid particles. The condensation of particles oceurs during the rise and cooling of the fireball. Ordinarily, the whole visible cloud is dispersed by wind in less than an hour. The condensation of vaporized material in the fireball takes place in reverse order of boiling points (Triffit 1959). Since the amount of radioactive material in the fireball is quite small in comparison to the amount of other materials, most of it condenses in or on particles of metal or soil which are more abundant. Thefirst particles available for this purpose are probably metallic. These may serve as nuclei for the condensation of materials having lower boiling points, or they may collide with and be absorbed by molten soil particles. In the cooler regions of the fireball, solid soil particles may provide surfaces for the condensation or absorption of radioactive materials (Miller 1960). The precursors of some of the radionuclides in fallout are noble gases, and these do not becomeassociated with solid particles until after they have decayed to form a moreactive kind of atom. This happens, for example, in the case of Sr-90 which is produced by the decay of Kr-90. As a result of this “isotopic fractionation’’ process, radionuclides having gaseous precur- yud by the ireball. which is us carried sors are more likely to be associated with relatively small particles which are carried to the upper regions of the mushroom cloud. Particles formed in this mannerare likely to be more prevalent in worldwide than in local fallout. Jb