Calibration of a Dual‐Energy Gamma Radiation System for Multiple Point Measurements in a Soil

A method is described to experimentally determine the calibration constants for gamma‐ray attenuation at multiple measurement points along a soil column as needed to simultaneously determine volumetric water content and dry bulk density at each of these points during subsequent experiments. Calibration constants for the soil solids, for both Cs‐137 and Am‐241, are defined as the product of the respective mass attenuation coefficients and the thickness of the soil through which the gamma radiation beam traverses. Calibration constants for water are similarly defined but include the additional term of water density. The calibration technique corrects for low energy Cs‐137 pulses present in the Am‐241 window and for resolving time. Although calibration constants are determined for each measurement point, the technique requires knowledge only of the initial average dry bulk density in the soil column and not at every point. The calibration constants and initial dry bulk density value for each measurement point are determined by an iteration procedure. Errors stemming from changes in the sources‐soil column‐detector configuration and/or pathlength when measuring at different points are therefore negated.