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P. Steier, M. Bichler, L. FiField, R. Golser, W. Kutschera, A. Priller, Francesca Quinto, S. Richter, M. Srncik, P. Terrasi, L. Wacker, A. Wallner, Gabriele Wallner, K. Wilcken, E. Wild:
"Natural and anthropogenic ²³⁶U in environmental samples";
Nuclear Instruments & Methods in Physics Research Section B, 266 (2008), Issue 10; 2246 - 2250.

The interaction of thermal neutrons with ²³⁵U results in fission with a probability of ~85% and in the formation of ²³⁶U (t_1/2 =2.3 x 10⁷ yr) with a probability of ~15%. While anthropogenic ²³⁶U is, therefore, present in spent nuclear fuel at levels of ²³⁶U/U up to 10^-2, the expected natural ratios in the pre-anthropogenic environment range from 10^-14 to 10^-10. At VERA, systematic investigations suggest a detection limit below ²³⁶U/U = 5 x 10^-12 for samples of 0.5 mg U, while chemistry blanks of ~2 x 10⁷ atoms ²³⁶U per sample limit the sensitvity for smaller samples. We have found natural isotopic ratios in uranium reagents separated before the onset of human nuclear activities, in uranium ores from various origins and in water from a subsurface well in Bad Gastein, Austria. Anthropogenic contamination was clearly visible in soil and rivulet samples from Salzburg, Austria, whereas river sediments from Garigliano river (Southern Italy) were close to the detection limit. Finally, our natural in-house standard Vienna-KkU was calibrated against a certified reference material (IRMM REIMEP-18 A).

http://dx.doi.org/10.1016/j.nimb.2008.03.002

http://publik.tuwien.ac.at/files/PubDat_175334.pdf