This study describes the effects of the detector-collimator on the gamma-ray response function for NaI(Tl) detectors with 2 x2 , 3 x3 and 5 x5 dimensions. In present study we have used five different detector-collimators and three gamma ray sources (241Am, 137Cs and 60Co). In all states, the time of counts and detector-source-distance were constant. We measured the gamma-ray response function, Full Width at Half Maximum (FWHM) and energy resolution in the full energy peak with and without detector-collimators. Then we perceived that the detector-collimator was very important in gamma ray spectrometry because, in the best state, FWHM and energy resolution decrease about 55%. In the end we obtained the best detector-collimator for each of the three NaI(Tl) detectors.

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