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.
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).
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.
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
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).
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.
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.
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).