Measurements of Pu and Ra isotopes in soils and sediments by AMS

Plutonium fallout from atmospheric nuclear weapons testing in the 1950s and 1960s constitutes an artificial tracer suitable for the study of recent soil erosion and sediment accumulation rates. Long-lived Pu isotopes provide an alternative tracer to the more widely used ¹³⁷Cs (t_1/2=30 a), the concentration of which is decaying at a rate that will limit its long-term application to these studies. For ^239,240Pu, the sensitivity of AMS is more than an order of magnitude better than that afforded by α-spectroscopy. Furthermore, AMS can provide a simple, direct measure of the ²⁴⁰Pu/²³⁹Pu ratio. Sample profiles from two sites along eastern Australia have been determined with both AMS and α-spectroscopy to provide comparative measurements of the sediment accumulation rate in water bodies and of the soil erosion rate. The two methods are in good agreement. The ²²⁸Ra/²²⁶Ra ratio potentially provides a probe for tracing the dispersion of uranium mining residues into the neighboring environment. Soil depth profiles of the ratio may provide information on the rate at which mining-derived radioactivity is spread by surface waters, and could be used to assess the effectiveness of remediation and rehabilitation technologies. AMS offers several advantages over the more usual α- and γ-spectroscopy techniques in that it can directly and quickly measure both isotopes in a sample of small size and with simple sample preparation. We show that AMS can be used to measure these isotopes of radium at the sensitivity required for environmental samples using RaC₂ ^- as the injected beam species.