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Measurement of Sediment Density by Attenuation of Transmitted Gamma Rays¹

ABSTRACT

The measurement of density by observing the attenuation of transmitted gamma rays was adapted to the measurement of sediment density. A dual probe, utilizing transmission techniques, was evaluated for use in measuring the density of sediments collected from runoff of plots or small watersheds. The equipment consists of a 7-mc. cesium¹³⁷ source placed in one access tube and a probe containing a NaI scintillation crystal, photomultiplier tube, and preamplifier in the second tube. The high-voltage power supply and scaler ratemeter for readout are supplied in a portable unit powered by silver-cadmium recharge-able dry cells. Cesium¹³⁷ gamma radiation is measured by employing electronic discrimination to eliminate all phostons of energies below about 0.65 Mev. Sediment densities were measured in silt boxes installed on a small watershed at the North Mississippi Branch Experiment Station, Holly Springs, Mississippi, following laboratory calibration tests. The dual probe employed measured densities of sediments to within 0.01 absolute density units. The vertical resolution of the apparatus is less than I inch so that density measurements of thin layers and of materials at interfaces can be made. In one field test sediment weights, of the order of 8,500 pounds, computed for a silt box on the basis of dual probe density determinations varied less than 1% from a gravimetric determination. Changes in sediment density with time, with additional increments of sediment, and with storms were effectively measured.

¹ Contribution from the USDA Sedimentation Laboratory, Soil and water Conservation Research Division, ARS, USDA, in cooperation with the Mississippi Agr. Exp. Sta. and the University of Mississippi. Presented at the Soil Sci. Soc. Am., before Div. S-6, Nov. 18–22, Denver, Colo.

² Research Soil Scientist and Agricultural Engineer, USDA Sedimentation Laboratory, Oxford, Miss.

Received for publication March 17, 1964. Accepted for publication August 6, 1964.