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Collimated Neutron Probe for Soil Water Content Measurements

Raytheon Services Nevada, Mercury, NV 89023

Alan L. Flint^*

U.S. Geological Survey, Mercury, NV 89023

*Corresponding author.

ABSTRACT

The use of uncollimated (undirected) neutron moisture meters is common in the biological and geophysical sciences. A collimated neutron probe was designed to enable measurements in specific directions from the access tube. To determine the size and shape of soil volume affecting the neutron counts, experiments were conducted to evaluate: (i) the vertical distance of soil above and below the probe that influences neutron counts, (ii) the horizontal distance away from the probe into the soil that influences neutron counts, (iii) the angle of soil viewed by the probe from the collimator, and (iv) the three-dimensional thermal-neutron density field. The distance and the angular dimensions of the volume of influence were defined as the horizontal distance of neutron penetration from the edge of the probe, the vertical distance above and below the center of the effective measurement point of the probe, and the angle from the center of the probe, which would allow the determination of relative water content to within 95%. The vertical distance was 0.5 m, the horizontal distance was 0.2 m, and the angle of soil viewed by the probe from the collimator was 120°. Thermal neutrons detected from distances or angles larger than these values influence the determination of relative water content by 5% or less.

Contribution from the U.S. Geological Survey, Denver, CO. This work was done in cooperation with the U.S. Department of Energy, interagency agreement DE-AI08-78ET44802.

Received for publication November 15, 1989.

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