Dedicated to Professor G. BECRERER on the occasion of his 65th birthdayThe application of a double-crystal diffractometer for the production of X-ray Schlieren topographs is described. The experimental arrangement is optimized on the basis of a wave-optical description for the investigation of shell targets for laser fusion experiments.

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The axes or cores of extensive air showers have been selected by observing in a cloud chamber cascade showers produced by a single high-energy (>5 x lo9 ev) electron or photon, and an upper limit of 5 m for the spread of such particles has been found. By placing trays of hodoscoped counters at various distances from this core selector it has been possible to determine the relation between density of ionizing particles and distance from the axis for showers of approximately 1 04-105 particles. T h e experimental arrangement and method of analysis used have certain advantages over those employed by other workers. I t is concluded that although the results of the present experiment and those of other workers are consistent with the Molikre structure function, not too much weight should be placed on this agreement.

The CuKα and CuKβ emission lines are remeasured with high accuracy. Analytical models of these profiles are obtained by fitting them with sums of n Lorentz functions. The wavelengths of the lines are determined in the metrical system with an accuracy superior to values obtained by other authors. The models are used for the X‐ray diffraction line profile analysis of polycrystalline materials and for the determination of lattice parameters with high accuracy. It is proposed to use the independently measured and fitted emission lineshape (wavelength distribution) in X‐ray line profile analysis for the modelling of the line profile to improve the conditions for determining the structure parameters of polycrystalline materials. An example for an application is given where the precise measurement of the CuKα emission lineshape is used to analyze molybdenum polycide samples. The lattice parameters of α‐Fe whiskers are determined with an accuracy about one or two orders higher than in literature. The values are given also in the metrical system.

The atomic positions of the silicon carbide (SiC) polytypes 6H and 4H differ slightly from an ideal tetrahedron. These small deviations can be investigated by X-ray diffraction of so-called 'quasiforbidden' reflections, which are very sensitive with respect to the extremely small variations in the structure. Nevertheless, an unambiguous calculation of the refinement parameters from the absolute values of the structure factors of the 'quasiforbidden' reflections is not possible. In the case of SiC-4H, there are two and, in the case of SiC-6H, six different structure models, which yield the same absolute values of the structure factors. In order to distinguish between these models, additional phase information about the measured reflections is needed. To achieve this, Renninger-scan (psi-scan) profiles in the vicinity of three-beam cases are used. These experimentally measured psi-scans are compared with theoretical calculated profiles for each model. Another method to distinguish the different models is to compare the bond lengths between atoms of the two polytypes, which have equivalent vicinities. For both SiC-4H and SiC-6H, an unambiguous determination of the structure refinement parameters was possible.