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

Accelerator Mass Spectrometry (AMS) is the analytical technique of choice f or the detection of long-lived radionuclides which cannot be practically an alysed with decay counting or conventional mass spectrometry. AMS allows an isotopic sensitivity as low as one part in 10(15) for C-14 (5.73 ka), Be-1 0 (1.6 Ma), Al-26 (720 ka), Cl-36 (301 ka), Ca-41 (104 ka), I-129 (16 Ma) a nd other long-lived radionuclides occurring in nature at ultra-trace levels . These radionuclides can be used as tracers and chronometers in many disci plines: geology, archaeology, astrophysics, biomedicine and materials scien ce. Low-level decay counting techniques have been developed in the last 40- 50 years to detect the concentration of cosmogenic, radiogenic and anthropo genic radionuclides in a variety of specimens. Radioactivity measurements f or long-lived radionuclides are made difficult by low counting rates and in some cases the need for complicated radiochemistry procedures and efficien t detectors of soft P-particles and low energy x-rays. The sensitivity of A MS 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 10(6)-10(9) times higher than decay counting and the size of the sample required for analysis is reduced accordingly. For example,C-14 is being an alysed in samples containing as little as 20 mu g carbon. There is also a w orld-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-sit u analysis of stable isotopes in geological samples, semiconductors and oth er materials. Unfortunately, the use of AMS is limited by the expensive acc elerator technology required, but there are several attempts to develop com pact AMS spectrometers at low (less than or equal to 0.5 MV) terminal volta ges. Recent advances in AMS will be reviewed with highlights from the scien tific programs at Lucas Heights and other AMS centres. (C) 2000 Elsevier Sc ience Ltd. All rights reserved.