Accelerator mass spectrometry (AMS) counts rare, usually long-lived isotopes after ions have been accelerated to high kinetic energies. A negative-ion source first forms an isotopic beam; mass and charge selection precede acceleration through a tandem or other electrostatic accelerator, and passage through a stripper removes electrons, breaks up molecular ions, and produces positive charge states ^[1]. Subsequent magnetic and electrostatic analysis separates the desired nuclide, while particle detectors identify individual rare-isotope ions and stable-isotope currents provide ratio normalization ^[2]. AMS therefore measures atom-number isotope ratios rather than decay rates or molecular fragmentation patterns. Its defining capabilities arise from molecular-isobar destruction, high abundance sensitivity, and single-ion detection at isotope ratios well below those accessible to conventional mass spectrometry ^[3]. The method is distinct from isotope-ratio mass spectrometry without an accelerator and from radiometric counting. Essential components are an ion source, staged mass filtering, high-energy acceleration and stripping, nuclide-specific interference rejection, and rare-ion detection.