Calculation of point isotropic buildup factors of gamma rays for water and lead

Exposure buildup factors for water and lead have been calculated by the Monte-Carlo method for an isotropic point source in an infinite homogeneous medium, using the latest cross sections available on the Internet. The types of interactions considered are: photoelectric effect, incoherent ( or bound-electron Compton) scattering, coherent ( or Rayleigh) scattering and pair production. Fluorescence radiations have also been taken into account for lead. For each material, calculations were made at 10 gamma ray energies in 40 keV to 10 MeV range and up to penetration depths of 10 mean free paths at each energy point. The results presented in this paper can be considered as modified gamma ray exposure buildup factors and be used in radiation shielding designs