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Resolving Time, Mass Absorption Coefficient and Water Content with Gamma-Ray Attenuation¹

ABSTRACT

A method is described that was used to experimentally determine an intensity correction for gamma-ray attenuation. The correction is determined by using the minute nonlinearity of a semi-log plot of gamma-ray intensity versus the thickness or the product of thickness and density of a material. The experimentally determined correction is expressed in terms of resolving time and varied with both material and counting rate. A correction of 5.0 µsec/count was determined experimentally as the best resolving time to correct the observed counting rates for water content determination. Part of the 5.0 µsec/count correction was evidently due to causes other than resolving time. Mass absorption coefficients of water and soil were 0.08105 and 0.07309 cm²/g, respectively, with no correction and were 0.0871 and 0.0773 cm²/g with a 5.0 µsec/count correction. The difference between gravimetric water content and gamma-ray water content dropped from 3.02% to 0.60% when this correction was used.

¹ Journal Paper no. J-5914 of the Iowa Agr. & Home Econ. Exp. Sta., Ames, Iowa. Projects no. 998 and 1653. Supported in part by US Atomic Energy Commission Contract At(11-1)-1269, Report no. COO-1269-17 and Office of Water Resources, US Department of Interior, as authorized under the Water Resources Research Act of 1964, Public Law 88-379.

² Formerly Research Associate, Iowa State University, Ames, Iowa; presently Assistant Professor of Soil Physics, Cornell University, Ithaca, N.Y.

Received for publication March 21, 1969. Accepted for publication May 2, 1969.