Basic principles of 14C AMS

The AMS technique is an extension of conventional isotope ratio mass spectrometry in which a magnetic field is used to separate the ionised carbon isotopes by their different masses which are quantified separately. The low natural abundance of 14C (one 14C-atom in 1012 to 1015 12C-atoms) prevents the use of conventional mass spectrometer measurements, because of much higher abundances of other ions of the same mass (14N and molecular ions like 12CH2, 13CH, ...).

In AMS, 14N is eliminated by the initial choice of a negative ion beam (no stable N-) and, after selection of mass 12, 13, 14 and acceleration to 2.5 MeV, molecular ions are destroyed in a gas stripper (Ar) set to strip the negative of a few electrons to an electric charge of +3. Repeated magnetic and electrostatic filtering reduces interfering ions to a level where 14C concentrations in the range 10-12 to 10-15 can be measured.


Schematic of the AMS system: the symbols indicate L a lens, Y-S one pair, XY-S two pairs of steerers (horizontal and vertical),
BPM a beam profile monitor, FC faraday cups, SL vertical slits, and A aperture (drawing by M.-J. Nadeau).