Several studies have been conducted to measure different type of gamma ray buildup factor for its importance in measuring the dose resulting from Electromagnetic rays use and its shielding In this study gamma ray buildup factor for two shielding materials water (Zeff =7.42) and graphite (Z=6) within the energy range 4-10 MeV and up to 5 thickness mean free path has been studied. To achieve this study a simulation computer program has been written (Visual Basic language version G) and applied depending on Monte Carlo simulation method.The basic idea of the program includes real radiation behavior description and prediction of its random movement through the media. This method can be used to simulate a traditional problem (classic) resulting from the fall of the beam on the flat slide works as attenuator for gamma rays. The radioactive source geometry adopted in the study is mono energetic normal plane source In this research the contribution of annihilation effect on the calculation of gamma ray dose buildup factor has been studied in the same energy and within the studied energy range. This study also examining a number of variables related to the program design called simulation variables like number of gamma scenarios, number of divisions for energy. This research is also including study the effect of energy for the incident photon, shield thickness, and atomic number for shielding material on the calculation of gamma ray buildup factor. The results indicated that the gamma ray dose buildup factor is inversely proportional with energy increase. This behavior means that when the energy within the studied energy range is increased, the probability of Compton effect decrease, since the Compton effect is inversely proportional with energy. Second, the penetrating ability for gamma radiation is also increase and this leads to an decrease in the probability of scattering with small angels and finally it is reflected on the calculation of dose buildup factor (the scattering with small angels plays a very important role in the calculation of buildup factor).For both graphite and water, although the results indicated that the calculated values of dose buildup for plane source when the contribution of pair production is ignored, less in comparison when the contribution of pair production is taken in consideration but the contribution of Compton effect is still more predominant than the effect of annihilation within the range of energy (4-10) MeV.The results obtained from the performed Monte Carlo simulation program in this study are very closer to the published experimental results (error = 10.4%) in comparison with Monte Carlo simulation results (error = 40.58%) obtained from some published researches. The comparison with experimental result is the best way to judge on the accuracy of the results obtains theoretically.
