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Monte Carlo procedures were used to calculate reduction factors for an upright cylindrical concrete barrier with a 10-ft radius exposed to gamma rays from infinite and finite plane 60Co sources. The factors were calculated for detector heights of 3, 13, and 23 ft and for barrier thicknesses of 20, 40, and 80 psf. Comparisons with reduction factors calculated using the 'engineering method' (NBS 76) showed that the two methods agreed within 10 percent for the infinite plane source while for the finite sources they differed as much as 45 percent. Angle distributions of the various exposure components inside the barrier were used to calculate directional response functions for both wall-scattered and non-wall-scattered radiation. Comparison with corresponding data given in NBS 76 showed large differences in some cases. The Monte Carlo data indicated that separate directional response functions should be used for radiation coming from above and from below the detector plane. However, substitution of these and other quantities derived from the Monte Carlo results into the engineering method generally did not improve the agreement with the reduction factors calculated directly by Monte Carlo. It was concluded that any errors in the engineering method data and assumptions must tend to compensate one another, and that similar studies must be made of other aspects of the engineering method in order to fully understand these implications. (Author)

A study was made of four models for calculating the effect of ground roughness on the gamma-ray energy and angular distribution in air above a fallout field. One model, called the 'buried-source model', assumes that a uniform thin layer of soil covers the fallout with the thickness of the layer corresponding to the degree of ground roughness. Another, called the 'mixed-source model', assumes that the fallout is mixed with a thin layer of soil. A third model, called the 'collimated-source model', assumes that the fallout is deposited on a smooth ground surface but that the gamma-ray emission is constrained to certain angular sectors. The last model, called the 'furrowed-surface model', assumes that the fallout is distributed over a ground surface which has uniform concentric circular furrows. Calculations of the energy and angular distributions of the uncollided and the singly scattered fluxes three feet above sources with one or more energies were performed using each of the models. Comparisons with measured data and consideration of the convenience of application indicate that the buried- and mixed-source models are preferable to the other two. The results indicate that the magnitude and the angular distribution of the uncollided flux is strongly influenced by ground roughness. The singly scattered flux, which can actually be increased by ground roughness, is much less sensitive to ground roughness than is the uncollided flux. (Author)