Surface and depth profile concentrations (down to 50 cm) of ²³²Th chain, ²²⁶Ra, and ⁴⁰K radionuclides were determined in undisturbed coastal and inland soils of La Plata city region, Argentina, through their gamma-ray activity using a high-purity Ge detector spectrometer. These results were compared with superficial activities determined in soils from the surroundings of the Centro Atómico Ezeiza (Ezeiza Atomic Center) located in Ezeiza, Buenos Aires Province, Argentina. The hyperfine and magnetic Fe phase's properties of soil profiles were characterized by Mössbauer spectroscopy, magnetic hysteresis loops and AC magnetic susceptibility. No dependence of the activity of the ²³²Th natural chain on depth was found, whereas variations for ²²⁶Ra and ⁴⁰K activities were observed. Positive correlations, determined by the Pearson correlation coefficients, were established between ⁴⁰K, ²²⁶Ra and ²³²Th activity concentrations for the whole set of soil samples. The annual external equivalent dose for adults was similar for La Plata and Ezeiza regions, with average values of 0.08 ± 0.01 mSv and 0.06 ± 0.02 mSv, respectively. The thermal dependence of the AC magnetic susceptibility revealed the existence of magnetite and hematite. The Mössbauer spectra of all soils were made up of signals associated with α-Fe₂O₃, a paramagnetic relaxation component, and Fe³⁺ and Fe²⁺ doublets. In addition, the spectra of inland soils revealed the presence of Fe₃O₄. A negative correlation was found between the activity concentrations and the α-Fe₂O₃ and Fe₃O₄ relative fractions, whereas a positive correlation was found between the Fe³⁺ relative fraction and the ⁴⁰K activity.
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