Calibration of a Neutron Moisture Meter in Leda Clay

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Abstract

<jats:p> A neutron moisture meter using a scintillation detector for slow neutrons was previously calibrated in materials carefully selected to avoid inaccuracies resulting from neutron absorption. Preliminary readings of moisture density in the field using this calibration, however, were not in agreement with values obtained by auger borings. Consequently the instrument was recalibrated; in the laboratory using samples of remolded Leda clay; and in the field, by taking readings throughout a summer in the natural material. Details of moisture content and density control are described. The new calibration curve is parallel to the original one, but considerably displaced from it by what appears to be a neutron absorption effect. Chemical analyses of the clay and pore water failed to detect any elements whose capture cross-sections for neutrons are large. They did however indicate a high percentage of iron whose capture cross-section is a little larger than those for elements occurring naturally in mineral soils. Although further research is required for confirmation of this observation, results of preliminary experiments point primarily to the presence of iron in quantities large enough to explain the discrepancy between the two calibration curves. </jats:p>

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