The assay of soil carbon with naturally occurring cosmic ray neutrons

The detection of gamma rays induced in soil by naturally occurring cosmic ray neutrons is explored with the Geant4 Simulation Toolkit to monitor carbon sequestration in soil. The simulated soil is a uniform mixture of minerals, air, water, and soil organic carbon. As the soil organic carbon increases from 0% to 15% by volume, the mineral matter decreases, and gamma ray counts from mineral-related isotopes decrease. Characteristic gamma ray energies from a variety of elements are collected near the surface with a germanium detector. Of these, the 2.224 MeV gamma ray from hydrogen is sensitive to changes in soil organic carbon as low as 0.12% after counting for the equivalent of 3.45 days. Counting longer is recommended to reduce the sensitivity of the primary 4.438 MeV gamma ray from carbon below its current value of 2.81% in the simulation.