Every year too many people are treated with radiation therapy and nuclear medicine all over the world. Neutron sources are used for many applications, especially in medicine such as: neutron therapy and neutron imaging. There is a linear relationship between absorbed dose and development of cancer. So, assessment of radiation dose and its related risks before exposure to radiation is an important task in radiation protection. International Commission on Radiation Protection reported that direct measurement of absorbed dose in human body is impossible. Hence the absorbed doses should be evaluated by using the new scientific facilities (Monte Carlo simulations).In this study, the absorbed doses are computed for different body organs and the effective doses are calculated for whole body on ORNL modified adult phantom. Calculations are performed for neutrons in energy range 10-9-20 MeV, under six irradiation conditions: AP-PA-RLAT-LLAT-ROT and ISO. MCNPX 2.4.0 code was used for this simulation. The cross sections were chosen from the ENDF/B-VI libraries. Also, kerma approximation and S(,) scattering treatment(for light water at 300K) were applied. Effective doses were calculated according to the methods recommended in ICRP publication 60 and ICRP publication 103. All of the data estimations in this work have an uncertainty less than 0.5%. Absorbed doses were evaluated for 26 organs in monoenergetic neutron beams (10-9, 10-8, 10-7, 10-6, 10-5, 10-4, 10-3, 0.01, 0.05, 0.1, 0.5, 1, 3, 5, 8, 10, 12, 15, 18 and 20 MeV). The results show that the largest and the smallest values of the absorbed doses related to thyroid in AP (86.1 PGycm2) and spleen in RLAT (0.129 PGycm2), respectively. The graphs of the effective doses were also represented for all irradiation geometries. The obtained results are different in various irradiation geometries. These differences have been clarified, especially in AP geometry rather than other irradiation conditions. The results indicate that determination of the type of irradiation geometry and the energy range of the source for patients by radiologist is necessary.

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