Figure 1

Schematic diagram of an accelerator mass spectrometry (AMS). Cesium (Cs) sputter ion source (A) contains the wheel with the graphite samples under high vacuum. Atomic Cs vapor is produced from a heated Cs reservoir and sprayed on to a heated ionizer surface, producing Cs⁺ ions that are accelerated towards the target held at -8 kV. The Cs⁺ ions sputter carbon atoms and ions from the target that are ionized to C^- ions as they pass through a condensed Cs layer on the cathode. Negative ions at m/z 13 (¹³C^-) and 14 (¹⁴C^-) are pulsed through an injection magnet or low energy mass spectrometer (B) into a tandem electrostatic accelerator (C). Negative C^- ions are accelerated towards the high-voltage terminal (+518 kV) in the center of the accelerator where they are converted to positive ions, C⁺ being the most abundant. The high-energy ion beam is focused to collide with argon gas electron stripper or a thin carbon foil, 0.02–0.05 μm thick (D) in a collision cell. Molecular charged ions such as ¹³CH^- and ¹²CH₂^- do not survive the electron stripping process and are converted to atomic species, and ¹⁴N^- ions decay on a femtosecond time-scale. The positive ions are repelled toward the high-energy exit of the accelerator held at 0 V. ¹³C⁺ and ¹⁴C⁺ ions are separated by momentum using a high-energy analyzing magnet or mass spectrometer (E). The beam currents of relatively abundant ¹²C and ¹³C are measured with Faraday cups (F). The ¹⁴C beam is focused by a quadruple and electrostatic cylinder analyzer and the atoms are counted in a gas ionization detector (G). The advantage of a gas ionization detector is that it measures energy loss in terms of ΔE/E which facilitates isotope separation. It is possible to optimize the detector to the energy-loss separation of the isotope.