A comparison of fallout 236U and 239Pu uptake by Australian vegetation

The isotopes 236U and 239Pu, both produced during nuclear weapons tests carried out in the 1950s and 1960s, are present in the environment and may be used as tracers for soil erosion studies. Although these radionuclides occur only at ultra-trace levels in nature, they can be readily measured by accelerator mass spectrometry with the 14UD heavy ion accelerator at the Australian National University. We have analysed a series of vegetation samples for their 236U and 239Pu concentration and compared the results with those found in the surrounding soil. 236U could be measured in all collected samples whereas 239Pu could not be detected in several vegetation samples due to its very low concentration, although it was readily detectable in the soil. We find that, relative to plutonium, 236U is preferentially taken up by plants with enrichment factors (236U/239Pu)veg/(236U/239Pu)soil that range between 7 and 52 in the present study.