In medical sciences, the individual ’s health can be controlled by determining the precise amount of body elements and taking care of probable changes in periods of time. The In Vivo Neutron Activation Analysis (IVNAA) has become the ‘gold standard’ method for the measurement of certain body elements which are unique constituents of body components. In this way the sample is irradiated by neutron so the elements of the sample (body) can be activated and the prompt gamma rays emitted from body are measured. One of the most important aspects in IVNAA is proper detection and prevention of neutron entrance to the detectors. If neutrons come into detectors they ’ll activate detector ’s elements so prompt and delayed gamma rays will be emitted and corrupt the measurement accuracy. To improve the measurement accuracy the time of neutron irradiation to the body must be increased, as a result the absorbed doze in body will increase. Designing a proper shield for the setup will prevent neutrons to come into detectors and the irradiation time may decrease. In this research, different compositions are studied for detector shield in IVNAA setup in order to choose the best composition for shield. To test the initial suggested shields which are chosen by the study of cross sections and librarian data, the real state is simulated by MCNPX code. The neutron source which is used in this setup is 241Am-Be, the phantom is soft tissue of human body and detectors are NaI. Among Boron and Lithium compositions which are the best elements for neutron absorbing, Lithium Oxide, Boron Carbide, Boric Acid and Boron trioxide are chosen. Finally 0.5 cm Boron Carbide is recommended for detector shield. This shield reduces 99.9% of thermal neutron flux and 40.9% of total neutron flux. Thereupon the background gamma rays decrease by the rate of 76% so the time of neutron irradiating and absorbed doze in body decreases in high degrees of 30%.

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