Background: Airborne γ-ray spectrometer has been used extensively over several decades for mineral exploration and geological mapping purposes to look for the peaks of potassium

uranium and thorium. And the low-energy ray is ignored. Purpose: In order to provide a basis for obtaining effective environmental radioactivity measurement results

Minimum detectable activity (MDA) values in monitoring ground radiation of the polycrystalline airborne γ-ray spectrometer need to be calculated. Methods: MDA is related with the detection efficiency. A Monte Carlo simulation was performed using the MCNP5 code for different radionuclides in the ground environment. Equivalent mass thickness was proposed to reduce variance

and the secondary source was used in the MCNP5 input. Results: The pulse height distributions of external detectors and internal detectors for 137Cs and 131I at different heights were obtained

which represent the counting rate decreased as the altitude increases. And the MDA of external detectors is better than that of internal detectors. Conclusion: The external detector is suggested to adopt in flight measurement for enhancing MDA.Cited