products. Much of the water condensed in the cloud gathering additional bomb products. As the particles fell they probably changed their con- position through reactions with atmospheric constituents, Their exact nature at the time of arrival at the earth's surface is not know. The measurements of the aerosols collected on the YAG's indicated small solid particles and larger liquid droplets, Results of the decontamination studies of the YaG's (Project 6.4) and special panels (Project 6.5) could be explained best by assuming a contaminant whose constituent radionuclides were largely in the ionic form, The gross fallout samples had little solid matter, In fact, the fallout was invisible both in the air and on the surfaces where it was deposited. nuclides was water soluble, 6.3 A large fraction of the radio- SITUATION F. UT One of the factors neéded to estimate the radiological situation in the fallout zone is the decay rate of the radiation field. This was determined by direct measurement on land areas wherever it was possible in moderate radiation fields, In other instances, it was estimated from measurements of the decay rates of samples collected in the areas of interest. The observed beta and gamma decay rate curves compared well with calculated curves based on the radiochemical composition of the samples, There was little difference in the decay rates of fallout samples collected at various distances after any single shot, This fact showed that fractionation was unimportant in determining the gross decay rate. Small variations were observed in the decay rates of samples from different shots. The decay rate changed considerably with time; at 60 days .after the induced activities had decayed to a negligible level,it achieved a relatively constant value consistent with the t-1-2 law, Radiochemical measurements on debris from detonations in CASTLE gave information on the shape of the fission yield curve, on fission product fractionations, on contributions of neutron induced radionuclides, and on the fraction of bomb in fallout material. The shape of the fission yield curve from these detonations was altered in respect to that from thermal neutron fission of 0°35, The valley of the curve was raised by a factor of about 20 while the heavy wing was raised by a factor of about 6 at mass 156, Fractionation of several fission products was found to occur. That of cr89 was the most extensive among the limited mumber of elements which could be studied. Neutron induced radionuclides were demonstrated to be very important contributors to the radioactive mixtures resulting from -he detonations, Most important were 0239-Np739, 0240, and 0237; Na24 was only a minor contributor, Ratios of amounts of the important induced radionuclides to amounts of fission products showed that at certain times the uranium and neptunium isotopes contributed as much as 50 per cent of the total beta activity. Since the energy of the Np gammas is low as compared with the average of those from fission products, its contribution to the total radiation field is less than indicated by the beta ratio. Values of fraction of the bomb falling out at certain locations were determined radiochemically. of Project 2.5a. These were useful in connection with the fallout studies 133