Accelerator mass spectrometry AMS 14 C dating off rock varnish provides minimum-limiting ages for landforms and archaeological artifacts in arid and semiarid lands that are undatable by conventional radiocarbon methods. Experiments on sample collection, the effect of different individuals preparing samples, the influence of different chemical-extraction procedures, the incorporation of carbonate detritus, possible contamination from rock underlying varnish, the role of bio-geochemical erosion of varnish, and other influences reveal procedures that yield reproducible results.
Conventional radiocarbon dates from charcoal beneath lava flows of Hualalai Volcano, Hawaii, and from arid sites in western North America provide controls to test varnish radiocarbon dating. Applications are illustrated here for fluvial, aeolian, peri-glacial, hillslope, lacustrine, and glacial geo-morphology, as well as rock-art research in archaeology.
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Volume , Number Previous Article Next Article. Research Article November 01, Accelerator mass spectrometry radiocarbon dating of rock varnish R.
This requires sufficient atoms to be present to provide a large enough decay rate, as described above. AMS, on the other hand, does not rely on radioactive decay to detect the 14 C. The AMS technique literally extracts and counts the 14 C atoms in the sample, and at the same time determines the amount of the stable isotopes 13 C and 12 C. As a consequence, a measurement that may take several days and require grams of sample using decay counting may take only 30 minutes and consume a milligram using AMS.
A small sample size may or may not be a decisive advantage in a particular case, depending on the task and the nature of the sample material. The real advantages of AMS lie in the possibilities it offers for doing completely new kinds of measurements and using new kinds of sample materials.
Accelerator Mass Spectrometry
A novel application of AMS is the measurement of 14 C tracer used at near-natural levels in biomedical and pharmaceutical research. While 14 C has long been used as a tracer for chemical processes and pathways, the amount of tracer required using decay counting can be hazardous to the researchers, pose contamination problems or, in some cases, itself influence the process being studied.
AMS allows very low levels of tracer to be used, completely avoiding these problems. AMS tends to be more expensive than decay counting because purchasing and maintaining a particle accelerator and its associated components is expensive.
Accelerator mass spectrometry radiocarbon dating of rock varnish | GSA Bulletin | GeoScienceWorld
The accelerator is used to help remove ions that might be confused with radiocarbon before the final detection. The sample is put into the ion source either as graphite or as carbon dioxide. It is ionised by bombarding it with caesium ions and then focused into fast-moving beam energy typically 25keV.
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- Accelerator mass spectrometry (AMS) measurement!
The ions produced are negative which prevents the confusion of 14 C with 14 N since nitrogen does not form a negative ion. The first magnet is used in the same way as the magnet in an ordinary mass spectrometer to select ions of mass 14 this will include large number of 12 CH 2- and 13 CH - ions and a very few 14 C - ions. The ions then enter the accelerator.
These are then accelerated down the second half of the tandem accelerator reaching energies of about 8MeV. The second magnet selects ions with the momentum expected of 14 C ions and a Wien filter checks that their velocity is also correct. Finally the filtered 14 C ions enter the detector where their velocity and energy are checked so that the number of 14 C ions in the sample can be counted.
- Accelerator Mass Spectrometry, C14 Dating, What is AMS?;
- Radiocarbon WebInfo - AMS.
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Not all of the radiocarbon atoms put into the ion source reach the detector and so the stable isotopes, 12 C and 13 C are measured as well in order to monitor the detection efficiency. Careful sampling and pre-treatment are very important stages in the dating process, particularly for archaeological samples where there is frequently contamination from the soil.
Before sampling, the surface layers are usually removed because these are most susceptible to contamination.