the ine er er ace te- nm. AN hours, it is reasonable to assume the major portion of each sample was taken at that depth. Benthic organisms were collected in three bottom dredges south and east of Bikini Atoll at depths of 1,400 to 3,600 meters. A few living sponges, gastropods, brittle stars, and foraminifera were obtained. Bottom-dwelling organisms in Bikini and Ailinginae Lagoons were obtained by skin diving and bottom samplers. Surface organisms were sought with lights and dip nets at every night station. How- ever, few specimens were obtained in this manner in the EPG open sea area. Flying fish were numerous during the day but avoided the lights at night. Diurnally migrating fish also failed to cometo the night light. Other marine organisms were obtained by line or spear fishing. A series of seventeen samples of the ocean floor were taken around Bikini Atoll with a gravity coring device. The sampies consisted of smal] bits of coral, basalt, manganese nodules, coarse coral] sand, shell fragments, and fine globigerina ooze (fossils of minute organisms of the order Foraminifera). Several of the cores were lost because of the inability of the core barre] to penetrate hard sediments, or because of the corecatcher's inability in retaining the coarse sand found at some of the stations. In the latter case, only a few grains were saved for identification of the bottom sediment and these were insufficient for accurate radioassay. The bottom water trapped above the cores was filtered and evaporated for assay, the suspended sediment removed from this water was saved for analysis. © A number of samples of the Bikini Lagoon floor were collected with a snap type of bottom sampler. One sample of sediment was obtained from Ailinginae Lagoon. The samples consisted chiefly of fine coral sand and foraminifera mixed with fragments of coral and shells. Miscellaneous samples of plant and animal life from Site Nan at Bikini Atoll and Sifo Island at Ailinginae Atoll were collected for analysis and comparison with the marine organisms. Equipment was set up for collecting air~-borne particulate matter aboard the Horizon. A series of samples were obtained at various localities by drawing 1,000 liters of air through a Type AA millipore filter. The Horizon surveyed the radioactivity levels of the EPG over a 6-week period end- ing 15 May 1956, prior to Shot Cherokee, the first major nuclear event of the 1956 test series. rod 1.5 INSTRUMENTATION 1.5.1 Data Requirements. The determination of the ambient radioactivity required the measurement of the activity and the radioisotopic concentration in a large number of samples. In the EPG, the background measurement was made difficult by the wide variation in character of samples which had to be taken. Gross beta counting may be used for comparing the activity of samples containing many of the fission products. When beta counting is resorted to, for the identification of the isotopes present in a sample, a combination of gross counting, chemical separation, half-life measurements, and beta-absorption curves is required. In addition, beta counting requires preparation of thin samples andclose control over geometry and self absorption. These processes require techniques which are difficult to perform rapidly aboard a ship. Gamma counting, on the other hand, is less restricted by self absorption and thus gamma counting simplifies field measurements of large specimens by eliminating extensive sample preparation. The determination of the total gamma count over a known 17