The exposure buildup factors (EBFs) have so far been calculated by computer simulations for point sources with spherical shields considering the effect of coherent scattering and with plane sources and stratified slab shields without effect of coherent scattering. In this thesis, the MCNP code (Monte Carlo N particle) is utilized to calculate the EBFs for normal plane sources in an absorbent monolayer and two layer environment of water and lead. Then the effect of the coherent scattering and fluorescence radiation are investigated. First, we will obtain the EBFs for energies range of 0.5 - 6 MeV under the geometrical conditions of the previous studies without coherent scattering effect. Then, considering importance of EBFs, we calculate them by taking into account all factors for range of energy of 0.1 - 6 MeV. All calculations of EBFs are done for monolayer and two layer environments. EBFS without coherent scattering are different in a single layer of water or lead as a low and a high atomic number environment respectively. EBFs increase as energy of incident photon decreases or thickness slab shield increases. Coherent scattering decreases the number of uncollided and increases the total number of photons reaching the detector and as a result, EBFs are increased. This effect is observed stronger in higher energies and weaker in lower energies. According to the MCNP results, the coherent scattering effect in water is not considerable for the energies above 3 MeV, and in lead is considerable for the energies up to 6 MeV. Along with the coherent scattering effect, the fluorescence radiations effects on the EBFs were investigated. The fluorescence radiations effect occurs in energies above 88 keV in lead. There are some considerable effects of the fluorescence radiations on EBFs, in massive elements like lead, below 200 keV energy. If the gamma ray energy is more than 88 keV, for 78.8% of photoelectric interactions in lead, an electron will be ejected from K layer and a fluorescence x ray with 78.53 keV energy is produced. These radiations are emitted homogeneously. For 0.1 MeV gamma ray, due to the production of secondary photons in the range of energy of the incident photons, EBFs are not increased as one expects. So the increase in EBFs due to the coherent scattering is considerably slowed down because of the fluorescence radiation. Kay words: MCNP , exposure buildup factor, gamma ray, coherent scattering, florescence radiation
