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

Abstract

Isotopic ratios of 14C at natural levels can be efficiently measured with accelerator mass spectrometry (AMS). In compact AMS systems, 13CH and 12CH 2 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 13CH and 12CH 2 in N 2 and He in the energy range of 80–250 keV. In N 2, cross sections were constant for energies above 100 keV with average values per molecule of (8.1 ± 0.4) × 10 −16 cm 2 for 13CH and (9.5 ± 0.5) × 10 −16 cm 2 for 12CH 2. In He, cross sections were constant over the full measured range of 80–150 keV with average values of (4.2 ± 0.3) × 10 − 16 cm 2 and (4.8 ± 0.4) × 10 −16 cm 2, respectively. A considerable reduction of the terminal voltage from the currently used 200 kV while using N 2 for 13CH and 12CH 2 molecule dissociation is not possible: the required N 2 areal densities of ∼1.4 μg/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 ∼0.4 μg/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 14C-AMS systems than available today.