measured at a number of widely separated points for as many different sets of detonation con- ditions as possible. Because these quantities represent the end result of a complex series of interactions between device, particle, and meteorological parameters, additional relationships between them would not only provide interim operational guides, but would also be useful as general boundary conditions to be satisfied by model theory. 2. The particle-size distributions with time reported herein should be further assessed to remove the effects of background dust collections and applied to a more detailed study of particle size-activity relationships. For future use, an instrument capable of rapidly sizing and counting fallout particles in the diameter-size range from about 20 to 3,000 microns should be developed. Several promising instruments are available at the present time, and it is probable that one of these could be adapted for the purpose. While appropriate collection and handling techniques would have to be developed as an integral part of the effort, it is likely that improved accuracy, better statistics, and large savings in manpower could he achieved. 3. Controlled measurements should be made of the amount of solid-particle activity which penetrates to depths greater than the thermocline at rates higher than ~3 to 4 m/hr. Supporting measurements sufficient to define the particle size and activity distribution on arrival would be necessary at each point of determination. Related to this, measurements should be made of radionuclide fractionation with depth for both solid and slurry particles; in general, the solubility rates and overall dispersion behavior of fallout material in ocean water should be studied further. Underwater gamma detectors with improved performance characteristics and underwater particle collectors should be developed as required. Underwater data are needed to make more-accurate estimates from measured contours of the total amount of activity deposited in the immediate vicinity of the Eniwetok Proving Ground. 4. A formation theory for slurry particles should be formulated. Separation procedures should be devised to determine the way in which the total activity and certain important radionuclides are partitioned according to physical-chemical st.te. Microanalytical methods of chemical analysis applicable both to the soluble and insoluble phases of such particles are also needed. The evidence is that the solids present represent one form of the fundamental radio- logical contaminant produced by nuclear detonations and are for this reason deserving of the closest study. The radiochemical composition of the various types of solid particles from fall- out and cloud samples should also receive further analysis, because differences related to the history of the particles and the radiation fields produced by them appear to exist. 5. A fallout model appropriate for shots producing only slurry particles should be developed. At best, the fact that it proved possible to locate the fallout pattern for shots of this kind, using a solid-particle model, is a fortuitous circumstance and should not obscure the fact that the pre~ cipitation and deposition mechanisms are unknown. Considering the likelihood in modern war- fare of detonations occurring over appreciable depths of ocean water near operational areas, such a modelis no less important than a mode! for the land-surface case. It would also be de- Sirable to expand the solid-particle model applied during this operation to include the capability of predicting radiation contours on the basis of conventional scaling principles or the particle size-activity relationships given earlier. 6. Theoretical and experimentai studies of radionuclide fractionation with particle type and Spatial coordinates should be continued. This is a matter of the first importance, forif the systematic variations in composition suggested herein can be established, they will not only make possible more accurate calculation of the radiation fields to be expected, but may also lead to a better understanding of the basic processes of fallout-particle formation and contamination. 7, A series of experiments should be conducted to determine the true ionization rates and those indicated by available survey meters for a number of well-known individual radionuclides deposited on various kinds of terrain. Although the absolute calibration of all gamma counters and a good deal of logistic and analytical effort would be required, the resulting data would be invaluable for comparison with theoretical results. Also in this connection, the proposed decay Schemes of all fission products and induced activities should be periodically revised and brought up to date. 155