Carbon distribution in soil is intricately linked to soil health. However, repeatable measurements of carbon distribution typically require destructive sampling and laboratory analyses. Soil carbon distributions in both natural and managed landscapes significantly vary due to numerous factors related to topography, mineralogy, hydrology, land use history, and vegetation. In order to accurately inventory soil C distributions and dynamics over time, we are developing a new technique that relies on neutron inelastic scattering to measure elemental distribution. This approach can be used to image a volume of approximately 50 cm × 50 cm × 30 cm (depth) with a few centimeters resolution, for example the root zone of a plant. To achieve this, we use neutrons created in a deuterium-tritium fusion reaction. The products of this reaction are an alpha particle and a neutron. Due to momentum conservation, both particles are emitted in opposite directions in the center-of-mass frame. This allows us to measure the neutron direction by detecting the alpha particle with a position sensitive detector. The neutron can then induce an inelastic scattering reaction on a carbon nucleus present in the soil, and this event produces a gamma ray with a characteristic energy for the carbon isotope. Using a gamma detector, we measure these gamma rays, which allows us to perform time-of-flight analysis between arrival times of the alpha and gamma particles. Using the information from both measurements (alpha and gamma), we can reconstruct the spatial distribution of the carbon atoms and other elements in soil. We will report on the design, potential applications, and limitations of the instrument. We will also report on initial results from laboratory experiments and progress towards future field experiments. 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.

Carbon sequestration in soils represents an important opportunity to reduce the amount of greenhouse gases in the atmosphere and thereby offsetting the effects of climate change. To monitor carbon sequestration accurate measurements of soil carbon are needed that can be repeated over several growing cycles. Furthermore, soil carbon is an important indicator of soil health; accurately measuring carbon distribution is therefore important for informing land use management practices. We are developing a new instrument that images a volume of approximately 50 cm × 50 cm × 30 cm (depth) with a few centimeters resolution by applying neutron scattering techniques. Contrary to current coring methods, this approach is non-destructive, samples a large area, and allows real-time analysis of the soil carbon density. In this technique a neutron and an alpha particle are created in a deuterium-tritium fusion reaction. Due to momentum conservation the two particles move in opposite directions. Creating the particles in a small point source allows us to calculate the direction in which the neutron is moving by tracking the associated alpha particle using a position sensitive detector. The neutron can then enter the soil and inelastically scatter off atoms in the soil, creating an isotope-specific gamma ray in the process. Measuring the energy of the gamma ray allows identification of the isotope. Measuring the time-of-flight between the alpha detection and the gamma detection together with the direction of travel of the neutron allows the calculation of the 3D position of the scattering center. Using this Associate Particle Imaging (API) technique 3D density plots of carbon, oxygen, silicon, and aluminum can be obtained. In this poster we present first results from applying API to pre-mixed and standard soil samples in a laboratory setting (field tests are planned in the future). We will compare measured data to neutron-transport simulations and discuss our data analysis algorithm to reconstruct the carbon density in the soil from API data. We will further discuss achievable resolution and time requirement for measurements in the field. 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.