CO 2 is collected from shells by reaction with phosphoric acid. The CO 2 is then reduced to graphite with H 2 at o C using an iron catalyst. At the Laboratory, aside from modern and background standards, routine in-house measurements are also made on standards of like composition and age to the sample being dated. This enables us to maintain a constant check on the accuracy of different pretreatment and CO 2 purification processes. 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.
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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.
Accelerator mass spectrometry (AMS) measurement
Before sampling, the surface layers are usually removed because these are most susceptible to contamination. Only very small quantities are required for the AMS measurement 30ug-3mg of carbon and so the damage to objects can be minimised. The chemical pre-treatment depends on the type of sample. As an example bones are treated as follows:.
After chemical pre-treatment, the samples are burnt to produce carbon dioxide and nitrogen. A small amount of this gas is bled into a mass spectrometer where the stable isotope ratios of carbon and nitrogen are measured.
At the kinetic energies typically used in an AMS system it is possible to use well-established nuclear physics techniques to detect the individual 14 C ions as they arrive at a suitable particle detector. This may be a solid-state detector or a device based on the gridded ionisation chamber.
AMS Dating - Radiocarbon Dating Laboratory: University of Waikato
The latter type of detector can measure both the total energy of the incoming ion, and also the rate at which it slows down as it passes through the gas-filled detector. These two pieces of information are sufficient to completely identify the ion as 14 C. The main advantage is the much smaller sample size that is needed to make a measurement.
Radiometric counting can only detect 14 C atoms at the rate at which they decay. This requires sufficient atoms to be present to provide a large enough decay rate, as described above.
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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.