the operetional success of the free-floating buoy station phase was sufficient to engourage the emrloyment of this fallout sampling technique at CASTLE Wy) IVY provided vaiuable data on the extent of the crosswind and upwind fallout and on the nature of the contaminant to be expected from the land surface detonations at CASTLE, 1.1.2 High Explosive Tests fallout. Six high explosive field tests have been conducted to study Charges varying from 250 to 50,000 1b of TNT were fired. Emphasis has been placed on shajlow underwater explosions.16/ Of a total of 38 stots, 26 were fired in shallow water; 5 in deep water; and 7 on land, both surface and underground. Non-radioactive cobalt and lithium were incorporated in the charges to trace the explosion products. Variables under study include energy yield, charge depth, explosion media, and wind. 1.2 JBJACTIVES The surface detonations of thermonuclear devices at Operation CASTLE were expected to produce significant fallout over considerable portions of the ocean at the Pacific Proving Ground. The primry pur- pose of Project 2.5a was to document these fallout areas and determine the militarily important radiation fields which would have resulted had all of the material been deposited on land. Specifically, Project 2.5a was designed to determine the following information for selected shots: a, Time and rate of fallout and final distribution patterns, b. Particle size ranges of fallout with respect to time and distance, c. Amount and distributionaf radioactive materials in fallout. d. Gross gamma decay rates, . The gathering of fallout data at CASTLE was & logical extension of previous fallout documentation. Variation in proposed yields as well as the opportunity to document surface water detonations for the first time made the study of fallout in this operation extremely important.