CALCULATED GAMMA-RAY DOSE DISTRIBUTIONS IN A PHANTOM EXPOSED TO FALLOUT AND SIMULATED FALLOUT
RADIATION RESEARCH ASSOCIATES INC FORT WORTH TX FORT WORTH
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Gamma-ray depth-dose distributions in a phantom exposed to fallout and to simulated fallout were calculated by the Monte Carlo method. The phantom consisted of a tissue equivalent vertical right cylinder of 60 cm height and 30 cm in diameter. The center of the phantom was 3 ft 8 in. 111.8 cm above a smooth ground surface uniformly contaminated with 235U fission products. The energy and angle distribution of the gamma rays incident upon the phantom were taken from previous Monte Carlo calculations. The depth-dose distributions were found to be relatively insensitive to fallout age over the period investigated 1 hour to 9 days. The dose rate at the center of the phantom is approximately 65 percent of the free-field dose rate, while that at the lateral surface is approximately 80 percent. Except near the extremities, the dose rate along the vertical axis of the phantom varies at approximately the same rate with height above ground as does the free-field dose rate. Approximately one-half of the dose rate at the center of the phantom is from photons which have suffered previous collisions in the phantom. An additional calculation was performed of the depth-dose distribution within a phantom positioned at the center of a 4 ft diameter by 5 ft deep foxhole located in a 1.12-hr fallout field. The distribution in the horizontal midplane of the phantom was found to be quite similar in shape to the above ground case except for a sharper dropoff of the dose away from the lateral surface. As would be expected, however, the axial distributions is quite different from that found above ground.
- Nuclear Radiation Shielding, Protection and Safety
- Radioactivity, Radioactive Wastes and Fission Products