Materials for x-ray refractive lenses minimizing wavefront distortions
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Introduction The design and operation of advanced synchrotron light beamlines rely on the availability of near-perfect optical elements, some of which are used to collimate or focus the incident beam. Common focusing elements1 include bent crystals, multilayers or mirrors, capillaries, waveguides, refractive lenses, and diffractive elements such as Fresnel zone plates or multilayer Laue lenses. Focused beam sizes on the order of a few micrometers are now routinely achieved; focused beam sizes of a few tens of nanometers were recently obtained at some experimental stations. This article examines focusing x-ray optics that exploit refraction mechanisms. The first lens of this type, a compound refractive lens (CRL), was proposed in 19942 and employed in 1996.3 Since then, CRLs have become widespread. An overview of previously and currently used x-ray lenses, ordered by manufacturing technique, is available in the Supplementary Material, available online. CRLs have several advantages. (1) They occupy a very small footprint; (2) they do not cause deviations in beam direction; and (3) they do not require any sorting apertures for suppression of unwanted diffraction orders. Moreover, they are easy and fast to align, and obtaining a microfocused beam for energies E ≥ 2 keV4 is straightforward.
Focusing and absorption of x-rays in the lens material is described by the refractive index:3 n = 1 − δ + iβ,
(1)
where δ is the real part of the refractive-index decrement, which is responsible for refraction. In the imaginary part, β is the absorption coefficient, which is responsible for the attenuation of the beam (due to absorption and scattering caused by the photoelectric effect and the Compton effect) of x-rays in the material. For hard x-rays, the dependence on the material is approximated by
δ β ∝ Z and ∝ Z 3 with Z being the atomic ρ ρ
number and ρ the element density. Materials with low Z, but high density are, therefore, desired candidates for CRLs.5 The refractive-index real-part decrement δ is a small number (∼10−5−10−6). Therefore, refractive lenses are fabricated with a pronounced curvature, and a series of stacked refractive surfaces are needed in order to produce a relatively short focal length (this is the origin of the word “compound” in CRL). Absorption in the thicker parts of the lens limits the practical numerical aperture (NA), defining the optics opening angle.
Thomas Roth, European Synchrotron Radiation Facility, France; and European X-Ray Free Electron Laser GmbH, Germany; [email protected] Lucia Alianelli, Diamond Light Source, UK; [email protected] Daniel Lengeler, RXOPTICS, Germany; [email protected] Anatoly Snigirev, X-ray Optics Laboratory, Baltic Federal University, Russia; [email protected] Frank Seiboth, Deutsches Elektronen-Synchrotron, Germany; and SLAC National Accelerator Laboratory, USA; [email protected] doi:10.1557/mrs.2017.117
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