A tunable focusing beamline for desktop x-ray microtomography

Abstract

In this article a desktop x-ray microtomography (μXCT) instrument is presented, which utilizes conventional diffraction tubes and tunable focusing optics. The beamline is based on an elastically bent cylindrical multilayer mirror, given by a coated and rhombic shaped Si wafer, which is placed on bearings with two of its tips and is actuated by a single transverse center force to a desired curvature and herewith focal length. This optical element is used in a grazing incidence reflection geometry, demagnifying the tube focal spot into an image line with width (w) and position following from a classical imaging equation (magnification ratio M). While the tube and the image (“focal”) position are kept fixed, the curvature and axial position of the mirror are adaptively controlled for different M values and Bragg angles θ̄ (i.e., pass-energies Ē), which results in a one-dimensional zoom-optical system. The specimen is placed in the high-depth focal region of the condensed beam for the CT-scanning procedure with the slice orientation given by the focusing direction. Minor modifications of the fundamental rhombic mirror shape also enable the establishment of imaging geometries with elliptical and parabolic cylindrical-type optical figures. Multilayer reflection inherently results in a small bandpass of photon energies (ΔE/Ē). Pass-energy Ē is preferably tuned to characteristic lines of the tubes in use (Cr, Cu, Mo target) with the option of also using the white x-ray spectrum. Numerical values of beamline specifications are characterized by: 0.1⩽M⩽1.0, 10⩽w⩽100 μm, 0.5⩽θ̄⩽2°, 5⩽Ē⩽30 keV, ΔE/Ē≲0.1. Photon intensity along the focal line is given by 106<N<2×107 (s mm)−1, depending on the type of tube, mirror reflectivity and M setting. The fundamental principles, the experimental setup and major components of the beamline are described and the theoretical and experimental performance in terms of photon flux, pass-energy bandwidth and beam geometry are evaluated. Examples of μXCT scanning are also given. In the current configuration, a fast scintillation counter behind an object slice collimator is used for photon detection, although the sheet-like geometry of the focused x-ray beam can be further used for parallel projection data acquisition along the nonfocusing direction of the optical system (i.e., for different object slices) by application of a suitable charge coupled device-type detector.