ERDA with an external helium ion micro-beam: Advantages and potential applications

Preliminary ERDA experiments at atmospheric pressure have been performed wi th our external microprobe set-up currently used for the analysis of museum objects by PIXE, RBS and NRA. The objective was to check the feasibility o f hydrogen (and deuterium) profiting with an external beam of 3-MeV helium ions. The standard scattering geometry (incident beam at 15 degrees with re spect to sample surface and emerging protons or deuterons at 15 degrees in the forward direction) was kept, but the thin foil absorber was replaced by helium gas filling the space between the beam spot and the detector over a distance of about 84 mm. Several standards prepared by ion implantation, w ith well known H or D depth profiles, were first analysed, which indicated that the analytical capability was as good as under vacuum. A striking feat ure is the much lower surface peak than under vacuum, a fact that enhances the sensitivity for H analysis near the surface. The same type of measureme nt was then performed on different materials to show the usefulness of the technique. As a first example, we have checked that the incorporation of H or D into sapphire crystals during mechanical polishing is below the detect ion limit. Another example is the measurement of the H content in emeralds which can be used as an additional compositional criterion for determining the provenance of emeralds set in museum jewels. The advantages and limitat ions of our set-up are discussed and several possible applications in the f ield of cultural heritage are described. (C) 2001 Elsevier Science B.V. All rights reserved.