AMS dating involves accelerating the ions to extraordinarily high kinetic energies followed by mass analysis.
Although more expensive than traditional radiometric dating, AMS dating has higher precision and needs small sample sizes.
Aside from archaeology and geology, AMS dating is also used in other fields like biomedical research and ocean sciences research.
There are two techniques in measuring radiocarbon in samples through radiometric dating and by Accelerator Mass Spectrometry (AMS). The two techniques are used primarily in determining carbon 14 content of archaeological artefacts and geological samples. These two radiocarbon dating methods use modern standards such as oxalic acid and other reference materials. Although both radiocarbon dating methods produce high quality results, they are fundamentally different in principle.
Radiometric dating methods detect beta particles from the decay of carbon 14 atoms while accelerator mass spectrometers count the number of carbon 14 atoms present in the sample. Both carbon dating methods have advantages and disadvantages.
Accelerator Mass Spectrometry
Mass spectrometers detect atoms of specific elements according to their atomic weights. They however, do not have the sensitivity to distinguish atomic isobars (atoms of different elements that have the same atomic weight, such as in the case of carbon 14 and nitrogen 14, the most common isotope of nitrogen).
Thanks to nuclear physics, mass spectrometers have been fine tuned to separate a rare isotope from an abundant neighbouring mass, and accelerator mass spectrometry was born. A method has finally been developed to detect carbon 14 in a given sample and ignore the more abundant isotopes.
How Does AMS Work?
There are essentially two parts in the process of radiocarbon dating through accelerator mass spectrometry. The first part involves accelerating the ions to extraordinarily high kinetic energies, and the subsequent step involves mass analysis.
There are two accelerator systems commonly used for radiocarbon dating through accelerator mass spectrometry. One is the cyclotron, and the other is a tandem electrostatic accelerator.
AMS Analysis via Tandem Accelerator
After pretreatment, samples for radiocarbon dating are prepared for use in an accelerator mass spectrometer by converting them into a solid graphite form. This is done by conversion to carbon dioxide with subsequent graphitisation in the presence of a metal catalyst. Burning the samples to convert them into graphite, however, also introduces other elements into the sample like nitrogen 14.
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 analysed in sequence.
Ions from a cesium gun are then fired at the target wheel, producing negatively ionised carbon atoms. These negatively ionised 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. 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 13. From these data, concentration ratio of the isotopes can be known to allow evaluation of the level of fractionation.
