Determination of Characteristic vs Anomalous 135Cs/137Cs Isotopic Ratios in Radioactively Contaminated Environmental Samples

A contamination with the ubiquitous radioactive fission product Cs-137 cannot be assigned per se to its source. We used environmental samples with varying contamination levels from various parts of the world to establish their characteristic Cs-135/Cs-137 isotope ratios and thereby allow their distinction. The samples included biological materials from Chernobyl and Fukushima, historic ashed human lung tissue from the 1960s from Austria, and trinitite from the Trinity Test Site, USA. After chemical separation and gas reaction shifts inside a triple quadrupole ICP mass spectrometer, characteristic Cs-135/Cs-137 isotope signatures (all as per March 11, 2011) were obtained for Fukushima- (similar to 0.35) and Chernobyl-derived (similar to 0.50) contaminations, in agreement with the literature for these contamination sources. Both signatures clearly distinguish from the characteristic high ratio (1.9 +/- 0.2) for nuclear-weapon-produced radiocesium found in human lung tissue. Trinitite samples exhibited an unexpected, anomalous pattern by displaying a low (<0.4) and nonuniform Cs-135/Cs-137 ratio. This exemplifies a Cs-137-rich fractionation of the plume in a nuclear explosion, where Cs-137 is a predominant species in the fireball. The onset of Cs-135 was delayed because of the longer half-life of its parent nuclide Xe-135, causing a spatial separation of gaseous Xe-135 from condensed Cs-137, which is the reason for the atypical Cs-135/Cs-137 fractionation in the fallout at the test site.

Automated summary

By analyzing the Cs-135/Cs-137 isotope ratios in environmental samples, it is possible to distinguish the sources of Cs-137 contamination.