Simulating the attenuation of in situ 40K gamma rays to monitor soil properties

Abstract Purpose The passive and non-destructive monitoring of soil carbon and soil moisture with low levels of naturally occurring gamma rays can assay carbon sequestration, improve irrigation management, and even aid in the timely delivery of chemical fertilizers that contain potassium. All are of interest to promote sustainable agriculture. Materials and methods The Geant4 Simulation Toolkit mimics the production of the 1.460 MeV gamma ray from the decay of Potassium-40 and explores the underlying behavior of gamma rays in different soils. Gamma rays are emitted at different depths in a modeled soil, and data is collected with a large sodium iodide detector placed 2 m above the surface. Results and discussion The detector response to the addition of either soil carbon or soil moisture is linear. Because soil carbon is only exchanged with the minerals that contain the source of the gamma rays, the negative detector response to adding soil carbon is stronger than that of adding water. Gamma rays that are successfully detected originate from a depth between 95 and 125 mm on average in the study. The average depth is greater when the soil has more soil carbon or air and smaller when it has more mineral or water. Conclusions Linearity in the detector response is helpful when assessing changes to soil carbon or soil moisture. Given where the detected gamma rays originate, proximal gamma ray spectroscopy with the 1.460 MeV gamma ray from 40 K is best considered when evaluating carbon and moisture content near the surface.