Test of negative ion beams from a microwave ion source with a charge exchange canal for accelerator mass spectrometry applications

A test facility has been constructed to evaluate negative ion beams from small gaseous samples for accelerator mass spectrometry applications. The positive ion beams from the microwave ion source are passed into a charge-exchange canal (CXC) where the ions exchange electrons with magnesium vapor and become negatively charged. Positive molecular ions were either neutralized or broken up into atomic ions and neutral atoms and molecules by collision processes. Most of the resulting particles were suspected to be neutrals. In studies with injections of CO 2 pulses, the resulting positive and negative 12 C current peaks gave a 0.09% yield of C − ions from CO 2 molecules, which includes a charge-exchange efficiency of 10%. Since nitrogen does not form a stable negative ion, 14 N background is virtually eliminated after the beam goes through the CXC, which is necessary for radiocarbon measurements.