CHARACTERISTICS OF RADIOACTIVE PARTICLES IN CLOSE-IN FALLOUT M. W. Nathans LFE Environmental Analysis Laboratories Richmond, California ABSTRACT The author is in the process of systematizing available data on a number of properties of fallout particulates from cratering events and from surface and near-surface bursts in order to provide a better understand- ing of the properties of the radioactive soil at the Test Sites. In this paper, the results of the work through January, 1978, are presented. Included are the results of observations on shape and color, and some of the systematics of radionuclide concentration in its dependence on the particle size over a size range extending from about 1 um to more than l mm. Plutonium data are very scarce, and the behavior of plutonium is inferred from the behavior of gross radioactivity. For a better understanding of the fallout characteristics of particles from different types of bursts, a general discussion of relevant phenomenology precedes the presentation and discussion of the data. INTRODUCTION The characterization of fallout particles containing transuranic elements and present in the soil of the Nevada Test Site is important to the achievement of the objectives of the Nevada Applied Ecology Group (NAEG). The first step was the preparation of a document containing an overview of the available data on the physical, chemical, and radiochemical properties of fallout particulates, and of the methods by which these data had been obtained (Nathans, 1976a). The essential contents of this document were presented at the Gatlinburg meeting in October, 1976 (Nathans and Leventhal, 1977). Among the conclusions reached were that many of the available data remained uncorrelated, and that they did not provide 549