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Spatial resolution of three-dimensional images obtained by synchrotron X-ray microtomography technique is evaluated using cyclic bar patterns machined on a steel wire. Influences of X-ray energy and t...he sample-to-detector distance on spatial resolution were investigated. High X-ray energies of 33-78 keV are applied due to the high X-ray absorption of transition metals. Best spatial resolution of about 1.2 μm pitch was observed at the sample-to-detector distance range of 20-110 mm and at the energy range of 68-78 keV. Several factors such as X-ray scattering and diffraction phenomena affecting the degradation of spatial resolution are also discussed.Over the past decade, synchrotron X-ray microtomography (XMT) has been performed on light metals such as aluminum with high spatial resolution close to the theoretical limit of the projection-type set-up (about 1 μm).1–3 However, only a few attempts have been made in the past to apply hard XMT to the transition elements such as ferrous metals due to their low X-ray transmission, even though the necessity of their three-dimensional (3D) observations has been recognized. Probability of the photoelectric absorption per unit mass of materials is approximately proportional to Z3/E3, where Z is the atomic number of the material and E is the energy of the incident photon.4 Photoelectric absorption increases with increasing atomic number and decreasing photon energy. This can lead to degradation of contrast and spatial resolution due to poor photon statistics.3 Such technological limitation in practical application of XMT to transition metals restricts utilizing advanced applications of XMT such as measurements of 3D/4D (3D with time sequence) crack-tip strain fields,5 local crack driving forces,6 and spatial distribution of chemical concentrations,7 which have been developed mainly for light metals. To solve this problem, therefore, the high energy X-ray is essential to acquire the transmitted images of the transition metals.Several attempts to the transition metals have been made using the high energy beam source. Everett et al.8 utilized white X-rays to observe elongated voids in HY-100 steel with a spatial resolution of 3 μm at X27A of the National Synchrotron Light Source (NSLS) in US. Lame et al.9 obtained a spatial resolution of 2.7 μm for sintered steel powders using white beam of 35–65 keV with a sample-to-detector (STD) distance of 200 cm at ID15 of the European Synchrotron Radiation Facility (ESRF) in France. Narrow fatigue crack of about 10 μm gap (300 μm long) in SUS630 was observed using 35 keV bending beam with a STD distance of 160 mm at BL19B2 of the Super Photon ring-8 GeV (SPring-8) in Japan.10 Cheong et al.11 (90 keV X-ray was used for field of view of 4.34 mm) and Shobu et al.12 (66.4 keV X-ray was used for field of view of 5.97 mm) used monochromatic X-rays to observe the creep voids or cracks in steels. Although no measurement or no description of spatial resolution is presented in their papers, the spatial resolutions of 19 and 35 μm, respectively, are expected at best, according to the descriptions of sample size and the number of projections. There are wide variations in visible resolution together with applied X-ray energies and STD distances.Despite its necessity, there are no systematic investigations into the up-to-date spatial resolutions of XMT, especially concerning influence of the applied X-ray energy and the STD distance. It would therefore be of significant value to attain the best achievable spatial resolution level for the transition metals. In this study, the spatial resolution of synchrotron XMT is evaluated, which further subsequent research carried out by the authors.13 This preliminary research consisted of limited STD distances of 50–200 mm at three different X-ray energies. The best 3D spatial resolution achievable for high X-ray energies is discussed with the two experimental parameters with more wide ranges, i.e., influence of X-ray energy and STD distance on spatial resolution is evaluated.XMT was conducted at the undulator beamline BL20XU of SPring-8, Japan. A monochromatic X-ray beam which is produced by a liquid nitrogen-cooled Si (511)–(333) double crystal monochromator was applied with a beam source-to-sample distance of 80 m. When 40 keV energy was used, therefore, the flux density of beam was about 4 × 1011 photons mm−2 s−1 at sample position.14 A cooled 2D-CCD (charge-coupled device) image sensor (C4880-41 S, Hamamatsu Photonics) of 4000 × 2624 pixels (pixel size of 5.9 μm × 5.9 μm) was used in 2 × 2 binning mode for acquiring transmitted X-ray images through a 10 μm thick scintillator of Lu2SiO5:Ce (LSO) and an ×20 objective. The field of view of the detector was 1000 μm × 656 μm. 1500 radiographs were recorded for one scan by scanning 180° with 0.12° step. Isotropic voxels with 0.5 μm edges were obtained in the tomographic volumes reconstructed from 2D projectional radiographs via the filtered back projection algorithm.In order to examine the effects of experimental parameters on the spatial resolution in all the three directions, the spatial resolution was evaluated using cyclic bar patterns which were machined on a SUS304 stainless steel wire of about 500 μm in diameter (transmission ratio I/Io is about 25% at 40 keV) using focused ion beam. The patterns consisted of cyclic gratings with a line pitch (spacing between two center lines of the adjacent bars engraved on the wire) of 1.02–11.90 μm (84.03–980.39 lp/mm (line pairs per mm)) in the vertical direction and 0.96–11.78 μm (84.89–1041.67 lp/mm) in the circumferential direction of cylindrical coordinates, as shown in Fig. 1. Visibility of resolved patterns in the vertical and the circumferential directions was evaluated by the subjective inspection, which was based on reasonable agreement with 5% modulation transfer function (MTF) contrast. For the resolution in the radial direction, the MTF which was derived from an edge response function (ERF) at the outer contour of the wire (i.e., the metal/air interface) was calculated with a threshold of 5% contrast ratio. The eleven different X-ray energies from 33 to 78 keV (with an interval of 5 keV) were used with varying the STD distance from 2 to 140 mm (nine conditions with an interval of 15 mm) to study influence of X-ray energy and the STD distance on spatial resolution.show more
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