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Accelerator mass spectrometry

AMS counts the atoms of C and C in a given sample, determining the C / C ratio directly. The sample, often in the form of graphite, is made to emit C ions (carbon atoms with a single negative charge), which are injected into an accelerator. The ions are accelerated and passed through a stripper, which removes several electrons so that the ions emerge with a positive charge. The ions, which may have from 1 to 4 positive charges (C to C), depending on the accelerator design, are then passed through a magnet that curves their path; the heavier ions are curved less than the lighter ones, so the different isotopes emerge as separate streams of ions. A particle detector then records the number of ions detected in the C stream, but since the volume of C (and C, needed for calibration) is too great for individual ion detection, counts are determined by measuring the electric current created in a Faraday cup. The large positive charge induced by the stripper forces molecules such as CH, which has a weight close enough to C to interfere with the measurements, to dissociate, so they are not detected. Most AMS machines also measure the sample's δ C, for use in calculating the sample's radiocarbon age. The use of AMS, as opposed to simpler forms of mass spectrometry, is necessary because of the need to distinguish the carbon isotopes from other atoms or molecules that are very close in mass, such as N and CH. As with beta counting, both blank samples and standard samples are used. Two different kinds of blank may be measured: a sample of dead carbon that has undergone no chemical processing, to detect any machine background, and a sample known as a process blank made from dead carbon that is processed into target material in exactly the same way as the sample which is being dated. Any C signal from the machine background blank is likely to be caused either by beams of ions that have not followed the expected path inside the detector or by carbon hydrides such as CH or CH. A C signal from the process blank measures the amount of contamination introduced during the preparation of the sample. These measurements are used in the subsequent calculation of the age of the sample.