A varying decay of this type is consistent with the presence of Np-239 (tia = 56 hr) and U-237 (ty = 160 hr), which are both generatedin significant quantities from neutron capture in uranium. After several half-lives, when the presence of these two radioisotopes no longer dominate the decay rate, it approaches the traditional t}.2 value. In the absence ofradiological survey data, the time-dependent decay rate is used in reconstructing the radiation environments on the ships covered in this report. Generally, radiological data on the residence islands of Enewetak and Bikini support a t!.5 decay rate between 48 and 480 hours after detonation; shipboard data indicate slightly greater decay rates (t-!-§ to t-!-9) during the same period. The steeper shipboard decay rates can be attributed to a combination of the increased effectiveness of "weathering" on a ship's surfaces (as opposed to island soil), and to decontamination being carried out onboardthe ships. The topside radiation environment was perturbed when a ship encountered contamination in addition to the fallout on its deck. Some of the ships considered in this report serviced vessels that had remained in heavy primary fallout. Mere proximity to such "hot" vessels raised the topside intensities and thus contributed to the dose of typical crewmembers. Determinations of intensity of the shine from proximate ships are based on the geometries of both vessels and radiation transport calculations that are further discussed in the Appendix. Similar . te techniques are used to adapt island intensity curves for shipboard use, as required. SHIP CONTAMINATION MODEL. The water in Bikini Lagoon became contaminated following the five detonations conducted there. As ships steamed or anchored in the contaminated water, radioactive materials began to accumulate on the hulls below the waterline and in the saltwater piping systems within the ships. As a result, radiation intensities below deck began to increase, adding to the crew's exposure. However, when comparedto the topside radiation environments resulting from Shots BRAVO and ROMEOfallout, this radiation was "considered more of an operational nuisance than a hazard” (Reference 7). The same phenomenon was observed on the ships at Operation CROSSROADS conducted at Bikini Atoll in 1946. A model was developed in Reference 4 to determine personnel exposure aboard the ships at CROSSROADS due to ship contamination. Although only limited lagoon water contamination data have been found for Operation CASTLE, waterintensities are derivable from nearby land measurements; thus, this model is applied to all of the ships participating at this operation. il