appropriately, the energy of a particular harmonic will not be passed through, e.g. at
8 keV, a Si coating with not allow 16 and 24 keV to pass.
As X-ray mirrors need to operate at grazing incidence, great care must be taken to
ensure that their surfaces are free from errors in their figuring (slope error) that can
result in texture in the profile of the focused beam. Additionally, the temporal beam
stability can be affected by any vibration transmitted through the mirror mount –
which can be greatly amplified due to the long optical lever from the source through
the optical components to the sample. These issues are especially important for
experiments requiring focusing on the order of 10 μm or less. A solution for these
issues is the use of X-ray compound refractive lenses, which focus the beam through
a process of refraction rather than specular reflection. As the refractive index for
X-rays is close to and slightly less than 1, the lenses need to be concave to achieve
focus rather than the convex form used for lenses to focus visible light. Differing
focal lengths can be achieved by stacking a series of identical lenses into long rows,
called a transfocator, with a different number of lenses used to bring a specified
energy into focus at the same point. Compound refractive lenses produce extremely
well-focused beams, with very little profile texture at ideal focus, and they are very
tolerant of vibration, as they do not couple to the optical lever to the same degree as
mirrors. There is some absorption by the lens material itself, as the X-ray beam must
pass through it, so the choice of material is critical.
The advantages of increasing X-ray flux, using the above methods, are clear.
However, there is a potential downside which the macromolecular crystallography
community have had to tackle since the beginnings of structure solution from singlecrystal diffraction at synchrotrons – that of the propensity for crystal decay in the
X-ray beam. In the early days of synchrotron chemical crystallography, this was not
an observable problem, due to chemical crystals being less prone to decay, the
relative strength of a second-generation synchrotron and the sensitivity of the
detection. However, with the technological developments since then, this effect is
now far more likely and regularly being observed. This phenomenon is discussed
further in Sect. 1.3.
End Stations: Detectors
The development of single-crystal diffraction techniques at synchrotron sources for
atomic-resolution structure determination has been pioneered by the protein crystallography community, where the advantages of synchrotron radiation over lab
sources were evident on early first- and second-generation sources [37]. As well as
driving the development of sources and optics, protein crystallography was also
instrumental in the advancement, or adaptation, of detector technology for the
recording of the diffraction data. It is critical to remember, however, that the needs
of a protein crystallography beamline and one optimised for smaller molecules are
not identical. The push for large-format detectors largely stems from the d-space
resolution necessary for protein crystallography at an X-ray energy which resolves
the closely packed reflections from large unit cells. Protein diffraction data has a
Leading Edge Chemical Crystallography Service Provision and Its Impact on. . .
81
8 keV, a Si coating with not allow 16 and 24 keV to pass.
As X-ray mirrors need to operate at grazing incidence, great care must be taken to
ensure that their surfaces are free from errors in their figuring (slope error) that can
result in texture in the profile of the focused beam. Additionally, the temporal beam
stability can be affected by any vibration transmitted through the mirror mount –
which can be greatly amplified due to the long optical lever from the source through
the optical components to the sample. These issues are especially important for
experiments requiring focusing on the order of 10 μm or less. A solution for these
issues is the use of X-ray compound refractive lenses, which focus the beam through
a process of refraction rather than specular reflection. As the refractive index for
X-rays is close to and slightly less than 1, the lenses need to be concave to achieve
focus rather than the convex form used for lenses to focus visible light. Differing
focal lengths can be achieved by stacking a series of identical lenses into long rows,
called a transfocator, with a different number of lenses used to bring a specified
energy into focus at the same point. Compound refractive lenses produce extremely
well-focused beams, with very little profile texture at ideal focus, and they are very
tolerant of vibration, as they do not couple to the optical lever to the same degree as
mirrors. There is some absorption by the lens material itself, as the X-ray beam must
pass through it, so the choice of material is critical.
The advantages of increasing X-ray flux, using the above methods, are clear.
However, there is a potential downside which the macromolecular crystallography
community have had to tackle since the beginnings of structure solution from singlecrystal diffraction at synchrotrons – that of the propensity for crystal decay in the
X-ray beam. In the early days of synchrotron chemical crystallography, this was not
an observable problem, due to chemical crystals being less prone to decay, the
relative strength of a second-generation synchrotron and the sensitivity of the
detection. However, with the technological developments since then, this effect is
now far more likely and regularly being observed. This phenomenon is discussed
further in Sect. 1.3.
End Stations: Detectors
The development of single-crystal diffraction techniques at synchrotron sources for
atomic-resolution structure determination has been pioneered by the protein crystallography community, where the advantages of synchrotron radiation over lab
sources were evident on early first- and second-generation sources [37]. As well as
driving the development of sources and optics, protein crystallography was also
instrumental in the advancement, or adaptation, of detector technology for the
recording of the diffraction data. It is critical to remember, however, that the needs
of a protein crystallography beamline and one optimised for smaller molecules are
not identical. The push for large-format detectors largely stems from the d-space
resolution necessary for protein crystallography at an X-ray energy which resolves
the closely packed reflections from large unit cells. Protein diffraction data has a
Leading Edge Chemical Crystallography Service Provision and Its Impact on. . .
81