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This study explores the effect of suppressed surface charges on the sedimentation behaviour and fabric of kaolinite clay. In particular, sedimentation behaviours were compared between negatively charged natural kaolin clays and surface-treated and hence electrically suppressed kaolin clays by methylene blue adsorption. The results clearly indicate that the negative surface charge impeded the sedimentation rate of kaolin by resisting particle aggregation. Such an effect appeared the most significant when the surrounding pH was less than the isoelectric point (IEP) of the mineral edges. Under a pH less than IEP, the flocculation time and the final void ratio remarkably dropped after the negative charge neutralisation. This was also corroborated by the scanning electron microscopy images, which further revealed the fabric transition from an edge-to-face flocculated structure to a face-to-face aggregated structure due to the absence of interparticle electrical forces. Conversely, there was no significant fabric change at pH higher than or near the IEP, even though the sedimentation behaviours were accelerated by forming larger flocs. This study imparts important insights into the fundamentals of microstructure and resulting sedimentation behaviour of clayey soils and how they can be changed in electrically neutralised clay. 

Abstract We present a background model for dark matter searches using an array of NaI(Tl) crystals in the COSINE-100 experiment that is located in the Yangyang underground laboratory. The model includes background contributions from both internal and external sources, including cosmogenic radionuclides and surface $$^{210}$$ 210 Pb contamination. To build the model in the low energy region, with a threshold of 1 keV, we used a depth profile of $$^{210}$$ 210 Pb contamination in the surface of the NaI(Tl) crystals determined in a comparison between measured and simulated spectra. We also considered the effect of the energy scale errors propagated from the statistical uncertainties and the nonlinear detector response at low energies. The 1.7 years COSINE-100 data taken between October 21, 2016 and July 18, 2018 were used for this analysis. Our Monte Carlo simulation provides a non-Gaussian peak around 50 keV originating from beta decays of bulk $$^{210}$$ 210 Pb in a good agreement with the measured background. This model estimates that the activities of bulk $$^{210}$$ 210 Pb and $$^{3}$$ 3 H are dominating the background rate that amounts to an average level of $$2.85\pm 0.15$$ 2.85 ± 0.15  counts/day/keV/kg in the energy region of (1–6) keV, using COSINE-100 data with a total exposure of 97.7 kg $$\cdot $$ · years.