adapted from a technology used with copper radiation by the protein crystallography
community. These were pioneered for rotating anodes, but very rapidly became
integrated as standard for the follow-on technology from sealed tube systems
[18]. The inclusion of focusing optics on all X-ray generator types for chemical
crystallography produced an increase in X-ray flux of around 6–10 times across the
board. Around this time microfocus sources became ubiquitous – these are essentially the same technology as the sealed tube but tuned so that the electron beam is
highly focused into a small spot on the anode. This results in an X-ray beam that is
considerably more focused and therefore has a greater flux density for the same
amount of power.
For the rotating anode, there is now even more potential. Within the last decade, it
has become possible to produce a range of focusing mirrors with different capabilities. In particular for their highest flux rotating anode, Rigaku Oxford Diffraction
can produce different optics – the NCS has two diffractometers on the same X-ray
source, one with a very high flux (VHF) mirror and the other with an ultra-high flux
(UHF) version. The UHF optic provides a sharper focus of all the X-rays emitted
from the rotating target, resulting in a considerable increase in flux density at the
sample. The NCS UHF system has a beam with a 70 μm focus at the sample, while
the VHF is 150 μm, with the former giving an intensity in the home laboratory that is
of a similar order to that of a second-generation synchrotron single-crystal diffraction facility.
The key to producing high flux X-rays by electron bombardment of a metal target
is the efficient dissipation of the heat generated by the process. This is achieved in
the rotating anode by having a moving target, and a faster, larger, target will produce
greater strength X-rays. However, new approaches through the use of liquid gallium
and indium alloys [19] as a target are particularly promising for improving the
dissipation of heat and thereby producing higher-powered X-rays. These sources
use a liquid metal, which is injected into the path of the electron beam and thereby
generates X-rays, while the metal is gathered, cooled and recycled for subsequent
X-ray generation [20]. This type of system is now marketed by the company
Excillum and has been applied to numerous X-ray diffraction, imaging and spectroscopy techniques. With wavelength of 1.35 Å and 0.51 Å for the gallium and
indium sources, respectively, it is possible to approximately mimic the traditional
silver (0.48 Å) or molybdenum (0.71 Å) source for chemical crystallography and
that of the system of choice for larger unit cell systems, copper (1.54 Å). Comparison
measurements show this type of source to be stronger than molybdenum-based
rotating anode systems.
X-Ray Detectors
Early crystallographic laboratories depended on photographic film recordings of the
diffraction data. This painstaking process was later augmented by systems which
used photographs for indexing and point detectors for the actual intensity data
collections – this combination provided the reciprocal space detail of a 2D detector
74
S. J. Coles et al.
community. These were pioneered for rotating anodes, but very rapidly became
integrated as standard for the follow-on technology from sealed tube systems
[18]. The inclusion of focusing optics on all X-ray generator types for chemical
crystallography produced an increase in X-ray flux of around 6–10 times across the
board. Around this time microfocus sources became ubiquitous – these are essentially the same technology as the sealed tube but tuned so that the electron beam is
highly focused into a small spot on the anode. This results in an X-ray beam that is
considerably more focused and therefore has a greater flux density for the same
amount of power.
For the rotating anode, there is now even more potential. Within the last decade, it
has become possible to produce a range of focusing mirrors with different capabilities. In particular for their highest flux rotating anode, Rigaku Oxford Diffraction
can produce different optics – the NCS has two diffractometers on the same X-ray
source, one with a very high flux (VHF) mirror and the other with an ultra-high flux
(UHF) version. The UHF optic provides a sharper focus of all the X-rays emitted
from the rotating target, resulting in a considerable increase in flux density at the
sample. The NCS UHF system has a beam with a 70 μm focus at the sample, while
the VHF is 150 μm, with the former giving an intensity in the home laboratory that is
of a similar order to that of a second-generation synchrotron single-crystal diffraction facility.
The key to producing high flux X-rays by electron bombardment of a metal target
is the efficient dissipation of the heat generated by the process. This is achieved in
the rotating anode by having a moving target, and a faster, larger, target will produce
greater strength X-rays. However, new approaches through the use of liquid gallium
and indium alloys [19] as a target are particularly promising for improving the
dissipation of heat and thereby producing higher-powered X-rays. These sources
use a liquid metal, which is injected into the path of the electron beam and thereby
generates X-rays, while the metal is gathered, cooled and recycled for subsequent
X-ray generation [20]. This type of system is now marketed by the company
Excillum and has been applied to numerous X-ray diffraction, imaging and spectroscopy techniques. With wavelength of 1.35 Å and 0.51 Å for the gallium and
indium sources, respectively, it is possible to approximately mimic the traditional
silver (0.48 Å) or molybdenum (0.71 Å) source for chemical crystallography and
that of the system of choice for larger unit cell systems, copper (1.54 Å). Comparison
measurements show this type of source to be stronger than molybdenum-based
rotating anode systems.
X-Ray Detectors
Early crystallographic laboratories depended on photographic film recordings of the
diffraction data. This painstaking process was later augmented by systems which
used photographs for indexing and point detectors for the actual intensity data
collections – this combination provided the reciprocal space detail of a 2D detector
74
S. J. Coles et al.