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Accelerator Mass Spectrometry (AMS)

Accelerator Mass Spectrometry
Accelerator Mass Spectrometry (AMS) is a technique for measuring the concentrations of rare isotopes that cannot be detected with traditional mass spectrometers. AMS works by injecting negatively charged carbon ions from the material being analyzed into a nuclear particle accelerator. The device consists of two linear accelerators joined end-to-end, with the joined section, known as the terminal, charged to a very high positive potential (greater than 3 million volts). The negative ions are accelerated towards the positive potential.

At the terminal they pass through a stripper, which is either a very thin carbon film or a tube filled with a gas at low pressure. Collisions of atoms in the stripper remove several electrons from the carbon ions, changing their polarity from negative to positive. The positive ions are accelerated through the second stage of the accelerator, reaching kinetic energies on the order of 10 to 30 million electron volts. The significance of this process for 14C measurements is that 14N negative ions are very unstable and do not exist long enough to reach the accelerator terminal. This eliminates the 14N ions that would have overwhelmed a mass spectrometer. Any other ions present that mimic 14C can be distinguished by different kinetic energies during the stripping stage. At the kinetic energies typically used in AMS, it is possible to use well-established nuclear physics techniques to detect the individual 14C ions as they arrive at a particle detector.

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