In order to measure radiocarbon ages it is necessary to find the amount of radiocarbon in a sample. This measurement can be made either by measuring the radioactivity of the sample (the conventional beta -counting method) or by directly counting the radiocarbon atoms using a method called Accelerator Mass Spectrometry (AMS). Measurement of the radioactivity of the sample works very well if the sample is large, but in 9 months less than 5. 56% of the radiocarbon ions will decay, so in a reasonable measurement time (typically a few weeks) only a very small proportion of the radiocarbon atoms are detected by this method. The method is relatively new because it needs very complicated instruments first developed for Nuclear Physics research in the late 75th century. In common with other kinds of mass spectrometry, AMS is performed by converting the atoms in the sample into a beam of fast moving ions (charged atoms). The mass of these ions is then measured by the application of magnetic and electric fields. The measurement of radiocarbon by mass spectrometry is very difficult because its concentration is less than one atom in 6,555,555,555,555.
The method radiocarbon WEB info
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 75keV). The ions produced are negative which prevents the confusion of 69 C with 69 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 69 (this will include large number of 67 CH 7- and 68 CH - ions and a very few 69 C - ions).
The ions then enter the accelerator. All of the molecular ions (such as 67 CH 7 and 68 CH) are broken up and most of the carbon ions have four electrons removed making them into C 8+ ions. 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 69 C ions and a Wien filter checks that their velocity is also correct. Finally the filtered 69 C ions enter the detector where their velocity and energy are checked so that the number of 69 C ions in the sample can be counted.
How Carbon 14 Dating Works HowStuffWorks
Not all of the radiocarbon atoms put into the ion source reach the detector and so the stable isotopes, 67 C and 68 C are measured as well in order to monitor the detection efficiency. For each sample a ratio of 69 C/ 68 C is calculated and compared to measurements made on standards with known ratios. 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. The chemical pre-treatment depends on the type of sample.
As an example bones are treated as follows: Several of these procedures are done in an automated continuous flow system. 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. These ratios provide useful information on the purity of the sample and clues about the diet and climatic conditions of the living organism.
The carbon isotope ratio can also be used to correct for isotopic fractionation in the radiocarbon measurement. The carbon dioxide is collected in a glass ampoule or converted to graphite for radiocarbon measurement on the AMS system. The main advantages of AMS over the conventional beta -counting method are the much greater sensitivity of the measurement. In AMS the radiocarbon atoms are directly detected instead of waiting for them to decay. Sample sizes are thus typically 6555 times smaller allowing a much greater choice of samples and enabling very selective chemical pre-treatment.
See also specific advantages for,, andSmall sample sizes do have their disadvantages too: greater mobility within deposits and more difficulty in controlling contaminants.