In this study, a Monte Carlo program has been developed to simulate the response function of the NaI(Tl) detector with all features for 60Co and 137Cs and to investigate the effects of detector housing material on response function. The pulse height spectra in a 2″ × 2″ NaI(Tl) detector due to these gamma ray sources have been measured. Comparison of the experimentally obtained and simulated spectra shows that there is good agreement between both spectra. Energy distributions of gamma photons that generate the backscattering peak in the response function were obtained and the contribution of single, double and multiple Compton scattering events to these distributions was investigated. © 2012 Akadémiai Kiadó.