Calculation of Gamma-Ray buildup factors up to 100 MFP by the Monte Carlo method

The point-kernel method is an analytical calculation method used to evaluate gamma-ray radiation fields and is widely applied in the fields of radiation protection and shielding design. The buildup factor is a critical parameter in determining the accuracy of the point-kernel method. However, the standard buildup factor datasets are constrained by limited range and insufficient consideration of certain physical processes, which restricts the application of the point-kernel method. This study aims to calculate buildup factors up to 100 MFP based on detailed physical models and advanced variance reduction techniques. Extensive statistical checks are conducted to ensure that the results are strictly unbiased. Additionally, the impacts of physical models, geometric configurations, and dose types on buildup factors are further evaluated. When the detailed physical model is considered, the BUF shows a maximum increase of 523 times compared to the standard data, making the standard database no longer conservative. In the low-energy region of highly scattering medium, the impacts of geometric configurations and dose types on BUF are observed to reach up to 80 %similar to 90 %. Finally, the origins of conservatism in the point-kernel method are discussed. Validation and testing are conducted using a typical model on the data which exhibited significant differences from the standard dataset. The results indicated that the new data surpassed the standard dataset in both conservatism and accuracy.