X-Ray Absorption Coefficients and Geometrical Collimation of the Beam

In recent years, in order to achieve high angular collimation of an intense x-ray beam in a measurement of an absorption coefficient, most investigators have used the action of a high-resolving-power monochromator to provide the collimation. In this case, if the absorber exhibits small-angle scattering (including multiple scattering) of significant intensity in a scattering cone as large as a few seconds of arc, the value of the coefficient depends upon the absorber position with respect to the monochromator. A few measurements are reported with a two-crystal spectrometer to illustrate the points discussed.