Double focussing, mirror-monochromator

Double focussing, mirror-monochromator cameras are optimized for maximum flux at the sample. This type of camera is hence mainly used for real time diffraction studies on biological samples and polymers (see Sect. 4). Such a camera is shown in Fig. 23. The first optical element could only be placed at 20 m... 

Fig. 23. Schematic design of a double focussing mirror-monochromator camera at DORIS The middle of the mirror is at 20 m from the source point. A bent, triangular monochromator crystal is used for horizontal focussing and a segmented mirror (quartz) for vertical focussing. The ionization chamber is designated by-I-...

An increase in sensitivity (x 4) is planned by the use of a double focussing monochromator, where many small crystals will be arranged on a curved surface, analogous to a parabolic mirror. This will provide an incident flux of almost 10 neutrons cm s . 

The polarisation state of the synchrotron beam has been discussed in section 4.4. The polarisation state of the beam at the sample is determined by the fraction of the parallel and perpendicular components reflected by the monochromator (there may be several reflections, i.e. two for a double crystal monochromator), and the relative fraction of the parallel and perpendicular components incident on the monochromator from the source. This depends on the vertical angular aperture of the source that the sample sees and can be calculated from the source characteristics the size of the vertical aperture is dependent on whether a focussing mirror is used as this increases the aperture subtended by the sample at the source. 

Photon beam position monitors are essential to ensure that after an injection the electron beam position is adjusted to allow the SR to strike the beam line optical components in a constant way. The wavelength output from a double crystal monochromator is especially sensitive to the vertical beam position. Also, the quality of the focus, from a toroid mirror, is especially sensitive to the horizontal beam position (figures 5.18(c) and (e)). On existing machines it is necessary to recalibrate the wavelength and the focussing of a beam line optical system after each injection. 

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