Dr Fiona Petchey is using carbon-14 (C-14) to date artefacts of historical importance excavated from the Wairau Bar archaeological site in Blenheim.
However, pre-1950 samples that are less than 100 years old or older than 60,000 years cannot be accurately dated. The reason for this has to do with the concentration of C-14 in living materials as well as the half-life of the C-14 isotope. Atomic bomb detonations since 1950 have boosted the amount of C-14 in the atmosphere and, as a result of this, a method has been devised to date recent samples.
How does C-14 get into living materials?
C-14 is formed in the upper atmosphere by nuclear reactions initiated by neutrons in cosmic radiation (14N7 + 1n → 14C6 + 1H1). Once formed, the C-14 reacts with oxygen to form 14CO2. This enters the carbon cycle, circulating through the atmosphere, oceans and biosphere. Plants absorb the CO2 and convert it to organic compounds, and in this way, C-14 becomes incorporated into living tissue. When the plant dies, C-14 will no longer be taken up. Radioactive decay still continues, however, with the C-14 activity decreasing over time.
How can it be used to date samples?
A 1 gram sample of carbon from living material shows an activity of about 14 disintegrations per minute. The half-life of C-14 is 5,730 years. If an excavated sample of plant or animal origin from an archaeological site had a measured activity of 7 disintegrations per minute (dpm), the age of the sample could be fixed at about 5,730 years ± 40 years. At 3.5 dpm, the age would be about 11,640 years and so on. After 50,000 years, the activity of C-14 in the sample would be extremely low, making it difficult to accurately date the sample.
Fiona’s C-14 dating techniques
In the C-14 dating laboratory that Fiona works in, two dating techniques are used.
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-14 atoms decay. A special detector called a spectrometer can ‘see’ these specks and, with the aid of a computer program, can count them and determine the date of the sample.
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. The carbon present in the gas is then converted to a small plug of a graphite/iron composite. 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-14 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. Read more in this NZ Herald news story here.
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