Comparison of Single-, Double-and Dual-Gamma Ray Measurement of Soil and Water

Abstract

A laboratory study was carried out to make a comparison of single-, double-and dual-gamma ray measurements of soil and water. The beam was produced by placing a 45-mCi source of ¹³⁷Cs behind a 100-mCi source of ²⁴¹Am and was collimated through a narrow slit. The equipment was designed to move vertically and horizontally, stop at any distance within an accuracy of ±0.1 mm, measure the counts of gamma rays and calculate the average intensity automatically. A scintillation probe was connected in parallel to receive both pulses in a band of 572-818 keV (for ¹³⁷Cs, a 662-keV peak) and pulses in a band of 30-86 keV (for ²⁴¹Am, a 60-keV peak).
The differences_ofAmass absorption coefficients of soils obtained through the single beam method and the dual beam method were mainly attributable to correction of the ²⁴¹Am background intensity in the dual beam method, which was caused by ¹³⁷Cs in the low energy band. The thicknesses of materials were well predicted with the single beam method and the double beam method but poorly predicted with the dual beam method. The bulk density of a volcanic ash soil column was estimated excellently with the double gamma beam method but the water content was slightly underestimated. Using the method of Gardner et al. theoretical variances of bulk density or water content could be estimated as functions of the thicknesses.