Chopter 2 DESIGN OF THE FIELD EXPERIMENT For the survey following Shot 5, the ATF-75 (Sioux) was hurriedly fitted with hydrographic gear and with improvised radiation detectors cap:ible of being towed and lowered vertically to a depth of 250 feet into the sea. Between H + 6 hours and D + 4 days an 800-mile traverse of the suspected downwind area was made with sections taken near radii 30, 50, 100, 150, and 200 miles. Hydrographic casts were made at stations evidencing distinctly active water; water samples were taken to depths as great as 2,400 feet. Surface-water samples were collected frequently along the traverse while the ship was underway. The survey following Shot 6 included taking surface-water samples from the sea in the downwind area and readings on T1B survey instruments. During this survey, 120 water samples were taken by the crews of two Task Force ships and completed at 0530, 16 May, and consequently it presents a good synoptic picture. Coverage was out to approximately 135 miles north of zero. A simultaneous aerial survey with gamma-detecting instruments was conducted by the New York Operations Office, AEC. 2.1 RADIATION MEASUREMENTS The underwater radiation measurements for Shot 5 were made by sealed Geiger instruments which were either towed or lowered to various depths at definite points in the area. In order to assure that a record was made of regions of intensity beyond the recording capacity of the submerged GM instruments, a rough monitoring device, termed the “pot” was suspended overthe side of the ship to record these high intensities. The pot was a standard ionization-chamber-type radiac set fitted in a steel tank having a gasketed lid. This steel tank was mounted on the grid floor of the hydrographer’s platform 6 feet above the sea. Wire leads from the radiac set in the tank carried its output to a microammeter located on a part of the deck sheltered from the spray. The pot was set permanently on a scale of 0 to 50 mr/hr and wasread every 5 to 20 minutes without resetting its drift. This surface monitoring was continued throughout the radiation survey in a relative sense rather than indicative of absolute intensity of radiation; however, the readings are valuable. 2.1.1 Instrumentation. The radiation measurements were made with three improvised underwater Geiger tube instruments. These were designated the Mark I, Il, and II; they were hurriedly assembled from the parts and materials available at the forward area; none but essential details were put into the construction. The Mark I was made by rewiring a standard Victoreen radiac Geiger counter so that it and all its appurtenances except the microammeter would fit into a cylinder. This cylinder was about 30 inches long and was made of seamless steel tubing having an outside diameter of 4 inches and a wall thickness of yy, inch. One end of the tube was closed by brazing a disc to it, and the other end was fitted with a flange to which was fastened a gasketed cover. A piece of heavy-duty rubber-covered portable-tool “cord” about 200 feet long connected the counter with its microammeter which was located in a sheltered spot on the deck. This cord also served as the towing cable for the Mark I. 17 A pressure-