the 3-muinute cumulative doses are used (Table 3.23). The possibility of radiation due to wnite water cannot be excluded from the closer stations; therefore, the calculated depth of the underwater detector in these instances must be regarded as a minimum possible depth. These depths are calculated by modifying the expression for the radiation intensity at a point above an infinite slab of uniformly distributed activity. In this case the radiation intensity at a depth beneath the water surface Iuw? due to a radiating cloud above the surface, is expressed by: law Where: = Jad [EE] a qj ° -Ewd RWS _ -El (— uyd bw [-El( pwd] Ja = source uitensity per umt volume of the cloud Ma = linear attenuation coefficient for air linear attenuation coefficient for water By K = a constant approximating the buildup factor in an expression of the form (1+K yd) yd = the pach length in water expressed in units of mean free path for gamma rays of a Stated energy d = depth of the detector below the water surface. The radiation intensity at the interface I, is given by the expression: Ja (1+K) 2A Thus, the ratio between I, and [, is: Taw = eT HWe BW TK Bi (wa) Ss +K Values of this ratio have been calculated for a 1-Mev gamma energy and are plotted as a function of depth in Figure 3.138. These values were used in conjunction with the 3-minute cumulative dose figures to estimate the depth of the underwater detectors given in Table 3.23. They indicate that the underwater detector bobbed up to or near the surface, probably because of the action of device-generated water waves, aithough there is Some suggestion that a combination of current and normal wave action may occasionally have brought the detector near the surface at later times. After analysis of the data, this hypothesis was experimentally verified. This behavior had not been previously noted, since the detectors were dropped into the water only after zero time, and coracle recoveries after Wahoo were performed principally by nonproject personnel. Because of the particular nature of both events, this attitude of the underwater detectors does not vitiate their records; in fact, this occurrence permits checks on the std-GITR records, which would not have been possible had the underwater detectors dropped to their planned depth. Radiation due to waterborne material other than that deposited from the base surgeis discussed here, since these phenomena are definitely a part of the total gamma records. The gamma records show evidence of two such sources, viz, (1) radiation due to water directly contaminated by the device (white water) and (2) radiation due to patches of radioactive foam. During the early recovery of the target ships about 2 hours after Umbrella, a patch of radioactive foam, which pinned a survey meter set for a maximum rate of 50 r/hr, was observed by Project 2.1 personnel. Although this report represents the only direct observation of radioactive foam, it seems probably that such a waterborne source would be generated both by the collapse of the column and by the violent upwelling of water immediately after the detonation. The presence of spikes in the std-GITR records, after the passage of airborne material 247