Accelerator mass spectrometry analyses of environmental radionuclides: sensitivity, precision and standardisation

Accelerator Mass Spectrometry (AMS) is the analytical technique of choice for the detection of long-lived radionuclides which cannot be practically analysed with decay counting or conventional mass spectrometry. AMS allows an isotopic sensitivity as low as one part in 1015 for 14C (5.73 ka), 10Be (1.6 Ma), 26Al (720 ka), 36Cl (301 ka), 41Ca (104 ka), 129I (16 Ma) and other long-lived radionuclides occurring in nature at ultra-trace levels. These radionuclides can be used as tracers and chronometers in many disciplines: geology, archaeology, astrophysics, biomedicine and materials science. Low-level decay counting techniques have been developed in the last 40-50 years to detect the concentration of cosmogenic, radiogenic and anthropogenic radionuclides in a variety of specimens. Radioactivity measurements for long-lived radionuclides are made difficult by low counting rates and in some cases the need for complicated radiochemistry procedures and efficient detectors of soft β-particles and low energy x-rays. The sensitivity of AMS is unaffected by the half-life of the isotope being measured, since the atoms not the radiations that result from their decay, are counted directly. Hence, the efficiency of AMS in the detection of long-lived radionuclides is 106-109 times higher than decay counting and the size of the sample required for analysis is reduced accordingly. For example, 14C is being analysed in samples containing as little as 20 μg carbon. There is also a world-wide effort to use AMS for the analysis of rare nuclides of heavy mass, such as actinides, with important applications in safeguards and nuclear waste disposal. Finally, AMS microprobes are being developed for the in-situ analysis of stable isotopes in geological samples, semiconductors and other materials. Unfortunately, the use of AMS is limited by the expensive accelerator technology required, but there are several attempts to develop compact AMS spectrometers at low (≤0.5 MV) terminal voltages. Recent advances in AMS will be reviewed with highlights from the scientific programs at Lucas Heights and other AMS centres