Biomedical subjects
Publications and source records attributed to Elisabeth Rossmanith.
The expressions derived for the calculation of multiple diffraction patterns for a perfect crystal in the framework of the kinematical theory are compared with the relevant result of the kinematical power-transfer equation deduced for a mosaic crystal. It is shown that the results of the two concepts differ appreciably.
The modifications of the kinematical and extinction-corrected intensity profiles caused by the divergence of the incident synchrotron-radiation beam and the diffraction by the monochromator system are considered. Expressions for the integral width as well as the full width at half-maximum are presented and discussed. The theoretical results are compared with observations.
The structure of aragonite was first determined by Lawrence Bragg in 1924 in what is the now standard space-group setting Pnma (No. 62). Subsequent studies have all taken his structure as their starting points, despite Bragg's own stated doubts and some earlier etching studies which indicated that the underlying symmetry may really be polar. We have reinvestigated the structure and found that there are many reflections with significant intensity among those that should be systematically extinct in Pnma. Some of these reflections have been subjected to further experimental analysis and have been shown not to be due to Renninger effects. A possible model that satisfies these observations is one where the true structure is in space group P1; and the structure is twinned about the three axial twofold rotation axes of Pnma. The space group P1 cannot be ruled out. Evidence for these conclusions is presented. The crystal chemistry of aragonite is revisited and described in terms of the stuffed alloy CaC. The carbonate group is confirmed to be non-planar in the crystal.
The kinematical derivation of intensity profiles is considered. It is shown that the shapes of the profiles depend not only on the shape of the crystal but also on the crystal system and axial ratios of the unit cell and on the angles between the direction of the diffracted beam and the cell axes. For all crystal shapes, the widths of the profiles are determined by the Lorentz factor and the mean thickness of the crystal in the direction of the diffracted beam. The similarity of the intensity distributions of cubiform and spherical crystals is discussed. It is shown that almost identical results are obtained for the kinematical intensity profiles of a plane parallel plate having a large crystal edge ratio bathed in the beam and the dynamical profiles of thin plane parallel plates with infinite lateral extension. Extinction-corrected profiles of the plane parallel plate are compared with the results of the dynamical theory. In the case of a spherical crystal, the primary-extinction correction factor, y(p), obtained with extinction-corrected profiles is compared with y(p) commonly used in structure analysis.