Soil bulk density evaluated by gamma-ray attenuation: Analysis of system geometry

The gamma-ray attenuation and computerized tomography have been proved to be efficient methods for soil physical properties measurements in the last decades. Soil bulk density (ρs) is a physical attribute that can be evaluated by these methods, which use the soil mass attenuation coefficient (μs). Before any ρs measurements, it is necessary to verify an interval of sample size at which the experimental μs value will not suffer significant variations that might affect the representativeness of such physical attribute. In this study, the effect of μs variation, as a function of the sample thickness increase and collimator sizes in ρs measurements, was analyzed. Two gamma-ray sources (137Cs and 241Am) were used. Tomographic images of two soils with different textures were obtained and the ρs maps were generated using μs distinct values. In summary, the results obtained with the 137Cs show that ρs remained fairly constant for thicker samples (x ≥ 10 cm) and smaller collimators (2 and 3 mm diameters). Regarding the 241Am source, the best ρs values were obtained for samples smaller than 4 cm and with bigger collimator sizes (3 and 4 mm diameters).

► Alterations in measured soil attenuation coefficient affects its bulk density.
► Sample thickness larger than 10 cm presents the best results for 137Cs.
► Sample thickness smaller than 4 cm presents the best results for 241Am.
► The influence of collimator size was also investigated.