Abstract: Radium-226 activity is determined by gamma spectrometry, based on the measurement of radon progeny short lived isotopes (214Pb and 214Bi) when Ra-Rn secular equilibrium is reached. Depending on the behaviors of radon emanation from the soil matrix, the homogeneous distribution of these radionuclides in measured soil samples may be lost, which may affect the quality of the obtained results if not taken into consideration. This paper highlights some assumptions regarding the distribution of radon daughters in the soil matrix and their effect on the detection efficiency of an HPGe detector using Mont Carlo simulation code.