X-ray focusing

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X-rays interact weakly with matter and are thus difficult to focus. The x-ray refractive index of most materials is extremely close to 1.0, which means that refraction is extremely weak and the conventional kinds of optics used in visible-light optics (glass lenses, metal mirrors, polarizers, etc.) are not applicable. Nevertheless, a variety of tricks can be used to direct and focus x-rays.

For instance, since x-rays undergo total external reflection at very shallow angles (grazing-incidence), one can use extremely flat materials to slightly reflect x-rays. For instance, silicon wafers coated with metal stripes are typically used to achieved the required flatness, while having a sufficiently large electron density so that the critical angle is reasonably large. Because these mirrors are being used at grazing-angles, the beam projection is quite large: x-ray mirrors must typically be 100 mm to 2 m in length along the beam. An x-ray mirror can be very slightly bent, in which case the curvature effectively acts as a focusing optic. (Note that the radius of curvature is typically 6-30 km!)

Focusing Optics

  • Mirrors: Grazing-incidence mirrors that are slightly curved in order to focus the x-ray beam.
    • Kirkpatrick-Baez Mirrors: Two mirrors oriented at right angles.
    • Wolter Mirrors: Can be used to form images of extended (non-point-source) objects. Cylindrically symmetric mirrors.
  • Capillary optics: Bundles of light-guiding pipes/capillaries can be used to focus.
  • Compound Refractive Lenses (CRL): Sequences of curved interfaces, to accumulate refractive effects and achieve focusing. (c.f. CRL)
  • Fresnel Zone Plates (FZP): Use diffraction effects (from rings of progressively different spacing/size) to focus beam.
  • Laue lens: Uses Bragg diffraction in order to focus beam (usually using a tilted crystal). Multilayer Laue lenses (MLL) can be used as effectively 1-D half-linear-zone-plates.
  • Kinoform lenses: Combine refractive and diffractive designs. (c.f. Detlef Smilgies presentation)

See Also