III. EXTERNAL ENVIRONMENTAL RADIATION ON BIKINI ATOLL External gamma radiation levels were found to vary considerably from island to island around the atoll. Typically, the levels on a given island ranged from very low near the lagoon and ocean shores to much higher near the center of the island. Hardly any natural radioactivity was detected at any of the field spectrometry measurement Sites or in any of the soil samples. This lack of natural radioisotope content was nct unexpected since the soil of coral atolls consists primarily of CaCOs. The isotopes contributing to the gamma radiation field varied considerably from island to island. On islands close to blast sites such as Eneman and Aomen-Iroij E° C0, +25 sn, and *5£ hu were predominant while on Bikini and Eneu islands the major emitters were *°’Cs, §° Co, *®8sp, and *°* Bu. ®°Cco was present in fair amounts everywhere, probably as a result of the many tests on steel barges in the lagoon. On several islands close to blast sites considerable quantities of rhodium isotopes were detected. Other isotopes such as *44 Ge, “5? my, °2n, and 2°7 Bi were also occasionally detected (by Ge(Li) spectrometry) in some of the soil samples. “4~am was detected in all the soil samples in varying quantities indicating the expected presence of its parent, 239 py. In addition to the gamma-ray exposure rates discussed in this report, the free air ionization and also the exposure to any potential residents would be increased by the contribution from f-ray emitters. Because of the fairly large *®7as concentrations found on many islands, one would expect correspondingly high #°sy concertrations, and °°sr would be by far the most important B-ray emitter present. Based on the measured *“°’Cs concentrations and radiochemical deter- | minations of ®°Sr concentrations (see Table 2) in a few of the sampled soils, ®°°sr concentrations appear to range from about 100 to 2000 dpm/gm for the first inch of soil as compared to corresponding concentrations on the order of 1 dpm/gm in the United States. These concentrations might increase the free air exposure by as much as several hundred