Grazing-incidence mirrors

The radiation from a storage ring is emitted in a broad sweep with a very narrow vertical spread. The beam is typically 1 cm high and 30 cm wide at a distance of about 10 m from the ring. An oscillating, grazing incidence, mirror is used to broaden it to the desired height. 

This uses a multipole wiggler and will have operational modes for focussed Laue diffraction work and monochromatic experiments. The small source sizes should allow an equivalently small focal spot from a grazing incidence mirror system. Exposure times in the microsecond range for a macromolecular crystal should be feasible. Depending on the current achieved in single bunch mode it may be possible, at least for smaller unit cell sizes, to record a Laue pattern from one of the single bunches with an intrinsic time resolution therefore of the bunch width. (Feasibility experiments of this kind have been conducted at CHESS but on an undulator (Szebenyi et al 1989).)... 

Figure 1. Effective area of two grazing incidence mirror systems. Both contain four modules, each with a 24cm diameter, 10m focal length and 80 cm long gold reflectors. The shell thickness is 0.7mm and the minimum spacing between reflecting surfaces is 1.2mm. One type (dashed line) is a conventional two reflection conical (or Wolter Type I) system of 45 shells. The other is a four reflection conical mirror that contains 58 shells.

The lobster-eye geometry X-ray optics offer an excellent opportunity to achieve very wide fields of view while the classical Wolter grazing incidence mirrors are limited by about 1 deg FOV. 

The dispersing element is a diffraction grating preferably used under conditions of grazing incidence (6 in Equation 3.9 about 89°) to improve the reflectance. The grating may also be concave to avoid the use of a focusing mirror. 

Newton-XMM is an X-ray telescope eqnipped with a set of nested mirrors designed to focns grazing-incidence X rays, a confignration which explains the name X-ray Mnlti-Mirror. It is an ESA project and was lannched by Ariane 5 in December 1999. It opens a window onto the nltrahigh temperatnre Universe with its explosions and stars ripped apart by black holes. Its spectroscopic targets are snpernova remnants and the gases that fill clnsters of galaxies. 

Figure 3. An optical arrangement for measuring polarized reflection-absorption spectra of a monolayer or thin film on the surface of a minor S at near grazing incidence. P is a polarizer, Ml, M2, M3 and M4 are mirrors.

The polishing, construction and alignment of the Chandra mirrors and optical bench were critical for achieving high angular resolution. First, because of the need for grazing incidence reflection, the effective telescope area is small... 

In the extreme case of grazing incidence, a field component exists only normal to the surface. Therefore an interaction is possible exclusively with transition moments or components thereof, orientated perpendicular to the surface. This anisotropy of interaction can also be explained by selection rules, which are based on symmetry consideration and include the mirror image of the analyte produced by the metallic surface. 

Fig. 26. Reflectivity of a quartz mirror as a function of the wavelength at different angles of grazing incidence . An optimum reflectivity at 1.5 A is obtained for an angle of 3 mrad. The mirror acts as a filter cutting at this angle shorter wavelength...

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