DNA 1240H-2 radiation from the water. However, it is estimated that the contaminated water patches would still represent n real hazard to small craft ae late as 1.5 hrs after burst, unless the patches were dissipated as a result of wind and wave action. After the first half hour, the decrease in dose rates in the contaminated water results because the radioactive particles are not concentrated in a mass on a flat surface, but are distributed at dif- ferent depths in the water and tend to disperse with the current, and because water is an extremely effective shield for gamma radiation. The half-value thickness of water (the thickness that will absorb half the gamma radiation incident upon it) for gamma energies characteristic of mixed fission products may be determined roughly by the equation 1/2 = ery then 0.693 nlEy) xt = Ky = half-value thickness (in em) of water % cm Ey = gamme-roy energy, which may vary between 0.5 and 2.0 Mev u(Ey)= linear absorption coefficient, which lies between 0.097 cm=? and 0.049 cm7+ for water, for 0.5 and 2.0 Mev respectively. The value of x)/p then lies between 7 and 14 cm, and thus, only a few ft of water will most effectively eliminate the gamma radiations of radioactive particles suspended in the water. Theoretical calculations have also been carried out 7 to determine the shielding effectiveness of an aircraft carrier to waterborne rad- *‘€ation sources. These calculations indicate that not only is the ship shielding highly effective, but also that the radiation from the water is negligible compared to other sources of radiation, even at times as early as 70 sec after burst. Further calculations ) indicate that consideration of radiation from waterborne activity is of academic interest only, because of the minor operational importance of the hazard from such activity aboard combatant ships. For example, computations were made of the percent of the in-situ water dose rate that would exist under worst conditions in a carrier. Results indicate that, assuming uniform activity distribution in a semi-infinite volume of water, this fraction would be only 8% of the in situ dose rate at a location next to the hull and just above the armor belt and waterline. Combining results of theoretical calculations with weapons-test data on dose rates from waterborne activity reaffirms the conclusion that negligible radiation from waterborne sources would penetrate combatant shipa later than 1 hr after burst. 17-104