A tunable focusing beamline for desktop x-ray microtomography

In this article a desktop x-ray microtomography (mu XCT) instrument is pres ented, which utilizes conventional diffraction tubes and tunable focusing o ptics. The beamline is based on an elastically bent cylindrical multilayer mirror, given by a coated and rhombic shaped Si wafer, which is placed on b earings with two of its tips and is actuated by a single transverse center force to a desired curvature and herewith focal length. This optical elemen t 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 an gles <(theta)over bar> (i.e., pass-energies (E) over bar), 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 i maging geometries with elliptical and parabolic cylindrical-type optical fi gures. Multilayer reflection inherently results in a small bandpass of phot on energies (Delta E/(E) over bar). Pass-energy (E) over bar 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 beam line specifications are characterized by: 0.1 less than or equal to M less than or equal to 1.0, 10 less than or equal to w less than or equal to 100 mu m, 0.5 less than or equal to<(theta)over bar>less than or equal to 2 deg rees, 5 less than or equal to (E) over bar less than or equal to 30 keV, De lta E/(E) over bar less than or similar to 0.1. Photon intensity along the focal line is given by 10(6)< N < 2x10(7) (s mm)(-1), depending on the type of tube, mirror reflectivity and M setting. The fundamental principles, th e 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 mu XCT scann ing are also given. In the current configuration, a fast scintillation coun ter behind an object slice collimator is used for photon detection, althoug h the sheet-like geometry of the focused x-ray beam can be further used for parallel projection data acquisition along the nonfocusing direction of th e optical system (i.e., for different object slices) by application of a su itable charge coupled device-type detector. (C) 1999 American Institute of Physics. [S0034-6748(99)04407-X].