Focussing monochromator

The source size varies for the different synchrotrons. At SRS it is 9.9 mm (horizontal) and 0.3 mm (vertical). Since the sample is about 20 m away, careful attention to monochromatisation and focussing optics is required so that the source size matches the crystal size. A curved crystal monochromator which is based on a triangular shaped crystal plate and uses germanium (111) reflection was first described by Lemonier et al. [225] at LURE. Similar focussing monochromators are now in use... 

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 . 

On DCI, Lemonnier et al (1978) established a focussing monochromator camera in 1976 (section 5.6.3.1) on which oscillation camera data sets were collected (e.g. from single crystals of tyrosyl t-RNA synthetase (Monteilhet et al 1978) or resolution and radiation damage tests were conducted (e.g. 6-phosphogluconate dehydrogenase (Helliwell 1979)). 

The irradiating X-ray beam cannot be focussed upon and scanned across the specimen surface as is possible with an electron beam. Practical methods of small-spot XPS imaging rely on restriction of the source size or the analysed area. By using a focussing crystal monochromator for the X-rays, beam sizes of less than 10 pm may be achieved. This must in turn correspond with the acceptance area and alignment on the sample of the electron spectrometer, which involves the use of an electron lens of low aberration. The practically achievable spatial resolution is rarely better than 100 pm. A spatial resolution value of 200 pm might be regarded as typical, and it must also be remembered that areas of up to several millimetres in diameter can readily be analysed. 

Fig. 4. Experimental arrangement on the XAS instrument 8.1 at the SRS, where a slitless monochromator is used and the crystals are mechanically bent to account for the vertical divergence (0.2 mrad) of the x-rays from the source. A toroidal mirror is placed after the monochromator, which provides the focussing...

For recording of the emission spectrum, the emitted radiation is focussed on the slit of a monochromator and intensities measured attach wavelength. Since sensitivities of photocells or photomultipliers are wavelength dependent, a standardization of the detector-monochromator combination is necessary for obtaining true emission spectrum This can be done by using a standard lamp of known colour temperature whose emission characteristics is obtained from Planck s radiation law. The correction term is applied to the instrumental readings at each wavelength. Very often substances whose emission spectra have been accurately determined in the units of relative quanta per unit wavenumber intervals are... 

X-ray Diffraction Studies. They are performed under vacuum with a Guinier type focussing camera equipped with a bent quartz monochromator giving a linear collimation of the CuK (X = 1.54 A) radiation (8). 

Hg. 21. Schematic design of a double monochromator small-angle scattering camera. The first monochromator is at 24 m from the source. I denotes two ionization chambers. No focussing elements are used... 

This camera does not use focussing elements. The cross section of the beam is defined by slits at the entrance of the instrument and close to the sample. As the first monochromator is — for architectural reasons — located 24 m from the source point, a considerable fraction of the beam is lost in order to reduce the beam size to the spatial resolution of the detector ( 2 x 2 mm). The recent introduction of segmented monochromators suggests to use the second crystal as a focussing element which would increase the intensity at the sample considerably. 

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-...

IR spectroscopy appears to be a very promising tool for oxidation studies. The rate of formation of carbon monoxide, carbon dioxide and formic acid and the disappearance of ozone was measured this way with the O3/O2 + CH4 system . The system used by Burt and Minkoff for the combustion studies is shown in Fig. 61. Light from a Nernst filament is split in two and passed alternately through two heated cells and F2 containing either fuel + N2 or fuel + Oj. The beams are rejoined and fed into a Wadsworth monochromator containing a CaF2 prism and finally focussed onto a thermopile, from which a particular signal may be amplified and recorded. 

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