INTRODUCTION EXPERIMENTAL DETAILS The locations from which the samples were collected are shown in Figs. l and 2. One sample of thatch from the roof of a hut on the island of Rongelap was measured. The rernainder of the samples received were of twotypes: gummed paper and polyethylene fallout collectors. ! The gummed papers were cut from their cardboard mounts and each folded to fit the bottom of a 100-ml lusteroid centrifuge tube. These tubes were then placed in the gammaionization chamber and the decay rate measured, The samples from the polyethylene fallout collectors were centrifuged in preweighed 100-ml lusteroid centrifuge tubes. These tubes were then placed in the gamma ionization chamber and the decay rate measured. The samples from the polyethylene collectors were centrifuged to separate the liquid from the solid. The liquid volume was measured in a graduated cylinder, acidified with hydrochloric acid, and concentrated by evaporation to a volume of less than 75 ml. The only possible error introduced by the procedure would be the loss of any iodine that may have been present. The weight of the solid was determined ona semi-microgramatic Balance. -l- UNCLASSIFIED eee ee ee ae ee wre During Operation CASTLE, fallout samples were collected from areas immediately adjacent to and 180 miles from the Pacific Proving Grounds. These samples were returned to this laboratory for gamma measurements in a 4-pi high-pressure ionization chamber, A number of samples were retained for periodic measurements of decay rate. This was done to evaluate the range of the exponent in the equation At = Agt™*, where At and Ag are the radioactivities at time t and at zero time respectively, This paper presents the data and the calculated values of the exponent for fallout samples from the first four CASTLE detonations,