Gamma-ray attenuation method as an efficient tool to investigate soil bulk density spatial variability

Abstract The spatial variability of soil bulk density ( ρ b ) was measured by using the volumetric ring method (VRM) and the gamma-ray attenuation method (GAM). Collimated radiation from 3.7 GBq of 241 Am was used to evaluate the soil mass attenuation coefficient and its bulk density. Circular lead collimators were adjusted and aligned between source ( D  = 1, 2 and 3 mm) and detector ( D  = 4.5 mm). Results of GAM for average ρ b provided good agreement with the corresponding values obtained gravimetrically. Variations in bulk density for different collimator dimensions can be attributed to multiple scattering after photons interaction with soil, mainly for 3 mm collimator size. The best result of ρ b by the nuclear technique was obtained when ρ b represents an average of the measurements for collimators of 1 and 2 mm. Another cause for the differences in ρ b by GAM and VRM is the heterogeneity of soil when the collimated beam can interact with stones or large air-filled holes or channels present in the sample. Therefore, the pattern of spatial variability obtained by VRM was confirmed by GAM for all collimator sizes. This result is a good indication that GAM can be used with success to analyze soil spatial variability.

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