Effects of soil properties on neutron scattering techniques for soil carbon measurement

Soil is a major storage reservoir in the terrestrial carbon cycle that is highly variable according to local vegetation, bedrock type, topography, and land-use history. Besides point-sampling by coring and lab analysis, a repeatable field-scale measurement of soil carbon heterogeneity is yet to be developed. Inelastic Neutron Scattering (INS) techniques involve the emission of high-energy (14.1 MeV) neutrons into the soil and detection of the characteristic gamma ray energies which are returned as a result of neutron scattering reactions. In the case of carbon, a 4.4 MeV gamma ray is emitted after excitation by a high-energy neutron. The probability of this gamma-emitting reaction varies with incident neutron energy, and becomes effectively zero at neutron energies less than 5 MeV. This requires an approximation of neutron scattering and energetic reduction in the soil column. To address this, we use the Monte Carlo Neutron Particle (MCNP) modeling software to simulate INS analysis under a wide variety of soil properties. Preliminary results show that water content in soil is approximately twice as effective as the bulk density of solid minerals at attenuating neutrons through the soil column. In contrast, variations in the chemical composition of solid mineral content demonstrates little impact on the process. Based on these observations, a simple model is developed to apply a correction factor which allows for the measurement of carbon concentrations at various depths in soil.

This study focuses on an INS setup using time-of-flight Associated Particle Imaging (API) which allows one to measure the 3D coordinates of the particle undergoing an INS reaction within an accuracy of a 5-10 cm. This allows for a focused study of how neutrons, and thus the carbon INS signal, attenuate with depth in the soil. We show how variations in soil properties affect the energetic reduction of neutrons with depth in soil, and how this propagates to maximum measurement depth and accuracy of the INS technique.

The information, data, or work presented herein was funded by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy, under Contract No. DE-AC02-05CH11231