Accelerator mass spectrometry: The new revolution in radiocarbon dating

Abstract

The development of accelerator mass spectrometry (AMS) during the last ten years has created a new revolution in the fields of radiocarbon ( 14C) dating and other types of radioisotope analysis. The radiocarbon-dating method can be used to determine the ages of many types of carbonaceous material of up to about 50 ka in age. An AMS instrument has been in operation at the NSF-Arizona Accelerator Facility for Radioisotope Analysis of the University of Arizona since 1982. It has been used almost exclusively to measure carbon-14. The greatest advantage of radiocarbon dating by AMS is the ability routinely to measure samples containing 1 mg or less of elemental carbon, while previous methods have required nearly 1000 times that amount. In addition, the ability of AMS to measure the numbers of individual 14C atoms present, rather than having to wait to detect the small numbers of 14C atoms decaying, makes the measurement time very much shorter for AMS. The precision attainable for a one-hour counting time for a sample ⪕5 ka in age is about ±0.6% or ±50 years. AMS radiocarbon dating is of value to multiple areas of Quaternary research. These include archaeology, oceanography, hydrology, geology, and paleoclimatology. While AMS dating of archaeological and geological samples was once seen as the biggest application of the method, studies of global change (atmospheric-oceanic-climatological changes in the past), with potentially important applications to predicting future changes, now contribute the largest share of samples for analysis by this laboratory. Other isotopes that have been analyzed using AMS include 10Be, 26Al, 36Cl, 41Ca, and 129I. The research applications of these isotopes are much the same as for 14C, but are useful for studying processes of time scales greater than the limit of 14C dating.