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.
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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.
Accelerator mass spectrometry (AMS) measurement
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.
Accelerator Mass Spectrometry
In order to measure radiocarbon ages it is necessary to find the amount of radiocarbon in a sample. This measurement can be made either by measuring the radioactivity of the sample the conventional beta -counting method or by directly counting the radiocarbon atoms using a method called Accelerator Mass Spectrometry AMS. Measurement of the radioactivity of the sample works very well if the sample is large, but in 9 months less than 0. The method is relatively new because it needs very complicated instruments first developed for Nuclear Physics research in the late 20th century.
In common with other kinds of mass spectrometry, AMS is performed by converting the atoms in the sample into a beam of fast moving ions charged atoms. The mass of these ions is then measured by the application of magnetic and electric fields.
Accelerator Mass Spectrometry
The measurement of radiocarbon by mass spectrometry is very difficult because its concentration is less than one atom in 1,,,, The accelerator is used to help remove ions that might be confused with radiocarbon before the final detection. 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.
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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.