ON THE EFFECT OF BEAM SPATIAL BROADENING IN ION MICROTOMOGRAPHY (IMT) IMAGE QUALITY

Ion microtomography (IMT) is a quantitative MeV ion beam technique capable of mapping spatial variations in specimen density. IMT images are obtained from ion energy loss measurements using computed tomography methods. Multiple scattering of individual ions results in cumulative small directional changes that can ultimately lead to significant spatial broadening of the focussed microbeam. Quantitative studies on the effect of beam spatial broadening on IMT image quality have not previously been performed and current reconstruction methods ignore this effect when computing cross-sectional density images. In this work, the effect of beam spatial broadening on IMT image quality is examined and an algorithm that partially corrects for it is described for specimens with suitable composition. The algorithm is initially tested by considering a single slice of a simulated test object in which spatial broadening out of the plane of the slice is neglected. This approximation has a negligible effect on the corrected image quality in this case since the test object geometry is taken to be identical over neighbouring slices. The correction algorithm is then applied to measured data collected from a single slice of a specimen having similar properties as the test object. Comparisons between the measured and simulated data show qualitative agreement. Results with spatial broadening corrections are presented for contiguous multi-slice data of other samples. It is shown that beam spatial broadening can have an appreciable effect on IMT image quality.