In this thesis, by means of MCNP code (Monte Carlo N Particle) that is based on monte carlo method, we are going to study the dependence of exposure buildup factors (EBFs) on depth of penetration for plane normal source in two layer shields of Pb, Fe, concrete and water. The dependence of EBFs on atomic number and energy in monolayer shields is also studied. So at first we studied the effect of atomic number on EBFs at 5 and 10 mean free path (mfp) in energy 0.5 and 10 MeV. In the next step we studied the effect of energy on EBFs at depth 1, 6 and 10mfp. In last step we obtained the EBFs in energy range of 0.5 – 10 MeV in three stages. First we obtained these EBFs without the effect of coherent scattering and bremsstrahlung and then by considering the effect of coherent scattering and at last we obtained them by considering the effect of both coherent scattering and bremsstrahlung radiation. According to the MCNP results, coherent scattering is important only in lower energies and high atomic number materials and we can ignore the effect of this interaction for low z-material and energies higher than 3MeV, because the cross section of this interaction at low energies is high. Also from the studying the effect of bremsstrahlung we can find that this interaction is important for high atomic number and in high energies and we can ignore the effect of this interaction for low z material in energies lower than 3MeV. For example coherent scattering at 4mfp for energy range of 0.5MeV – 10MeV in Pb changes EBFs from 28.29% in 0.5MeV to 0% in 10 MeV. These changes in Fe are from 8.47% in 0.5MeV to 0% in 10 MeV, in concrete from 2.43% in 0.5MeV to 0% in 10 MeV and in water from 1.11% in 0.5MeV to 0% in 10MeV. The bremsstrahlung in previous energy range and depth of penetration in Pb changes EBFs from 0.43% in 0.5MeV to 95.27% in 10 MeV. These changes in Fe are from 0.18% in 0.5MeV to 24.42% in 10 MeV, in concrete from 0.14% in 0.5MeV to 7.62% in 10 MeV and in water from 0% in 0.5MeV to 4.5% in 10 MeV. Therefor when we plot the EBFs by diagram we can see that the coherent scattering is important just for energy 0.5-1MeV and in this energy range bremsstrahlung is negligible and by increasing the energy to 3MeV, the effects of both are similar to each other and when we increase energy further, bremsstrahlung will become more important and coherent scattering negligible. Keyword : MCNP, monte carlo, exposure buildup factor, coherent scattering, bremsstrahlung, gamma ray, mean free path.
