Accelerator mass spectrometry

A mass spectrometer fronted by a high-energy particle accelerator (typically a 500 keV to 6 MeV tandem). Counts individual carbon-14 atoms directly rather than waiting for them to decay. Needs only a milligram of carbon and a few hours of run time, against grams and weeks for conventional liquid scintillation. The modern global standard for radiocarbon dating, including for questioned documents.