Minimum detectable activity for NaI(Tl) airborne γ-ray spectrometry based on Monte Carlo simulation

The determination of the effective minimum detectable activity (MDA) of radionuclides by a detection system plays an important role in environmental radiation monitoring. In this study, the responses of an NaI(Tl) airborne γ ray spectrometry (AGRS) system to different radionuclides ( 137 Cs and 131 I) were investigated using the Monte Carlo technique. The MDA values were determined under different conditions according to the counting spectra obtained from the Monte Carlo simulation. The equivalent mass thickness method was applied to the Monte Carlo modeling for monitoring ground radiation to reduce statistical uncertainty. The secondary source method was used to monitor both air and ground radiation. A quadratic relationship was found between the MDA and activity concentration. An exponential relationship was found between the MDA and altitude. The MDA of a specific radionuclide from external detectors was found to be superior to that obtained from internal detectors under the same conditions. The MDA values in an NaI(Tl) AGRS system under different conditions can be estimated based on the results of this study.