The analysis of trace body elements with neutron activation method called IVNAA, for practical use must have the gold standard. So, the dose received by the patient body from non-essential beam such as undesirable gamma rays must be reduced.Because the total dose is proportional to exposure time and exposure time is proportional to the thermal flux, the ratio of thermal neutron flux to dose per one neutron emitted from the source is defined as device sensitivity. Therefore increase the sensitivity of measurement leads to decrease total dose received by the body. So, in this research the effect of gamma shield that was placed in device, on the dose received by the body and the sensitivity parameter is studied. Finally, the optimized device according to the sensitivity index is determined. Studies were performed on the IVNAA facility that was optimized by neutron activation center at Ferdowsi University in uniformity of activation rate. The most important source for producing undesirable rays is Gamma rays produced in 241 Am-Be neutron source and the secondary gamma rays which being produced in neutron collision with the components of IVNAA system. Tungsten, Lead, Bismuth, Mercury, Iron and Nickel with different thicknesses (In the range 0.5 to 6 cm) were selected and were placed in two separate positions: around the source and around the patient. With MCNPX 2.4.0 code, the absorbed dose and dose equivalent per one neutron emitted from the neutron were calculated on cubic water phantom in the dimension of 32*16*100 cm3. To have statistical error of order one-tenth percent, 356 programs were performed in whole time 712 hours. The results showed that absorbed dose and dose equivalent per neutron emitted from a source by placing the gamma shield is reduced. W in both situations causes the most reduction in comparison wit other elements. Also, the sensitivity of the device by placing the shield around the source is increased. But, all elements with low thicknesses around the patient, except for Bi and Pb, reduce sensitivity, and then by increasing their thickness, this parameter increases. Bi and Pb shield around the patient in all thicknesses, increase sensitivity. Generally, the sensitivity by shielding around the source in comparison with shielding around the patient has further increased. The best elements around the patients are Lead, Bismuth and Tungsten, respectively and around the source are W and Hg. Finally, considering sensitivity, the best setup is the system with Tungsten shield around the source and then Mercury in the same position with a radius of more than 4 cm were calculated.

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