A HIGH PRECISION SPECTROMETER FOR THE ABSOLUTE DETERMINATION OF X-RAY ABSORPTION EDGES AS CALIBRATION STANDARDS. J. Stümpel, P. Becker, Physikalisch Technische Bundesanstalt, Bundesallee 100, Braunschweig, Germany

The general characteristics and spectrometric features of a high resolution four-crystal reflection x-ray monochromator with wavelength analysis installed at the HASYLAB beamline L at DESY are presented.

The monochromator is part of a spectrometer developed to calibrate x-ray absorption edge spectra in the energy range of 6 - 36 keV with a relative uncertainty [[Delta]][[Epsilon]]/[[Epsilon]] from 10^-5 to 10^-6. This requires an extremely effective suppression of harmonics and also a negligible instrumental influence in order to obtain almost intrinsic spectra. As the results show, the monochromator fulfills the requirements, including very high stability.

One essential advantage of the (+ - - +)-setting is that the monochromator itself limits the divergence of the primary radiation, which is very useful for energies above 5 keV where the natural divergence of the synchrotron radiation exceeds the width of the crystal reflection curve. This setting needs no further optics such as slits to improve the resolution, which is therefore not influenced by vertical movements of the primary radiation source.

Harmonic suppression can be achieved by detuning the channel-cut nondispersive monolithic part slightly out of its parallel-sided position and is described by the detuning angles dl,2. For reflections at netplanes with Miller's indices all odd, the structure factor of the diamond structure leads to the forbidden harmonic with n = 2 in the Bragg-equation n[[lambda]]B = 2d sinJB. It is therefore advantageous to choose reflecting netplanes with odd indices, (e.g. (111) or (311)).