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I. IntroductionX-ray attenuation coefficient measurements made on single-crystal silicon specimens by participants in the International Union of Crystallography X-ray Attenuation Project are presented for the energy range 8-60 keV. Twelve laboratories using eight different experimental configurations have provided data for analysis. A comparison is made between measurements using the different techniques at those characteristic wavelengths of interest to crystallographers.Comparison of these measurements with available theoretical cross sections suggests that a model in which the thermal diffuse scattering cross section is used instead of the Rayleigh scattering cross section for the calculation of the theoretical total scattering cross section gives better agreement with the experimental values. No basis was found for preferring one of three current theoretical tabulations of photoelectric absorption cross section over the others.* Present address:

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DIRECT MEASUREMENTS OF TRIPLET PHASE RELATIONSHIPSpropafenon and butafenon. Suitable three-beam cases with almost equal intensities of the reflections involved were selected from a data set measured prior to the three-beam measurements. The structures were later solved by direct methods.There is one essential point open for further investigation. We have stated in the preceding theoretical paper (Weckert & HiJmmer, 1990) that in Laue-Laue and Laue-Bragg diffraction geometry Pendell6sung effects occur. Then the energy flow into the primary diffracted beam depends on the thickness of the crystal plate and the ~-scan profiles with constant triplet phase depend on the crystal dimensions. By averaging over the Pendell6sung the phase information is lost. Experimental, we use non-cut crystals with grown faces, and the crystals are bathed by the incident beam. Therefore, the crystal thickness varies over the cross section of the diffracted beam. As a consequence, we guess that the Pendell6sung effects are averaged out, so that the Laue-Laue and LaueBragg parts give no phase information. In addition, these are affected by anomalous absorption. Thus, we assume that only parts of the primary diffracted beam that are Bragg reflected carry the phase information.In conclusion, the achievable accuracy of experimental triplet phase determination should be sufficient to solve structures that cannot be solved otherwise by a combination of measured triplet phases and direct methods.These results were reported at the l lth European Crystallographic Meeting in Vienna (Weckert & HiJmmer, 1988) and in part at the Fourteenth International Congress of Crystallography in Perth (Hiimmer, Bondza & Weckert, 1987; Weckert, Bondza & Hiimmer, 1987

The x-ray mass-attenuation coefficient of copper was measured at 108 energies between 5 and 20 keV using synchrotron radiation. The measurements are accurate to between 0.09 and 4.5 %, with most measurements being accurate to better than 0.12%. The imaginary component of the form factor of copper was also determined after subtracting the attenuation contribution due to scattering. Measurements were made over an extended range of experimental parameter space, allowing us to correct for several systematic errors present in the data. These results represent the most extensive and accurate dataset of their type for copper in the literature and include the important and widely studied region of the K-edge and x-ray absorption fine structure. The results are compared with current theoretical tabulations as well as previous experimental measurements and expose inadequacies in both.