Update to ANSI/ANS-6.4.3-1991 for low-Z and compound materials and review of particle transport theory

摘要

The ANSI/ANS-6.4.3-1991 Gamma-Ray Attenuation Coefficients and Buildup Factors for Engineering Materials Standard (herein known as ANS Standard), contains derived gamma-ray attenuation coefficients and buildup factors for selected engineering materials and elements for use in shielding calculations (ANSI/ANS-6.1.1, 1991). The current status of the ANS Standard is withdrawn due to the lack of any updates since its last publication. Since the last update of the ANS standard, more accurate codes for particle transport and cross sectional data have become available. As a result, this study was carried out to update gamma-ray buildup factors for iron and water as a function of source energy that are presented in ANS Standard by using ENDF/B-VI.8 photo-atomic cross-section library data in Monte Carlo N Particle 5 (MCNP5) version 1.40. Another code, Anisotropic Source-Flux Iteration Technique (ASFIT), was used as a benchmarking tool to validate the MCNP5 results. The results from MCNP5 did vary from those of the ANS Standard with the greatest differences for iron and water being 10.48% and 13.83%, respectively. These differences can be attributed to the following: (1) Differences in cross section libraries; (2) Method of solution for the codes; (3) Physics Assumptions; (a) Bremsstrahlung; (b) Coherent Scattering (4) Calculation methods; (5) Standard deviation.To obtain buildup factors for points not calculated, fittings were calculated using a fitting function technique. After the buildup factor for each source energy was plotted as a function of mean free path (mfp), the data was fitted via Microsoft Excel. The fitting method chosen for each was a polynomial of the sixth order to ensure the highest accuracy and closest to 1 value for the regression coefficient, R 2 . Tables and figures representing energy absorption buildup factors and fitting functions are attached to this study.