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Methoden und Instrumentierung Poster: Mi., 14:00–16:30 M-P33
CCD-Calibration for Accurate High Resolution Crystal Structure Analysis
Simone K.J. Johnas 1 , Edgar Weckert 1
1 HASYLAB at DESY, Notkestr. 85, 22607 Hamburg
Hard X-rays, about 25 keV and higher, are of advantage for high resolution crystal
structure analysis, up to about 0.5 ˚A resolution, because of minimization of systematic
errors like absorption of the X-rays in the crystal and extinction. For crystals of large
unit cells only 2D-CCD-detectors enable to collect the necessary intensity data within
reasonable time.
Presently available CCD-detectors are optimized for photon energies of about 12 keV,
where photons are almost totally stopped by the fluorescence phosphor independent
of the incident angle. For higher photon energies a smaller portion of the diffracted
photons is absorbed within the phophor leading to systematic errors of the measured
intensities as a function of the angle of incidence on the detector surface. This is due
to the different path lengths of the photons in the phosphor, see Fig.1.
This effect can account for a factor of more than 2, and needs to be corrected especially
in the case of accurate structure determination like charge density structures. A
generaly applicable correction procedure based on reference data sets of well known
compounds will be presented as well as the influence of this correction on various high
resolution data sets and the derived structural parameters.
Fig. 1: Scheme in which way the diffraction angle affects
the absorption path of the incoming photon in
the pixel at a given wavelength.