Abstract: Using the Monte Carlo simulation code Geant4, Energy absorption buildup factors in water under the infinite medium model and the infinite slab model were both calculated. The results of the two different kinds of buildup factors were compared and analyzed, and the quantitative relationship between them was established. The relationship is not influenced by the photon energy and mean free path (MFP), and the buildup factors under the infinite plate model can be calculated by using the corresponding values under the infinite medium model through a few parameters, which can effectively reduce the errors caused by the different models in the actual calculation and improve the practicability of the buildup factors. The work in this paper can provide reference for the related research in other materials.

Key words: buildup factor, slab model, infinite medium