We used the x-ray-extended range technique to measure the x-ray mass attenuation coefficients of silver in the 15-50 keV energy range with a level of uncertainty between 0.27% and 0.4% away from the K-edge. The imaginary part of the atomic form factor of silver was derived by subtracting the scattering component from the measured total mass attenuation coefficients. Discrepancies between the measured mass attenuation coefficients and alternative theoretical predictions are discussed. Atomic

more »

... factors can be determined experimentally by various methods, such as interferometry, total external reflection, refraction through prisms, fitting diffraction profiles, or by direct measurement of attenuation [1] [2] [3] [4] [5] [6] [7] [12] [13] [14] [15] . Each method has particular advantages and disadvantages [16] . Determination of the real component of the atomic form factor involves the use of Bragg diffraction or of the coherent (or elastic or Rayleigh) scattering process. Calculations of this scattering process can be found in [17] . The imaginary component of the atomic form factor, which is proportional to the photoelectric absorption coefficient, can be determined very accurately from measurements of attenuation but requires correction for scattering and fluorescence contributions [18] . Since heavy elements are more absorbing, experimental work dealing with these elements requires the use of quite thin foils, especially at energies of interest to crystallography. Accordingly, accurate determination of the local thickness of specimens becomes much more challenging. This is one of the main reasons for the dearth of good quality data for heavy elements in the literature. At high energies, the contributions from scattering processes become commensurate with those from the photoelectric absorption. Accurate allowance for scattering is therefore required in order to extract the photoelectric component from the measured mass attenuation coefficients. Our earlier work targeted low to medium Z elements at relatively low energies. However, the techniques of these experiments are not entirely adequate for experiments with high Z elements, especially at higher energies. In particular, the investigation of the scattering contribution, which is more significant in this higher energy range, requires some modification of the original experimental set-up. This paper presents the results of our measurement of the x-ray mass attenuation coefficient of silver in the 15-50 keV energy range. The level of the accuracy of the measurement (0.27%-0.4% away from the K-edge and up to 0.7% in the K-edge region) makes it possible to compare these results with alternative theoretical predictions. The imaginary part of the atomic form factor of silver was derived by subtracting the scattering component from the total attenuation coefficients and using the resultant photoelectric mass absorption coefficients in the optical theorem. Experimental details The experiment was conducted at the third-generation APS synchrotron radiation source, beamline 1BM [19] . Figure 1 shows the experimental arrangement [12, 14] . The beam was monochromatized by a double-reflection 400 silicon monochromator and was collimated to a 2 × 2 mm 2 cross-section. The incident and attenuated intensities were detected by two ion chambers. The flow of nitrogen gas through the two ion chambers and the electronic settings were optimized for stability of the readings and for counting statistics [20] .