U(238) is a radionuclide present in the earth’s crust that provides 65.9% of annual average radiation dose and represents 99.27% of total uranium. U(238) has been determined in samples from undisturbed topsoil in the Zacatecas State, Mexico, using several analytical techniques. ICP-MS and α-spectrometry produced the most reliable results. Uranium concentrations found in topsoil samples (0.4–3.7 mg kg−1) were similar to its average crustal abundance (2.8 mg kg−1) and indicate high homogeneity without evidence of enrichment. The average concentration of 2.1 mg kg−1 can be established as uranium background level in the topsoil of Zacatecas (Mexico).
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