A dual source gamma ray traversing mechanism suitable for the non-destructive simultaneous measurement of bulk density and water content in columns of swelling soil

Abstract

A traversing mechanism is described in which 100 mCi sources of americum-241 and caesium-137 are alternately brought into line with a sodium iodide (thallium) scintillation counter connected to a single-channel pulse height analyser equipped with two preset base lines. The equipment is capable of scanning a 10 cm diameter soil column equipped with sensors to a height of 200 cm, and facilities are provided for automatic indexing at preset intervals to an accuracy of 0.01 cm. Non-linear counting losses in the spectrometer system are described, and were found to be dependent on the window setting of the pulse height analyser. With the window setting adjusted to cover the apparent spread of the gamma peak, counting losses were negligible. Mass absorption coefficients of various materials were obtained using a compartmented box. Measured mass absorption coefficients of water, silica sand, and a chernozemic soil were 0.201, 0.247, and 0.303 cm2 g-1 respectively for americum-241, and 0.0826, 0.0746, and 0.0728 respectively for caesium-137. The precision of the apparatus was demonstrated by determining the moisture characteristic and bulk density characteristic of 0.5-1.0 mm aggregates of Narrabri soil during absorption and desorption. Gamma ray attenuation measurements and direct volumetric measurements were comparable. The standard deviations in the gamma ray measurements were found to be 0.03 g cm-3 for bulk density and 0.04 cm3 cm-3 for water content. Most of this inaccuracy arose from lack of precision in the americum-241 mass attenuation coefficient for the soil. By contrast, changes in moisture content and bulk density, at any level, of 0.004 cm3 cm-3 and 0.004 g cm-3 respectively, were statistically very significant.