Monte Carlo simulations to estimate the background spectrum in a shielded NaI(Tl) γ-spectrometric system

The paper describes a Monte Carlo simulation technique to estimate the background γ-radiation for a low-level counting setup with NaI(Tl) detector inside an iron shield box. With monoenergetic γ-sources ranging from 300 to 2000 keV at intervals of 100 keV we have got a matrix of transmitted flux data calculated using a general purpose Monte Carlo code. The intrinsic efficiency of the detector is also calculated. This matrix of data is folded with the measured spectrum outside the setup to estimate the observed spectrum in the NaI(Tl) detector. We have studied various combinations of splitting and Russian roulette to arrive at an optimal combination based on the figure of merit of the Monte Carlo results. The observed discrepancy between the measured and calculated spectra is mainly due to porosity of the iron shield box.