where D, is the deposit dose rate at some distance d, D, is an intercept constant representing a virtual deposit dose rate at zero distance, and a is a constant representing the decrease in deposit dose rate with distance. For the data presented in Figures 3.32 and 3.33 these constan:s are: a = 3.6x 107 ft7', D, = 5,400 r/hr at 1 minute for Wahoo, and D, = 260 r/hr at 1 minute for Umbrella. These plots cannot be considered in themselves sufficient evidence for an exponential decrease of deposited material with distance, but they do indicate that there is es- sentially no significant deposition beyond 15,000 feet for Wahoo and at 11,000 feet for Umbrella. The value of Dy for Wahoo is 21 times that for Umbrella, which could be caused by the com- bined effect of a heavier Wahoo deposition and the higher general background for all Wahoo samples. Total depositions over the shot arrays estimated from IC collections do not, however, indicate a significantly greater deposition after Wahoo. Finally, the few data points along upwind and crosswind radii show an abrupt decrease in deposit dose with distance as might well be expected. The sum of IC collections attributed to base surge plotted against distance from surface zero (Figure 3.34) shows even wider scatter yet a similar decrease with distance. A straight line with nearly the same slope as that obtained from the GITR background data may be drawn through the Wahoo points; the slope and intercept values are: @ = 3.0x 1074 ft7' and D, = 1,100 r;hr at 1 minute. The fact that this line appears to fit the Umbrella data points is undoubtedly coincidental. If the close-in Umbreila stations are ignored, the Umbrella IC collections show little tendency to vary with distance from Surface zero and are again about a factor of 10 lower than the Wahoo collections. In the case of Station U 2.7 on Umbrella, the very large IC collection suggests that this sta- tion. was involved in primary throwout (Appendix F). The additional fact that the early collections made at this station exhibit a decay curve that is characteristic of depositions accompanied by large amounts of water would seem to bear out this assumption. Photographic evidence for Umbrella is poor, but analysis of Wahoo plume trajectories (Figure 3.35) would indicate that the maximum throwout radius for that shot is about 1,800 feet. The possibility of a greater throwout radius existing for Umbrella appears unlikely; therefore, this explanation for the large collection at U 2.7 is not acceptable. Collections made abdard the DD-592 platform also indicafe such a heavy deposit accompaniéd™ by large amounts of water (Section 3.5.2). Reference 90 postulates that heavy rains of short duration fall from the base surge soon after its formation, the exact time being dependent upon the average size of the original base surge droplets. Although the time of both observed depositions is much earlier than that calculated in Reference 90, it is still possible that these two Stations were exposed to such rainfall if original droplets of 20 to 50 microns are assumed. On the basis of these two records, heavy rains possibly accompanied by large amounts of radioactive material may be postulated to distances of 3,000 = 500 feet. Such heavy deposition of water would probably cross-contaminate (Appendix F) the uppermost IC trays, which mayaccount for the fact that these IC collections appear to have continued after passage of the base surge (Figures 3.48, 3.61, and 3.62). If such deposition actually did occur, the meter survey and the GITR background data indicate that the majority of this material washed off the coracle decks, probably within a short space of time. The IC trays were counted in an end-window gamma counter, consisting of a 1'4-inch-diam- eter Nal thallium-activated crystal, % inch thick, mounted in a lead shield (Technical Associ- ates Lead Shield, Model LS-6) with appropriate photomultiplier and scaling circuits (Nuclear Instrument and Chemical Corporation, Model 162 scaler backed up with a Model 182 scaler). The crystal was shielded with 4 inch of aluminum to eliminate all contributions from beta activity. All trays were counted on Shelf 5 (distance: 84 mm from shelf support to bottom of cryStal); the efficiency for gamma 0.7-Mev photons is approximately 0.35 percent for this position. The efficiency of the crystal system for a given sample can best be determined empiri- Page 108 deleted. ™~ Yy 107