Development of New Gamma-Ray Buildup Factor and Application to Shielding Calculations

Abstract

The PALLAS (discrete ordinates-integral transport) code was improved to include secondary sources, such as bremsstrahlung and fluorescence, to assure accurate and reliable results. The point buildup factors for high-Z materials were calculated with this code in the energy range of 0.015~15 MeV up to 40 mean free paths. The buildup factors for low-Z materials in the low energy range, which are most difficult to calculate, were calculated with PALLAS and were validated by comparison with the results of the EGS4 (point Monte Carlo), and ANISN codes. A function to calculate coherent scattering was added to the EGS4 code. It was suggested that neglecting coherent scattering and assuming free-electron Compton scattering can result in an error in the buildup factor. A fitting formula, the geometric progression (G-P) form, was developed to reproduce the data in codes used for shielding design. This formula can reproduce the data over the full range of distance, energy and atomic number within a few percent. The above cited buildup factor data and the G-P formula have been adopted for proposed standard ANS-6.4.3 “Gamma Ray Attenuation Coefficient and Buildup Factors for Engineering Materials”. The G-P fitting function has been implemented in the CCC-493/QAD-CGGP and CCC-494/G33-GP codes available from the Radiation Shielding Information Center (RSIC) at Oak Ridge National Laboratory. These are used for γ-ray shielding calculations throughout the world. Also, the data and the method of evaluation for 1 cm dose equivalent, introduced into the Japanese law according to ICRP recommendations, have been offered by this team.