Body Composition analysis (BCA) by in vivo elemental measurement has proved considerable value to clinicians over the last forty years. Most of the works in this field has been performed by Prompt γ-rays in vivo neutron activation analysis (IVNAA) technique due to its “gold standard” to determine certain chemical body elements. The IVNAA facilities have produced a significant amount of knowledge in studying diseases, such as osteoporosis, obesity, AIDS, cancer, anorexia, renal disorders, and aging. To prevent receiving the dose from γ-rays produced in system by patient, an efficient strategy is to apply γ-shield. In previous publication, absorbed dose, dose equivalent and sensitivity factor were considered for cubic water phantom.The Monte Carlo simulation of BCA facility using MCNPX 2.4.0 code was performed. The results determine the optimum thickness and γ-shield composition. In this study, the γ-shield is described in three separate positions: 1) top & bottom of the patient, 2) around the 241Am-Be neutron sources and 3) latter item together with covers surrounding inner collimator walls. Then its influence on reducing effective dose equivalent is examined for 5 year old ORNL phantom. With regard to effective dose equivalent, the optimum position is 6cm thick tungsten layer on top & bottom of the patient. Gall bladder has a maximum amount of dose equivalent among all tissues in no shield state and then breasts and lungs receives maximum dose equivalent. However, when the W shield is added to free shield IVNAA facility, breasts and lungs receives maximum dose equivalent and gall bladder is in second order. However, as the importance of received dose of radiation to make the most common cancers, we are considered to the percentage of reduction of dose equivalent in lungs, gonads, bone marrow and bone surface. Decreasing dose equivalent on these organs with optimum shield is 66.47%, 65.83%, 65.57% and 65.48% rather than without it, respectively. Finally, total effective dose equivalent in no shield state is 3.17×10-2 ± 2.14×10-4 mSv/min. This parameter decreases about 66.25% in comparison with no shield state.

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