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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 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.
Hence, because of its ability to analyze samples even in minute amounts, accelerator mass spectrometry is the method of choice for archaeologists with small artifacts and those who cannot destroy very expensive or rare materials. Due to the sensitivity of accelerator mass spectrometers, carbon dating small particles like blood particles, a grain, or a seed have been made possible.
Accelerator mass spectrometry also takes less time to analyze samples for carbon 14 content compared to radiometric dating methods that can take one or two days. An accelerator mass spectrometer has a run time of a few hours per sample.
Accelerator mass spectrometry (AMS) dating is an advanced technique used to measure the Carbon content of materials. It involves accelerating the ions to. Sample Suitability: AMS or Radiometric Dating? Advantages of AMS Radiocarbon Dating over Radiometric Analysis by LSC: AMS dating is an advanced method compared to radiometric dating using liquid scintillation counters (LSC).
Lastly, it must be noted that AMS measurements usually achieve higher precision and lower backgrounds than radiometric dating methods. An accelerator mass spectrometer, although a powerful tool, is also a costly one. Establishing and maintaining an accelerator mass spectrometer costs millions of dollars. 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. 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. 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.
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. 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:.