WORLDWIDE EFFECTS OF ATOMIC WEAPONS ORIGIN AND NATURE OF RADIOACTIVE DEBRIS would expect that the degree of fractionation of the fisston products would ’ tends to concentrate on or near the surface of the particles as if the radioactive material had been picked up after a solid particle of CaO had been formed. Particles of this kind probably represent large CaO fragments that were notin the fireball at the time of the initial condensation but were swept through the cloud later in solid form. The CaO that was vaporized and condensed along with the fission fragments is probably much too finely divided to be sectioned and examined in the manner used in the 20 change markedly with yield, but one could not say that the over-all fractionation would be either increased or decreased. We must take indi- vidual chains and study the effects of longer time before condensation. In the case of Sr’° we would expect less fractionation with respect to materials such as Ce™*’ and Zr”as the yield is increased, but perhaps more fractionation of Sr*° with respect to Sr°*. SURFACE OR UNDERGROUND BURST In the case of bursts where the fireball touches the earth, large quantities of the surface materials are swept into the resulting cloud. (This includes the tower in tower shots.) When the region is composed mostly of silicates, molten silica (SiO,) will form, which is then swept around through the cloud in the typical toroidal type of motion. Molten silica is an extremely good solvent for the metallic oxides and hence thesé matetials are dissolved and eventually solidified in the glass as the silica cools. These large particles appear to concentrate in the upper stem and the lower part of the mushroom. A large percentage of the total activity ts apparently captured by thesesilicates but falls out within a very short time. A small fraction remains as very small particles, perhaps even in molecular form for those substances that have gaseous precursors, gnd is airborne for long periods of time. This fine material represents a large fraction of the total fallout on the East Coast from bombs exploded in Nevada. Since silica glass remains soft down to 1500°K,it may collect small particles very efficiently for a long time after the explosion. This property, together with the tendency of the silica to react chemically with the metal oxides, explains the principal difference between the Nevada fallout and that from the Pacific Islands, where calcium carbonate predominates. ‘The carbonate materials are converted at relatively low temperatures to calcium oxide (CaO) and this may or may not melt or decompose, depending on the time at which it enters theOF Ut Chote are ner T= point, 2873°K; vaporization temperature, 3800°K; decomposition, 4000°K. ) In any event, CaO is a much poorer solvent for metallic oxides thansilica and hence would beless efficient in collecting small particles of condensed matter. In the larger particles formed by thetests in the Pacific, the activity aa study of the Pacific tests. This hypothesis could betested by measuring the specific activity of the calcium in such particles and comparing it with the specific activity of calctum in fine particles or even in total samples taken from the cloud after a few hours. The particles in the fireball should have large amounts of induced calcium activity, whereas those swept into the cloud later should have much fess. REFERENCES 1. Brewer, Leo, “Thermodynamic Properties of the Oxides and Their Vaporization Processes,”’ Chem. Ret, Vol. 52, No. 1, 1953. 2. Average value taken from Smyth, Atomic Energy for Military Purposes, Princeton University, 1946, p. 72. 3. Hunter, H. F.. anD N. E. BALtou, “Fission-product Decay Rates," Nucleontcs, Vol. 9, No. 5, November, 1951, 4. Ketiocc, W. W., R.R. Rapp, ann S. M. Gareneten, “Close-in Fallout,” to be published in the J. Mererol.