Calculation of buildup factors of gamma source with continuous energy spectrum for water, iron, and lead by Monte Carlo Code

The exposure buildup factors are important for determining the shield thickness of gamma and X-ray. Without taking this factor into account, the estimated thickness of the shield is less than what is expected which leads to an excessive received radiation exposure of staffs or patients. Given that the exposure buildup factor for the monoenergetic homogenous point sources have been calculated by others, the calculation of these factors for continuous energy spectrum source through the Monte Carlo MCNPX code has not been studied so far. In this study, we attempted to investigate the difference in the exposure buildup factor due to a continuous energy spectrum source compared to a monoenergetic source. Therefore, the exposure buildup factor of a continuous energy spectrum source with 1 MeV endpoint energy (a typical energy spectrum) and a 1 MeV monoenergetic source for the shield of water, iron and lead up to depths 10 mean free path is calculated. Significant difference was observed in the obtained values. So that, the relative difference between the exposure buildup factor of radiation from monoenergetic sources and a source with the continuous energy spectrum for 1 to 10 mean free paths of water, iron, and lead are 38% to 39%, 17% to 30%, and 8% to 50%, respectively. Accordingly, in order to calculate the shield thickness for a continuous energy spectrum source in comparison with a monoenergetic source, a significant distinction should be made.