Isobaric molecules suppression by a surface stripper for the AMS system with ion energy below 50keV: a numerical study

Full text: The technology for downsizing AMS systems has approached a stage in the development of ¹⁴C-AMS systems with a footprint below 2m x 2m (ion energy below 50 keV without a tandem accelerator). One of the challenges is to overcome high background which is attributed to poor vacuum conditions originated from a gas stripper used to destroy interference isobaric molecules such as ¹²CH₂, ¹³CH. As the system scale is reduced, the position of the gas stripper becomes closer to a magnetic filter. As a result, it is difficult to maintain the pressure of the filter low enough to prevent various collisional processes inside the filter. A novel stripper technique based on ion–surface interactions, which would be termed "surface stripper", was proposed as a gas free stripper [1]. Under grazing angle incidence of ions on a single crystal surface, the ions can be specularly reflected by a repulsive planer potential of the first atomic plane. During the reflection, the ions interact with surface electrons and atoms, which may result in charge transfer and dissociation of those isobaric molecules. In addition, the specular reflection not only ensures suppression of the large angle scattering, but also is of advantage to avoid damage to the crystal surface or the surface stripper. We numerically estimated the suppression efficiency of the surface stripper in the case of the grazing incidence of the ¹³CH with energy of tens of keV onto a single crystal surface. The present study is focused on the effect of the collision with the surface electrons on the dissociation. The energy distribution of the surface electrons along the trajectory was derived using a local density approximation. In addition, the model involves a softening of the C-H bond due to the electrostatic screening by the surface electrons. The obtained survival probability of ¹³CH falls to below 1/10¹² at which the level is required for the practical use. This suggests the sufficient dissociation capability of the surface stripper.