isa small drop in the peak yields accompanied by an increase in the symmetric fission probability. The game nuclide distribution might be expected in the fallout material, and this is found to be roughly true under certain conditions. In other cases, the elements formed initially partially separate with respect to one another so that samples of fallout may differ in composition aong themselves and also from the distribution curve characte ristic for the event. an ™ Fractionation is a term that has been applied to this phenomenon. It is used to signify alteration in nuclide composition of some portion of the debris that renders it nonrepresentative of the progucts as a whole. The R-values, which are commonly used for reporting radiochemical data on cloud and fallout samples, are useful indices of fractionation. The R-value for any nuclide ts defined aa the ratio of the number of atomsof thia nuclide to the number of atoms of a reference substance (usually Mo”) in the sample divided by the same ratio for the products of thermal neutron fission of u*§_ Atoms that do not separate from the reference substance have R-values appropriate for the type of detonation, while enrichment or depletion are manifested by positive or negative deviations from the characteristic value. Knowledge of the causes and mechanism of fractionation is still largely incomplete at the present time. One effect that seems to be indicated by the available data may occur in the iso- paric chains near mass numbers 90 and 140, which contain rare gas nuciides as prominent chain members. Because of their half-lives and independentfission yields, they comprise a considerable fraction of the total chain yield during the period when the environmental material is con- densing. If the rare gas atomsthat collide with the liquid drops of environmental material are not held by strong forces, as appears probable, the particles formed at this stage will be depleted in the nuclide chains in question. A variety cf types of particles have been observed in the local fallout at previous test series (References 8 through 13). For land surface shots in the Pacific they have been mainly of three kinds: irregular grains, spherical solids, and fragile agglomerated flakes. The grains were not, in general, uniform throughout but consisted of layers or shells of calcium oxide, calcium hydroxide, and calcium carbonate formed by the decarbonation, hydration, and recarbonation processes going on in the fireball and subsequently. The majority of them were white or transparent, but some were yellow or brown. Many of the flaky aggregates were observed to disintegrate spontaneously into smaller particles within a few hours after collection. In addition to these primary types, a fourth kind was noted consisting of smail black spheres of calcium iron oxide (2CaO-Fe,0;). These were usually observed adhering to the surfaces of the large grains but occasionally were found isolated (Reference 12). For detonations over ocean surfaces, the fallout collected consisted of droplets of salt slurry 50 to 300 microns in diameter. These contained about 80-percent salt, 18-percent water and 2-percent insoluble solids by volume. The major part of the radioactivity was found in the in- soluble solids portion. The fallout deposited at more distant points has not been as well charac- terized but is believed to be composed of minute spheres formed by condensation of the environmental material from the vapor plus a very fine, unfused dust swept up into the cloud from the area around the shot point (Reference 14). The availability of the radioactivity in the fallout for assimilation into the biosphere depends to a large extent on its solubility in aqueous or slightly acid media. Determination of the soluble fraction is therefore an important problem, and Solubility studies have been reported on fallout from several of the shats during Operations Castle and Redwing. For Castle fallout, it was found that the soluble fraction was strongly dependent on the detonation environment, being a- round 0.05 for land shots and 0.58 to 0./3 for shots fired from a barge (Reference 15). The solubility in seawater of the fallout from the reef shot (Tewa) during Operation Redwing was investigated in two ways: by leaching of particles placed on top of a glass wool column and by centrifuging a suspension of the fallout material (Reference 13}. The soluble fractions found by these two methods were 0.08 and 0.18, respectively. An ultrafiltration method was used for determining the solubility of fallout from the land shor (Zuni). About 25 percent of the total gammaactivity and Np**? were soluble in seawater, and 5 percent of the total gamma activity was soluble in rainwater.