well adapted for crystallography, especially when element-specific resonant data or
simultaneous spectroscopic data is needed or if higher energies are required to
penetrate in situ apparatus. In moving from second- through to third-generation
synchrotrons, the source size has continually been reducing, due to accelerator
physics improvements as well as the choice of insertion device becoming more
commonly an undulator rather than higher divergence wigglers and bending magnets. Currently, most synchrotrons worldwide are considering or implementing low
electron emittance upgrades, also known as brightness limited upgrades. This
movement has centred around reducing the electron beam emittance; the primary
way of achieving this is to reduce the angular bend per magnet by replacing one
bending magnet with many lower field magnets. Variants on the multi-bend achromat storage ring design have been implemented in MAX-IV [33], will be
implemented on SIRIUS and will be integrated into many existing facilities as
upgrades, e.g. ESRF-EBS [34], APS-U, ALS-U and PETRA-IV. The choice of
sources possible for crystallography beamlines will change in this fourth-generation,
high-brightness, low-emittance regime: bending magnet sources will rarely be
feasible for crystallography, wigglers will be chosen if a smooth continuum is
necessary, and undulators will yield even higher brightness and flux. With source
size reduction, it becomes easier to focus the beam to a smaller spot at the sample and
thereby increase the flux density. Sub-micron spots will be routinely possible;
however, when operating a rotation-based data collection approach in this regime,
the limitations will be the speed and accuracy of the diffractometer and the ability to
centre the sample.
Optics
For studies requiring a single well-defined wavelength, the energy is usually selected
by a monochromator, which exploits the Bragg diffraction from crystals to select the
required energy and direct the monochromatic beam to the downstream optics. The
design of synchrotron monochromators varies considerably, but usually they involve
a pair of matched crystals, a so-called double-crystal monochromator (DCM). These
are often silicon polished to the (1 1 1) Bragg plane and, with the first crystal held at
an angle to the primary beam, to select the energy, while the second crystal is
positioned parallel to the first so that the monochromated beam is directed parallel
to its original path, with a spatial separation from the main, unmonochromated beam.
It is important to match the vertical beam divergence to the natural rocking width of
the silicon reflection to maximise the flux – in higher divergence sources, a mirror
may be used upstream of the monochromator to collimate the incoming X-rays, but
the construction of this mirror imposes a restriction on the maximum photon energy
of the beamline. The typical bandpass of a monochromator with this configuration is
0.1%, which is a very thin slice of the energy available from the source. As the angle
of the first crystal is changed to select a specific energy, the second single crystal can
be moved both horizontally and vertically to maintain the position of the X-ray beam
leaving the monochromator – providing a ‘fixed exit monochromator’. The huge
Leading Edge Chemical Crystallography Service Provision and Its Impact on. . .
79
simultaneous spectroscopic data is needed or if higher energies are required to
penetrate in situ apparatus. In moving from second- through to third-generation
synchrotrons, the source size has continually been reducing, due to accelerator
physics improvements as well as the choice of insertion device becoming more
commonly an undulator rather than higher divergence wigglers and bending magnets. Currently, most synchrotrons worldwide are considering or implementing low
electron emittance upgrades, also known as brightness limited upgrades. This
movement has centred around reducing the electron beam emittance; the primary
way of achieving this is to reduce the angular bend per magnet by replacing one
bending magnet with many lower field magnets. Variants on the multi-bend achromat storage ring design have been implemented in MAX-IV [33], will be
implemented on SIRIUS and will be integrated into many existing facilities as
upgrades, e.g. ESRF-EBS [34], APS-U, ALS-U and PETRA-IV. The choice of
sources possible for crystallography beamlines will change in this fourth-generation,
high-brightness, low-emittance regime: bending magnet sources will rarely be
feasible for crystallography, wigglers will be chosen if a smooth continuum is
necessary, and undulators will yield even higher brightness and flux. With source
size reduction, it becomes easier to focus the beam to a smaller spot at the sample and
thereby increase the flux density. Sub-micron spots will be routinely possible;
however, when operating a rotation-based data collection approach in this regime,
the limitations will be the speed and accuracy of the diffractometer and the ability to
centre the sample.
Optics
For studies requiring a single well-defined wavelength, the energy is usually selected
by a monochromator, which exploits the Bragg diffraction from crystals to select the
required energy and direct the monochromatic beam to the downstream optics. The
design of synchrotron monochromators varies considerably, but usually they involve
a pair of matched crystals, a so-called double-crystal monochromator (DCM). These
are often silicon polished to the (1 1 1) Bragg plane and, with the first crystal held at
an angle to the primary beam, to select the energy, while the second crystal is
positioned parallel to the first so that the monochromated beam is directed parallel
to its original path, with a spatial separation from the main, unmonochromated beam.
It is important to match the vertical beam divergence to the natural rocking width of
the silicon reflection to maximise the flux – in higher divergence sources, a mirror
may be used upstream of the monochromator to collimate the incoming X-rays, but
the construction of this mirror imposes a restriction on the maximum photon energy
of the beamline. The typical bandpass of a monochromator with this configuration is
0.1%, which is a very thin slice of the energy available from the source. As the angle
of the first crystal is changed to select a specific energy, the second single crystal can
be moved both horizontally and vertically to maintain the position of the X-ray beam
leaving the monochromator – providing a ‘fixed exit monochromator’. The huge
Leading Edge Chemical Crystallography Service Provision and Its Impact on. . .
79