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SOIL PHYSICS

Thermal Neutron Computed Tomography of Soil Water and Plant Roots

^a 716 W. 16th Ave., Spokane, WA 99203
^b McClellan Nuclear Radiation Center, 5335 Price Avenue, Bldg. 258, McClellan, CA 95652
^c Dep. of Land, Air, and Water Resources, Univ. of California, Davis, CA 95616

^* Corresponding author (jwhopmans{at}ucdavis.edu).

Neutron radiography is a noninvasive imaging technique that measures the attenuation of thermal neutrons, as is done with x-ray and -ray radiography, to characterize the internal composition of materials. Neutron and x-ray imaging are complementary techniques, with neutron imaging especially well suited for materials containing H atoms and other low-atomic-weight attenuating materials. Although neutron computed tomography (NCT) techniques are routinely used in engineering, relatively little is known about their application to soils. We developed new techniques that use thermal neutron attenuation to measure the spatial and temporal distribution of water in soils and near roots at near 0.5-mm spatial resolution or higher. The neutron source was a Mark II Triga Reactor at McClellan Nuclear Radiation Center in Sacramento, CA. After calibration using both deuterated and regular water, the effects of beam hardening and neutron scattering could be corrected for, provided that the total path length for a soil–water mixture does not exceed 1.0 cm, limiting soil sample thickness to about 2.5 cm. Using regular water, for a wide range of soil water content values, experiments demonstrated that NCT is sensitive to small changes in soil volumetric water content, allowing estimation of the spatial distribution of soil water, roots, and root water uptake. Although the spatial resolution of the applied NCT system was 80 µm, an error analysis showed that the averaging measurement volume should be not less than about 0.5 mm for the uncertainty in volumetric water content to be minimized to near 0.01 m³ m^–3. A single root water uptake experiment with a corn (Zea mays L.) seedling demonstrated the successful application of NCT, with images showing spatially variable soil water content gradients in the rhizosphere and bulk soil.

Abbreviations: CCD, charge-coupled device • CT, computed tomography • MNRC, McClellan Nuclear Radiation Center • NCT, neutron computed tomography