GObel mirrors

Rotating anode, conventional optics Rotating anode, Gobel mirror optics Synchrotron, bending magnet (DORIS, A2) Synchrotron, insertion device (ESRF, ID2)... 

Laboratory X-ray sources emit highly divergent radiation. With conventional optics the major part of this radiation is discarded by a slit system and a monochromator. Both components can be replaced by a Gobel mirror [73,74], Figure 4.5 shows its construction and application. As a result a parallel and highly monochromatic primary beam is received. Replacement of conventional incident beam optics (cf. Fig. 2.2) by a Gobel mirror increases the primary beam intensity by a factor of 10-50. 

Nanobeam optics with beam diameters of several nanometers are presently developed at the ESRF. Using a Kirkpatrick-Baez optical system (cf. Fig. 4.9) beam diameters of 80 nm have been achieved. The Kirkpatrick-Baez system is made from two successively reflecting, orthogonal mirrors that are bent into elliptical shape by mechanical benders. The focused flux is strongly increased by deposition of a graded multilayer structure similar to that used with the parabolic Gobel mirror. 

Typical exposure time is 4 - 8 hours using a rotating anode source, Gobel mirror, and a bent ID detector for simultaneous recording of the complete curve. 

Most of the routine work in structural analysis is performed with D5000 Siemens diffractometer equipped with a Gobel mirror and an energy-dispersive detector. Raman micro-spectrometry has been recently introduced with a Labram infinity spectrometer with two laser sources, fitted with a horizontal output adapted to the investigation of vertical items like paintings or statues. For the most fine structural investigations, experiments are conducted with EXAFS, XANES or diffraction lines from various synchrotron facilities (ESRF at Grenoble, BESSY at Berlin, LURE at Paris). 

GRO 98a] GROSS M., HAAGA S., FIETZEK H., HERRMANN M., ENGEL W., Measurements in parallel-beam geometry achiev a Gobel mirror at a laboratory source . Mater. Sci. Forum, Trans. Tech. Pub., Switzerland, vol. 278-281, p. 242-247,1998. 

Figure 4.5. Beam shaping by a Gobel mirror, (a) The effective part of the mirror is a graded multilayer from materials with high contrast, (b) after parabolic bending the device converts divergent radiation emitted from a source point into a parallel beam...

A GObel mirror is designed for a specific X-ray waveiength. The design concerns the choice of the two materials for the multilayer, their thickness and the gradient along the mirror. Before application the graded multilayer (Fig. 4.5a) is bent and positioned (Fig. 4.5b) so that the source point of the radiation source is in the focus of the parabola. 

Duran, A., Herrera, L.K., Jimenez de Haro, M.C., Justo, A., and Perez-Rodriguez, J.L. Nondestructive analysis of cultural heritage artefacts from Andalusia, Spain, by X-ray diffraction with Gobel mirrors. Talanta. 76 no. 1 (2008) 183-188. 

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