Measurement of Attenuation of 137Cs and 241Am Gamma Rays for Soil Density and Water Content Determinations

Abstract

AbstractSoil density and water content can be determined concurrently by measuring the attenuation of two different gamma energies. A dual source containing 241Am and 137Cs was shown to be suitable for these measurements. Multichannel pulse‐height analyzers were evaluated to determine their accuracy under experimental conditions for simultaneous measurements of soil density and water content. NaI(Tl) and Ge(Li) detectors were also evaluated. The measurements were made with a collimated beam, 1 cm in diameter, of 662‐keV gamma rays and 59.6‐keV gamma rays from a source containing 70 mCi of 137Cs and 29 mCi of 241Am. Measured mass attenuation coefficients were 0.08565, 0.07803, and 0.07887 cm2/g for cesium gamma rays and 0.20493, 0.27100, and 0.33480 cm2/g for americium gamma rays with water, Cecil, and Houston Black soil, respectively. The higher resolution of the Ge(Li) detector and a 4096‐channel pulse‐height analyzer had no significant advantage with peaks as widely separated as 662 keV and 59.6 keV. A NaI(Tl) detector and a 400‐channel pulse‐height analyzer were satisfactory. With the multichannel pulse‐height analyzer, it was simpler to adjust the area under the americium peak for the Compton contribution from the cesium radiation than with a single channel pulse‐height analyzer. Water content and density changes in both a swelling and nonswelling soil illustrate the utility of the system.