The dissociation of (CH)-C-13 and (CH2)-C-12 molecules in He and N-2 at beam energies of 80-250 keV and possible implications for radiocarbon mass spectrometry

Isotopic ratios of C-14 at natural levels can be efficiently measured with accelerator mass spectrometry (AMS). In compact AMS systems, (CH)-C-13 and (CH2)-C-12 molecular interferences are destroyed in collisions with the stripper gas, a process which can be described by dissociation cross sections. These dissociation cross sections determine the gas areal density required for sufficient attenuation of the interfering molecular beams, and are therefore key parameters in the effort to further reduce the terminal voltage and thus the size of the AMS system. We measured the dissociation cross sections of (CH)-C-13 and (CH2)-C-12 in N-2 and He in the energy range of 80-250 key. In N-2, cross sections were constant for energies above 100 keV with average values per molecule of (8.1 +/- 0.4) x 10(-16) cm(-2) for (CH)-C-13 and (9.5 +/- 0.5) x 10(-16) cm(2) for (CH2)-C-12. In He, cross sections were constant over the full measured range of 80-150 key with average values of (4.2 +/- 0.3) x 10(-16) cm(2) and (4.8 +/- 0.4) x 10(-16) cm(2), respectively. A considerable reduction of the terminal voltage from the currently used 200 kV while using N-2 for (CH)-C-13 and (CH2)-C-12 molecule dissociation is not possible: the required N-2 areal densities of similar to 1.4 mu g/cm(2), consequential angular straggling and a decreasing 1+ charge state fraction would reduce the ion beam transmission too much. This is not the case for He: sufficient molecule dissociation can be obtained with gas densities of similar to 0.4 mu g/cm(2), for which angular straggling is relatively small. In addition, the 1+ charge state fraction still increases at lower stripping energies. Thus, the usage of He for stripping and molecule dissociation might allow the development of even smaller C-14-AMS systems than available today. (C) 2010 Elsevier B.V. All rights reserved.