The records for the DD-593 during Umbrella (Figures 3.193 and 3.194) also represent a central transit. The ship was anchored with the stern toward surface zero and the port side toward the hot line. No movement was observed after the shot. The records are comparable to those obtained on the other destroyers. The valley occurring around 4.2 minutes corresponds to the passage of the base surge center over the ship. The minimum dose rates for all shipboard records of the central decrement are discussed in Section 3.3.2. The higher minimum dose rate recorded by the starboard GITR is consistent with the ship’s orientation, since this instrument would be more influenced by the inner edge of the base surge. The prolonged upwind surge transi. :s again probably due to superstructure turbulence effects. As during Wahoo, the bow GITR record fring surge transit is abnormally high, a fact which is further discussed in Section 3.4.3. The iater records (Figures 3.205 and 3.206) show increased dose rates due to waterborne sources from 60 to 84 mimres and again from 2.3 to 2.6 hours. may be due to radioactive foam. The earlier peaks The later peaks indicate a speed of 0.3 knot, which is similar toe the previously calculated speeds for white water. speed nearly three times this value. 3.4.2 Variation of Shipboard Dose with Position. The earlier peaks, however, indicate a Two means of extending the measurement of gammaradiation exposure to additional weather deck positions were attempted. Approxi- mately 20 NBS film packs (Section 2.2.5) were placed in specially designed holders aboard each of the target ships, as shown in Figures 1.12 through 1.14. The packs were placed exactly 3 feet above the deck and, in the case of film packs associated with GITR’s, they were mounted On a pipe stand so that the film would be exposed to the same gamma environment as the GITR detector itself. These film pack stations represented the first attempt to extend GITR measurements to other locations. The second extension consisted of a pattern of marked meter survey points within 3 feet of every film pack, augmented by approximately 30 additional marked points distributed over all important weather decks (Figures 1.12 through 1.14). As soon after the shot as radiological safety permitted, all marked points were surveyed with Cutie Pies (Model CP-3DM beta-gamma survey meter, Reference 103) calibrated ona Co* range within 6 days of the survey. At each point, four meter readings were taken exactly 3 feet above the deck with the meter probe pointed at 90° intervals relative to the bow. These survey readings were averaged to compensate for possible variations caused by the ship’s superstructure. The NBS film packs were also recovered at the time of this meter survey. Data obtained from all shipboard film packs together with the survey results are given in Tables 3.30 and 3.31. A control group of similar NBS film packs were exposed ona Co* range at times sufficiently close to shot time so that no specific correction would be required for latent image fading (Reference 49). The film packs recovered from the target ships were then interpreted by means of these control films (Section C.4). Although film pack doses show the usual relationship to the GITR cumulative dosesif the effects of superstructure shielding are taken in account, no relationship can be established between the survey readings and film pack doses. This lack of correspondence is probably due to the fact that the principal radiation exposure occurs during transit of the base surge while wind and washdown obscure any possible regularity in deposition. A detailed analysis of the film pack doses indicates that the ship’s superstructure has a detectable influence on the total gamma dose and thus probably on dose rate. Plots of the recorded film pack dose versus frame number give a characteristic curve shape for each ship regardless of ship’s attitude or distance from surface zero (Figures 3.207 through 3.212). The regularity of these curve shapes is definite evidence of superstructure effect. If the free-field radiation is assumed to be uniform (this assumption can be valid only for relatively small masses of radiating cloud if all evidence is considered), the dose received by film packs would be influ- enced by the unobstructed solid angle subtended at the film. Consequently, the approximate solid angle subtended by each major component of the ship’s Super structure was calculated for each film pack location, using the DD-592 plans and photographs. This simplified treatment makes no allowance for variation in the shielding charac284 i (Text continued on Page 311)) /=*