Radiocarbon dating mass spectrometry


  1. Accelerator mass spectrometry (AMS) measurement
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  3. Accelerator mass spectrometry radiocarbon dating of rock varnish | GSA Bulletin | GeoScienceWorld
How the accelerator mass spectrometer works – Ian Clark, University of Ottawa

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.

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.

Accelerator mass spectrometry (AMS) measurement

AMS, on the other hand, does not rely on radioactive decay to detect the 14 C. 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. 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 neighboring 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 that swamp the carbon 14 signal. 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. 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 graphitization 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 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.

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These metal discs are then mounted on a target wheel so they can be analyzed in sequence. Ions from a cesium gun are then fired at the target wheel, producing negatively ionized carbon atoms.

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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.

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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.

Accelerator mass spectrometry radiocarbon dating of rock varnish | GSA Bulletin | GeoScienceWorld

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 From these data, concentration ratio of the isotopes can be known to allow evaluation of the level of fractionation.