Mineralogical composition of hardsetting soils and its effect on the radiation attenuation characteristics

Abstract

One important group of soils that has not been well studied yet belongs to the hardsetting horizons. These soils have been characterized through basic physical property measurements (porosity, bulk density, particle size distribution). XRD and XRF analytical techniques and the interaction of radiation (X and γ-rays) through the soil can also be interesting tools for a better understanding of the properties of these soils. This work presents the chemical and mineralogical properties of Brazilian hardsetting soils and their effects on the radiation attenuation characteristics. The mass attenuation coefficient (μm) of hardsetting soils was calculated by using the XCOM computer code based on the chemical composition of the soils. Soil samples derived from different regions of Brazil were collected in the hardsetting horizons. Each sample was mashed, sieved, and submitted to dispersion and physical fractioning process by sedimentation. Semi-quantitative elemental analysis of the soils was accomplished through EDXRF. Based on the soil chemical composition analyses, the contribution of the photoelectric absorption and incoherent and coherent scatterings were investigated. The mineralogical composition was determined through the Rietveld method combined with X-ray diffraction (RM-XRD) data. The soil mass attenuation coefficient decreased increasing the photon energy as expected. A great decrease was observed in the low photon energy region, where the contribution of the photoelectric absorption was dominant. Regarding the intermediate photon energy region, where the contribution of the incoherent scattering was dominant, only a slight decrease was observed in μm with photon energy. Strong positive and moderate negative linear correlations were found between μm and Fe2O3 and SiO2 oxides. The highest mineral percentages found for the hardsetting soils were quartz, kaolinite, and halloysite. Strong positive (photoelectric absorption) and negative (coherent and incoherent scatterings) correlations were found for goethite. Increase in the percentage of goethite and kaolinite influences increase in μm. The same behavior could be observed in relation to the contribution of the photoelectric absorption to μm. On the other hand, an increase in the amount of these two minerals presented a decrease in the contribution of the coherent and incoherent scatterings to μm. Therefore, in the absence of XRF analyses, the mineralogical soil measurements carried out through RM-XRD might be an interesting tool to infer the radiation attenuation characteristics of hardsetting soils. Such results bring a new insight into the radiation interaction through soils due to the analysis of the influence of the minerals in the photon attenuation.