Doves 7 samples were also collected in test compartments following Shot Umbrella, © and analysia of the samples indicated that 90% to 95% of the activity in the samples was due to particles with radii of less than 1 micron. It was demonstrated at previous tests that washdown is very effective with small particles carried in an invisible mist, and washdown was opersting on the test ships at both shots. Thus, it is possible that had wash- down not been opereting, radioactive material might have been deposited and remained on the weather decks of the test ships. No data were obtained at distances such as 10 to 20 miles from surface zero, to permit "scaled" comparison with data from the barge shots of previous operations. ration Wi : Tne YAC-39 encountered an invisible cloud of airborne radioactive material between H + 16 and H + 19 min. Residual contamination wae left on the ship, but decay and the washdowm system reduced the radiation levels quite rapidly, so that at H + 1 hr, the average gamma dose rate on the weather deck was about 9 mr/hr. The YAG~LO avoided the "cloud," and made numerous traverses of the contaminated area on D and D + 1 days, but encountered no fallout. It was estimated that very little residual activity remained on the hull of the ship. 2! 17.6.3 Weapons-Test Data for Shielded Locations 1. Water-Surface Burste Operation Castle: A study was undertaken to obtain data on the effectiveness of ships’ structures in shielding interior compartments from gamma radiations during and after a contaminating event. Data for this study were obtained from Shots 2, 4, and 5, and the recorded dose and dose-rate values at exterior and interior locations on the test ships are presented graphically in Chapter 2 of Ref. ©. Results of analysis of the data, presented in Chapter 3 of Ref. 40, indicate that the shielding factor (the dimensionless ratio of the dose rate or dose within a compartment to that measured above the weather deck) at locations between the 2nd deck and weather deck were in the range from 0.1 to 0.2 on YAG 40, and from 0.15 to 0.30 on the washdown-protected YAG 39. In superstructure compartments on both ships, the shielding factors generally were in the range from 0.1 to 0.6. It was pointed out that the shielding factor actually represents shielding from all sources of radiation ~ transit, deposit, and water-borne. However, it was concluded that “shielding factors on the YAG-hO are believed to be a good approximation to the shielding factors for activity deposited on the deck surfaces." ration Redwing: Dose and dose-rate values recorded at exterior and interior shipboard locations for the Operation Redwing shots indicate the extent to which the ships' structures attemated the gamma radiations emitted by radioactive material surrounding am deposited on the ships. The dose to 30 hr in the upper No. 2 hold of the YAG 39 was BEST AVAILABLE COpy 17-75 ~ wm neem em