A high-precision spectrometer for the absolute determination of X-ray absorption edges as calibration standards

Abstract

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 ΔE/E 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 is achieved by detuning the channel-cut nondispersive monolithic part slightly out of its parallel-sided position. In preliminary tests the operation of the set-up according to the principle was demonstrated. The thermal load of the first crystal caused a temperature increase of approximately 5 K. After reaching temperature equilibrium reproducible results are received. In a measured section of 18–30 degrees of arc, the deviation of the monochromator position from that measured by the goniometer is less than the resolution of the monochromator angle decoder. The FWHM of the rocking curves is in the range of the expected values. An example of the calibration procedure for the copperK-edge is given as well as a comparison of the remeasured absorption edge energies with the previously tabulated data.