A review of external microbeams for ion beam analyses.

Ion microbeam facilities are analytical tools with high spatial resolution exploiting MeV ion beams. The interactions of beam particles with atoms and nuclei of the target induce the emission of characteristic radiation, the energy of which provides signatures of the compositional and/or structural properties of the target; Ion-Beam Analysis (IBA) techniques, based on the detection of such radiation, enable characterisation of samples of interest, e.g. in material and earth sciences, cultural heritage, biology, medicine, and environmental studies. External beams, obtained by extracting the particles into the atmosphere through a thin window, have many attractive features, e.g. non-destructive/non-invasive analysis and ease of working, so many laboratories have dedicated beam-lines to ex-vacuo IBA analyses. External microprobes have made it possible to obtain probes in the micron range by adopting strong focusing lenses, ultra-thin windows for beam extraction, and short/ultra-short external path of beam particles in light gases; they have also made possible the use of new external IBA techniques, e.g. BS, ERDA, STIM, and IBIC. By adopting systems to raster scan the beam over the sample, imaging capabilities have also become available for ex-vacuo analysis. External scanning microprobes + IBA techniques have enabled the characterisation of samples with high spatial resolution, comparable with that achievable in-vacuum for thick samples, avoiding sample damage.