Accelerator Mass Spectrometry AMS is a technique for measuring the concentrations of rare isotopes that cannot be detected with conventional mass spectrometers. The original, and best known, application of AMS is radiocarbon dating, where you are trying to detect the rare isotope 14 C in the presence of the much more abundant isotopes 12 C and 13 C. The natural abundance of 14 C is about one 14 C atom per trillion 10 12 atoms of 12 C. A nuclear particle accelerator consists essentially of two linear accelerators joined end-to-end, with the join section called the terminal charged to a very high positive potential 3 million volts or higher. Injecting negatively charged carbon ions from the material being analysed into a nuclear particle accelerator based on the electrostatic tandem accelerator principle. The negative ions are accelerated towards the positive potential.
Mass spectrometry radioactive dating
This problem is solved in the tandem accelerator at the stripper —if three or more electrons are removed from the molecular ions the molecules dissociate into their component atoms. The kinetic energy that had accumulated up to now is distributed among the separate atoms, none of which has the same energy as a single 14 C ion. It is thus easy to distinguish the 14 C from the more intense "background" caused by the dissociated molecules on the basis of their kinetic energy.
Accelerating the ions to high energy has one more advantage. 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. 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.
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. A small sample size may or may not be a decisive advantage in a particular case, depending on the task and the nature of the sample material. The real advantages of AMS lie in the possibilities it offers for doing completely new kinds of measurements and using new kinds of sample materials.
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 25keV.
The ions produced are negative which prevents the confusion of 14 C with 14 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 14 this will include large number of 12 CH 2- and 13 CH - ions and a very few 14 C - ions.
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.
Accelerator mass spectrometry (AMS) measurement
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. 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. These ratios provide useful information on the purity of the sample and clues about the diet and climatic conditions of the living organism.Radiocarbon dating on ANSTO’s VEGA accelerator
The carbon isotope ratio can also be used to correct for isotopic fractionation in the radiocarbon measurement.