Added: Thanh Mckenzie - Date: 28.07.2021 12:55 - Views: 43427 - Clicks: 4332
Dr Fiona Petchey is using carbon C to date artefacts of historical importance excavated from the Wairau Bar archaeological site in Blenheim. However, pre samples that are less than years old or older than 60, years cannot be accurately dated. The reason for this has to do with the concentration of C in living materials as well as the half-life of the C isotope. Atomic bomb detonations since have boosted the amount of C in the atmosphere and, as a result of this, a method has been devised to date recent samples. Once formed, the C reacts with oxygen to form 14 CO 2.
This enters the carbon cycle, circulating through the atmosphere, oceans and biosphere.
Plants absorb the CO 2 and convert it to organic compounds, and in this way, C becomes incorporated into living tissue. When the plant dies, C will no longer be taken up.
Radioactive decay still continues, however, with the C activity decreasing over time. A 1 gram sample of carbon from living material shows an activity of about 14 disintegrations per minute. The half-life of C is 5, years.
After 50, years, the activity of C in the sample would be extremely low, making it difficult to accurately date the sample. Liquid scintillation spectrometry involves converting all of the carbon in the sample to a liquid called benzene. A special chemical is added to the sample that produces tiny specks of light called scintillations when carbon atoms decay.
Accelerated mass spectrometry AMS is the other technique used. It involves converting the carbon in only a very small piece of the artefact to carbon dioxide gas by controlled combustion. This is then sent to the US for analysis in a device known as an accelerated mass spectrometer. The AMS can count all of the C atoms in the sample resulting in increased sensitivity.
The University of Waikato's radiocarbon dating laboratory helped to identify the remains of a woman found in New South Wales 45 years after she went missing. in this NZ Herald news story here.
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