Preliminary evaluation of (135)Cs/(137)Cs as a forensic tool for identifying source of radioactive contamination

Ratios of the fission products (135)Cs and (137)Cs were determined in soil and sediment samples contaminated from three different sources, to assess the use of (135)Cs/(137)Cs as an indicator of source of radioactive contamination. Soil samples from the Chernobyl exclusion zone were found to have to be heavily depleted in (135)Cs ((135)Cs/(137)Cs approximately 0.45), indicative of a high thermal neutron flux at the source. Sludge samples from a nuclear waste treatment pond were found to have a (135)Cs/(137)Cs ratio of approximately 1, whereas sediment collected downstream from a nuclear reactor was highly variable in both (137)Cs activity and (135)Cs/(137)Cs ratio. Comparison of these preliminary results of variability in radiocaesium isotope ratios with reports of Pu isotope ratios suggests (135)Cs/(137)Cs similarly varies with fuel and reactor conditions, and may be used to corroborate other methods of characterizing radioactive contamination.