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.
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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. 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.
Radiocarbon dating
Only very small quantities are required for the AMS measurement 30ug-3mg of carbon and so the damage to objects can be minimised. 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.
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The negative ions are accelerated towards the positive potential. At the terminal they pass through either a very thin carbon film or a tube filled with gas at low pressure the stripper , depending on the particular accelerator. Collisions with carbon or gas atoms in the stripper remove several electrons from the carbon ions, changing their polarity from negative to positive.
The positive ions are then accelerated through the second stage of the accelerator, reaching kinetic energies of the order of 10 to 30 million electron volts. The ion source also inevitably produces negatively charged molecules that can mimic 14 C, viz. These ions are stable, and while of relatively low abundance, are still intense enough to overwhelm the 14 C ions. Burning the samples to convert them into graphite, however, also introduces other elements into the sample like nitrogen When the samples have finally been converted into few milligrams of graphite, they are pressed on to a metal disc.
Reference materials are also pressed on metal discs. These metal discs are then mounted on a target wheel so they can be analyzed in sequence.
Radiocarbon WebInfo - AMS
Ions from a cesium gun are then fired at the target wheel, producing negatively ionized carbon atoms. These negatively ionized carbon atoms pass through focusing devices and an injection magnet before reaching the tandem accelerator where they are accelerated to the positive terminal by a voltage difference of two million volts. At this stage, other negatively charged atoms are unstable and cannot reach the detector. The negatively charged carbon atoms, however, move on to the stripper a gas or a metal foil where they lose the electrons and emerge as the triple, positively charged carbon atoms.
At this stage, molecules that may be present are eliminated because they cannot exist in this triple charged state. The carbon atoms with triple positive charge further accelerate away from the positive terminal and pass through another set of focusing devices where mass analysis occurs. In mass analysis, a magnetic field is applied to these moving charged particles, which causes the particles to deflect from the path they are traveling.
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If the charged particles have the same velocity but different masses, as in the case of the carbon isotopes, the heavier particles are deflected least. Detectors at different angles of deflection then count the particles. At the end of an AMS run, data gathered is not only the number of carbon 14 atoms in the sample but also the quantity of carbon 12 and carbon From these data, concentration ratio of the isotopes can be known to allow evaluation of the level of fractionation.
The greatest advantage that AMS radiocarbon dating has over radiometric methods is small sample size. Accelerator mass spectrometers need only as little as 20 milligrams and as high as milligrams for certain samples whereas conventional methods need at least 10 grams in samples like wood and charcoal and as much as grams in bones and sediments. Accelerator mass spectrometers typically need sample sizes lesser than conventional methods by a factor of 1, Radiocarbon dating is a destructive process.