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

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