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Systematic and Random Errors in Dual Gamma Energy Soil Bulk Density and Water Content Measurements¹

ABSTRACT

Soil bulk density and water content may be obtained concurrently through measurement of the attenuation of gamma photons from two different gamma ray sources and simultaneous solution of the resulting attenuation equations. Errors in the soil bulk density and water content measurements result from random emission from the sources, random errors in attenuation coefficients and soil column thickness measurements, presence of a small, higher energy peak in the ²⁴¹Am spectrum, and counting equipment dead time. Using gamma photons from ²⁴¹Am at 0.060 Mev and from ¹³⁷Cs at 0.662 Mev, attenuated in 10-cm soil columns, the standard deviation in both water content and bulk density measurements is primarily due to random emission and is about 0.007 g/cm³ for 10⁶ counts measured in air and about 0.005 g/cm³ for 2.5 x 10⁶ counts. However, as larger counts are used the precision of measurement of column thickness and soil and water attenuation coefficients becomes limiting. If, by obtaining large counts, the variance due to random emission is reduced to the same magnitude as that associated with measurement of column thickness or attenuation coefficients, precision limit would be about 0.0035 g/cm³ at midrange values of water content and bulk density. At high count rates instrument dead time corrections are made to get accurate counts. Gamma rays scattered down from the small 0.103 Mev peak in the ²⁴¹Am spectrum result in attenuation dependent attenuation coefficients for which appropriate corrections must be made. For narrow collimating slits the spatial resolution is only a little greater than slit thickness. Collimater and shielding requirements are shown.

¹ This investigation was supported in part by the US Atomic Energy Commission Project At (451-1-1543) and by Western Regional Research Project W-68, and is published as Scientific Paper no. 3741, Washington Agricultural Research Center, Pullman.

² Professor of Soils and Soil Scientist, Assistant Professor of Biophysics and Assistant Soil Scientist, and Senior Experimental Aide, respectively, Washington State University, Pullman, 99163.

Received for publication October 8, 1971. Accepted for publication January 11, 1972.