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A Simple Grazing Incidence X-Ray Lens
Published online by Cambridge University Press: 06 March 2019
Abstract
Currently, making use of the phenomenon of total external reflection of x-rays by matter, lenses are being designed and constructed for use in x-ray telescopes, x-ray microscopes and small angle x-ray scattering cameras. One of the lenses under consideration for a biomedical application is a right-circular cylinder of nominal wall thickness coated with gold. Since the particular biomedical application is not demanding with respect to resolution and image quality in comparison with an x-ray telescope or an x-ray microscope, the imaging properties of the simple right-circular cylinder are satisfactory and the device economical to produce. A detailed study of its properties was undertaken. One of the most stringent requirements on the internal grinding and polishing of such a lens is the roundness of the cylinder. An interferometric method using a laser source enabled the diameter to be determined to one part in 106. Several tests of optical quality vere performed at 6328A in the visible and 8.34A in the X-ray region. A centered stop prevented the direct beam from reaching the screen. A point source of highly coherent 6328A radiation was located on the axis of the cylinder. The divergent beam of light striking the inside walls of the cylinder gives rise to a non-localized Airy-type diffraction pattern. If the surface finish is rough the diffraction pattern will show a star-like pattern at its center. Lack of roundness is indicated by poorly shaped diffraction fringes of low quality. Other optical tests at 6328A in the near and far field regions as well as in the “focal' plane also yield information concerning the roundness and smoothness of the optical element. The previous optical tests when repeated with 8.34A x-rays should give essentially the same general diffraction and interference effects noted with the 6328A radiation. However, most effects are not observable because of the poor resolution of x-ray film, Image forming tests clearly show the presence of barrel and pincushion type distortion.
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