For the purpose of rapid detection of internal contamination in emergency situations, a portable body counter is used. To estimate minimum detection activity (MDA) for a portable whole body counter; the Monte Carlo code has been used. In the case of outdoor open fields, natural background radiations were simulated. This counter has a chair geometry equipped with a NaI (Tl) detector (5cm x 5cm) inside a lead shield collimator consists of a set of lines and a continuous component with monenergistic γ sources ranging from 300 to 2000 keV at intervals of 100 keV. This matrix of data is folded with the measured spectrum outside the setup to estimate the observed spectrum in the detector. We evaluated the variation of the flux in the detector transmitted through the lead collimator and chair shield for different densities of lead and different photon energies. Computational data were used to estimate the MDA of the monitoring system. This method is less expensive to design and test a counter system for low-level counting of γ emitting radionuclides than experimental methods.