Description of multilayered gamma-ray exposure buildup factors up to 40 mfp by the approximating model

An approximating formula recently proposed by the authors for γ-ray buildup factors of multilayered shields was applied for very thick shields up to 40 mfp. For this purpose, modifications were made to the model and the fitting method to improve the data reproducibility. The previous model was expanded so that it included both the plane-normal and point isotropic geometries. The verification test of the modified model was made for three materials; water, iron and lead. The separately published data of double-layered shields for point isotropic buildup factors calculated by EGS4 from 0.1 MeV to 10 MeV were used as well as newly calculated data at 1 MeV for the plane-normal geometry, and data for the point isotropic geometry of triple-layered shields at 1 and 10 MeV. The present formula generally shows a very good reproducibility of the multilayer buildup factors, even in case of very thick shielding problems. The observed error between the approximating description and the EGS4 data is 15% in the intermediate energy range, about 30% in the higher energy range, and 35% at 0.3 MeV. However, the error in the approximation reaches a factor of 4 in the worst case at 0.1 MeV. (author)