Page 91 - Macromolecular Crystallography

80 MACROMOLECULAR CRYS TALLOGRAPHY

different focal spot sizes ranging from 0.1 × 12 mm 2 beam in the horizontal direction, and then strikes
2
through to 0.5×10 mm with loadings ranging from the second mirror imparting focusing in the verti-
1.2 kW through to 18 kW accordingly for a copper cal direction. X-ray mirrors of different types (such
anode target. For this generator the electron gun as toroidal mirrors and Franks optics) had been
has been totally redesigned in order to minimize used extensively in diffraction studies on fibres from
discharge and hence improve stability. The anode biological specimens, but the alignment of these
is also a new direct drive model which has a low earlier mirrors was difficult and could result in a
vibration. degree of X-ray scatter whilst aligning. In 1991,
Two generators that have become popular are the Zbyszek Otwinoski and Jerry Johnson produced a
Rigaku MSC 007 and 007HF, which are very high set of mirrors which came to be known as the Yale-
brilliance X-ray generators incorporating a 70 µm × MSC mirrors, where driving of the motors which
70 µm focal spot. The possible loading on the 007HF bent the mirrors could be controlled remotely, hence
is higher than the 007 by using a changed bearing protecting the aligner. The mirrors were in turn
design and spinning the wheel at 9000 r.p.m instead housed in a X-ray impermeable box, which was
of 6000 r.p.m in order to increase heat dissipation. flooded with helium in order to the reduce air scat-
ter of the X-rays and reduce air oxidation of the
mirror nickel coating. Using these mirrors in com-
5.2.2 Bruker AXS X-ray generators
bination with an area detector became very popular
Bruker AXS make a high-flux 27 kW generator and meant that the beam could be focused on the
knownastheMicrostar. Itisbeltdriven, hasa100 µm crystal and, at the same time, maximizing the flux
focal spot, and includes novel features such as vari- falling on it.
ableanodespeedstoaidinpoweringupthegenerator Subsequently, manufacturers have developed
depending on the state of the vacuum. The out- other mirror systems, two of which are in common
2
9
put of this generator is 1.7 × 10 photons/mm /sec use. The first are multilayer optics. Here, instead
and is configured with Helios multilayer optics and of the X-rays being focused by a bent mirror with
the Platinum 135 CCD (Charged Couple Device) to the X-ray radiation at grazing incidence, the focus-
form the Proteum X8 system. Very recently, they ing is performed by a curved sandwich of layers
have released the Microstar Ultra whose intensity of substrate, which are alternately coated with a
2
is 8 × 10 10 photons/mm . The increase in intensity light and heavy element. The layers of the ‘sand-
has been achieved by increasing the rotation rate wich’ act as a Bragg d-spacing, imparting diffraction
of the anode and remodelling it to increase heat from successive layers. In place of a second mirror,
dissipation. the complete composite formed of the multilayer, as
described above, is glued a similar sandwich rotated
◦
by 90 so that the incident beam is, in effect, focused
5.3 X-ray mirrors
over the full length of the multilayer mirror. Multi-
Historically, macromolecular data collection on layer mirrors are marketed by OSMIC, with different
diffractometers embodied a rotating anode genera- specifications compatible with a wide range of X-ray
tor which was coupled to a graphite monochromator sources, and the Helios optics is marketed by Bruker
in order to filter off the unwanted K β radiation. AXS and is configured to their Microstar generator
After the monochromator, the X-rays then passed to form the Proteum X8 system. The second is a mir-
through a collimator before striking the crystal. ror system that uses an ellipsoidal, monocapillary
An inherent problem with the use of a graphite optic for X-ray focusing; it is marketed byAXCO and
monochromator is that it greatly attenuates the beam is called the S70PX. It fits very neatly into a small
and only a fraction of the flux arrives at the crys- space between the source and the crystal. Unde-
tal. One way around this is to use X-ray mirrors, sired K β radiation may be filtered off by using nickel
which are coated with nickel in order to filter off filters of a range of thicknesses if it is found that
the K β radiation, arranged so that the beam strikes the K β spots are interfering with the indexing of the
the first mirror at grazing incidence, focusing the K α ones.