Technical concepts
The main technical challenges of SAXS are to separate the strong primary X-ray beam from the weak scattering information close
to the primary X-ray beam and to increase the intensity of the scattered X-rays.
Figure 2: A typical setup for a small-angle X-ray scattering instrument includes a source, collimation system with optics
(i.e., mirror), sample holder, and detector. The area that the beam enters after passing the mirror is typically evacuated
to reduce unwanted scattering from gas molecules.
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Scientists have used different concepts to achieve these goals. However, all SAXS instruments feature an X-ray source, a collimation
system focusing optics, a sample holder, a beam stop, and a detector (see Figure 2).
X-ray sources
To date, copper anodes, producing Cu–Kα radiation, have been the best compromise between sensitivity and absorption. The following
three technologies are currently in use:
- Sealed-tube sources
- Rotating-anode sources
- Microfocus sources.
In sealed-tube sources, the electrons are accelerated toward a fixed anode. Part of the energy is emitted as X-rays, but the
majority is converted to heat. In sealed-tube sources, the anode is fixed, so a limited amount of energy can be dissipated
before the copper melts. Rotating-anode sources exceed this limit and avoid local overheating (5).
Microfocus sources use electromagnetic lenses to focus the electron beam to a fine point, thus producing a high-brilliance
X-ray beam.
Scientists typically consider operating costs, energy consumption, and flexibility when choosing an X-ray source. They also
must determine whether the source will match with other components of the SAXS system.
X-ray optics and collimation systems
Focusing optics increase the photon flux through the sample. Visible-light lenses and electromagnetic fields such as electron
beams cannot focus X-rays. Multilayer mirrors reflect X-rays on a layered surface under the Bragg condition (see Equation
3). Depending on the layers' thickness, a certain wavelength positively interferes (i.e., reflects) at a specific angle (5).
By reflecting certain wavelengths at certain angles, multilayor mirrors can produce monochromatic X-rays. The mirror's curvature
produces the desired beam focus, typically parallel or converging.
The monochrome and focused beam then travel through the collimation system to produce a precise beam profile. The following
two collimation methods are common:
- Point collimation (i.e., a pinhole system)
- Line collimation (i.e., a slit system).
Point-collimation systems send a point-shaped beam through a series of three pinholes. The first two pinholes produce a parallel
beam, and the third pinhole blocks unwanted scattering produced by previous pinholes.
Line-collimation systems use a line-shaped beam and employ slits instead of pinholes. The block collimator, a compact and
effective design to eliminate parasitic scattering by the collimation system itself, was suggested by Kratky (4).
Figure 3: Scattering patterns of silver behenate. The liquid crystalline structure with clear concentric rings in the case
of a point-shaped beam profile (i.e., point collimation) (right) becomes smeared when using a line-shaped beam (i.e., line
collimation) (left). The line-collimated experiment yields higher intensity. Reversing the smearing effect requires knowledge
of the beam profile and position.
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Line collimation increases the photon flux through the sample by replacing a single point-shaped beam with the equivalent
of many such points arranged in a line. The increase in intensity comes at the cost of a smearing effect on the scattering
image (4). Figure 3 shows the effects of line collimation.
Desmearing data is a mathematical operation that requires knowledge of the beam profile that causes the smearing effect (6–8).
New technologies report the beam profile and position so that desmearing becomes a simple and precise routine, thus alleviating
earlier concerns (9, 10).
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