Significant Differences in Reported Gamma-Ray Buildup Factors

Significant differences have been observed between Goldstein and Wilkins (moments method) and ASFIT (anisotropic source flux iteration technique) buildup factors in the materials of high atomic number (Z) for 6- and 8-MeV gamma rays at depths greater than 10 mfp. Comparison has been made between the two, and quantitative differences are presented for tin, tungsten, lead, and uranium in the gamma-ray energy range of 3 to 10 MeV up to a depth of 20 mfp. It is believed that these large differences are a sequel to certain deficiencies in the Goldstein and Wilkins method of reconstructing the spatial distribution of the scattered flux in these cases. The closer agreement between the modified moments method values and the present results is cited.