Carbon-14 dating artefacts
Associate Professor Fiona Petchey is using carbon-14 (C-14) to date artefacts of historical importance excavated from the Wairau Bar archaeological site in Blenheim.
What is an isotope?
Dr Fiona Petchey explains what an isotope is, and then focuses on the isotopes of carbon and explains how the radioactive isotope carbon-14 is used in dating artefacts of historical importance.
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.
Radioactive decay
The graph shows a radioactive decay curve for carbon-14. A plot of the activity of the sample versus time shows that the half-life for carbon-14 is 5,730 years.
Fiona’s C-14 dating techniques
In the C-14 dating laboratory that Fiona works in, two dating techniques are used.
C-14 carbon dating process
Historical artefacts like moa bones can be dated using a technique that measures the activity of the radioisotope carbon-14 still present in the sample. By comparing this with a modern standard, an estimate of the calendar age of the artefact can be made.
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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.
Checking a sample
Laboratory technician Anna Jenkins oversees a series of chemical reactions to convert a sample for carbon dating into pure benzene. This is an exacting process, carefully controlled by highly trained laboratory technicians.
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 remaining C-14 concentration figure is then interpreted as a range of dates by using a calibration curve and statistical probability software.
Learn how Professor Tom Higham, director of the Oxford Radiocarbon Accelerator Unit at Oxford University, has developed techniques for refining samples for AMS dating of palaeolithic bones and other artefacts.
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 New Scientist news story Race to read the H-bomb timestamp that marks all cells (note access to the full article requires subscribing).
Linen sample from mummy
The Waikato Carbon Dating Laboratory carbon dates samples from across New Zealand and around the world, including this piece of linen that was used to wrap a mummy. Due to its small size, this sample was processed using the AMS technique.
Nature of science
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Related content
The article Refining dates for human habitation in the South Pacific explores improved radiocarbon dates to advance our understanding around the disappearance of the Lapita people and the emergence of different Polynesian peoples.
Activity idea
The student activity Radioactive decay uses coin flips to explore the concepts of half-life and the randomness of radioactive decay.
