Review of X-ray Optics for Synchrotron Beamline Development

There are multiple x-ray optics available for synchrotron development, including:


Summary of the performance of Common X-ray Optics

Of these, the most commonly employed synchrotron optics include: CRLs, zoneplates, and KB mirrors. Sigray's novel paraboloidal capillary x-ray mirror lens provide numerous advantages (in terms of numerical aperture, chromaticity, efficiency, price, etc.) over these optics, as summarized in the table below.

To best provide understanding of the relative performances of the optics, we will review the basic concept of each x-ray optic and its advantages and disadvantages below.


refractive lenses | compound refractive lens (crls)

CRLs have developed significantly in the past few years, thanks in part to efforts by several European Synchrotron Radiation Facility (ESRF) research groups in parabolic-shaped CRLs. These lenses are generally produced by generating holes along the optical axis in a low atomic number material.

Advantages:

  • Ease of Alignment (Preservation of the x-ray beam trajectory)
  • Compact
  • Potential for high resolution
  • Stable to angular vibrations & in-line with beam

Disadvantages:

  • Chromatic: Not usable for spectroscopy or other energy-varying techniques
  • Low to mid efficiency (particularly for lower energy x-rays due to absorption)
Compound Refractive Lens (CRLs)

Compound Refractive Lens (CRLs)


mirrors | Kirkpatrick-baez (KB) mirrors

KB Mirrors are one of the most commonly used optics at synchrotron beamlines and typically comprise an upstream and downstream pair of perpendicular reflecting mirrors. These mirrors are typically large and heavy, and require precise systems and aids for the alignment of the two mirrors and to achieve the high degree of stability and vibration isolation required.

Advantages: 

  • Can achieve very high resolution (e.g. down to 20-50 nm)
  • Achromatic

Disadvantages:

  • Cannot be easily moved in and out of the beam
  • Deviates the direction of the beam
  • Large size necessitates significant capital expenditure on alignment and motion control systems in addition to price of the mirror
  • Long focal length results in a longer beamline, which can be costly

A complete comparison of Sigray's optics and KB mirrors can be accessed here

Kirkpatrick Baez mirror pair

Kirkpatrick Baez mirror pair


Fresnel Diffractive Zone Plate for focusing x-rays

Fresnel Diffractive Zone Plate for focusing x-rays

diffractive optics | fresnel zone plates

Fresnel zone plates (FZPs) are comprised of concentric circular zones of two different materials with different absorption or phase shifting properties such that x-rays are diffracted at the interface of the zones onto a focal point. ZPs manufactured with smaller minimum zone widths (outermost zone) correspond to better spatial resolution and superior numerical aperture. Recent developments include stacked/multilayer ZPs for improved efficiency. 

Advantages:

  • Can achieve very high resolution
  • Large numerical aperture
  • Easy handling and can be moved in and out of the beam

Disadvantages:

  • Chromatic
  • Low efficiency, particularly for mid to high energy x-rays

Waveguides | tapered capillary & Polycapillaries

Tapered capillaries are x-ray condensers utilize multiple internal reflections within a conical capillary. Due to their limited efficiency and large divergence of the outgoing beam (resulting in small working distances), tapered capillaries are no longer commonly used in new beamline developments.

Polycapillary x-ray optics are a bundle of capillaries that provide higher collection efficiency than the single tapered capillaries but function more as a "wave guide" in which photons are reflected multiple times within each capillary. These lenses are more commonly used in laboratory instruments. For laboratory system development, Sigray also produces twin-paraboloidal laboratory optics with superior performance to polycapillaries.

Tapered x-ray capillary optic

Tapered x-ray capillary optic

Polycapillary x-ray optics

Polycapillary x-ray optics


sigray paraboloidal capillary x-ray optics

Sigray offers custom synchrotron x-ray condensers in addition to its laboratory line of twin paraboloidal x-ray mirror lens. 

More information can be accessed at the webpage here and through a downloadable technical guide here. 

Advantages:

  • Extremely efficient due to single bounce (>85%)
  • True achromatic imaging optics (not a wave guide)
  • Large numerical aperture 
  • Multiple condensers can be used within the same beamline, with each optic optimized for different conditions or techniques
  • Ease of alignment: In-line with the beam and compact nature