Improvement of in-situ gamma spectrometry methods by Monte-Carlo simulations

Performing in-situ measurements of gamma radiation originating from soil requires adequate detection efficiency curves, which can be obtained by Monte-Carlo simulations. In simulations, soil density of 1.046 g/cm³ was used, with the following elemental composition of soil in which gamma radiation was generated: O - 47%, Si -35%, Al - 8%, Fe - 3.9%, C - 2.1%, Ca - 1.4%, K - 1.3%, N - 0.6%, Mg - 0.6%, N - 0.1%. Soil matrix was represented by cylindrical volume of 1.5 m diameter and 0.5m thickness, while germanium detector was placed at 1 m height above the soil. The simulated gamma spectrum, originated from K-40, as well as from members of Th-232 chain, and daughters of Ra-226, was obtained. Homogeneous distribution of various radionuclides (Ra-226, Th-232, K-40) in soil matrix is considered in this work. Gamma spectra obtained in simulations were analyzed, and together with simulated detection efficiency data they provide comparison with real experimental measurements and practical application of results derived by Monte-Carlo simulations. As a result of this work, the corresponding detection efficiency curve for HPGe detector was obtained, which can be applied for in-situ measurements of radionuclide concentration in soil, assuming uniform radionuclide distribution. In order to validate our simulation results regarding detection efficiency, we performed in-situ measurements of soil radioactivity and compared the obtained activity concentrations with laboratory measurements. We found a good agreement, within activity concentration uncertainty, between in-situ measurement results and average values of activity concentrations obtained by laboratory measurements.