Response functions of the 3*3 in NaI detector, which is mainly used in PGNAA applications, have been calculated by using MCNP-4C code. Calculated results are compared with measured data by using standard g-ray sources and prompt g-rays from pure element samples to check their accuracy. Prompt g-rays from pure element samples were used for this determination in the range from 1.942 to 10.829 MeV by use of 241Am–Be neutron source and g-rays from radioisotope sources were used in the range from 0.081 to 4.438 MeV. Through the precise modeling of the detector structure, the agreement between both results has been improved. A surprising result is that in the PGNAA method the agreement between the MCNP simulation and experiment will be better by using a suitable neutron shield for NaI detector in order to prevent the activation of NaI (Tl) and a proper g-shield to attenuate the high-rate 4.438MeV g-ray, 241Am–Be g-ray component.

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