Measurement of body composition by total body in vivo neutron activation analysis is providing a valuable tool for clinical research in many areas of study. Changes in body composition may be diagnostic for the progression of disease or the results of therapy. Prompt γ-rays in vivo neutron activation analysis (IVNAA) has been widely used in recent years. Three major factors in this method are: 1) proper detection which leads to an accurate measurement 2) receiving low dose by the patient and 3) safety of facility for operator. As neutron sources are mostly mixed with γ-rays and these particles are very penetrating, the leakage is remarkable without proper shields. Thus, one of the important features in this method is optimization of shields to reduce operator receiving dose. Not only, must an appropriate shield reduce the operator receiving dose, but also it must have the less effect on detected spectrum. Because all parts of setup can be activated, the emitted γ-rays may be counted in detectors and increase background. So the selected shields may affect on gamma spectrum. In this research, several shields have been considered for an IVNAA setup. It has been tried to choose the best shield which have a low effect on detector spectrum and cause lower dose for operator. Four different shields (concrete, Epoxy colemanite resin, paraffin borated with Bismuth layer (PE-Bi layer) and paraffin borated with fine pieces of Bismuth (PE-Bi)) have been simulated by MCNPX code. The PE-Bi shield decreased the absorbed dose to 86% in compare with no shield state and 81% in compare to concrete. Also the rate of equivalent dose is reduced 97% in contrast to no shield and 94% to colemanite. The annual equivalent dose for an operator who works 8-hour a day, would be 4.04 mSv. The neutron flux decreases 100 times in the presence of PE-Bi while it has the lowest background in γ-spectrum among other suggested shields.

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