Abstract
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.