SMALL-ANGLE X-RAY-SCATTERING USING COHERENT UNDULATOR RADIATION AT THE ESRF

A simple approach for producing a high-coherent-flux X-ray beam for sm all-angle-scattering studies used at the Troika beamline of the Europe an Synchrotron Radiation Facility is reported. For such small-angle st udies it is permissible to reduce the longitudinal coherence length of the beam, thus increasing the energy bandpass and intensity of the be am, because there is only a small optical pathlength difference. By us ing mirrors and filters to cut unwanted energies from the undulator ha rmonic structure, a high-flux beam of >10(9) photons s(-1) through a 5 mu m-diameter pinhole at 8.2 keV with a bandpass of 1.3% can be produ ced. The coherent properties of this beam have been measured by analyz ing a static speckle pattern from an aerogel sample imaged by a direct ly illuminated CCD camera. The speckle size and contrast are compared with the expected values based on a statistical analysis of the intens ity distribution of speckle patterns obtained using partially coherent conditions. The expected widths of the spatial autocorrelation are fo und, but there is an apparent incoherent fraction of the beam which re duces the measured contrast. The method presented is to be used as a t ool to optimize conditions for diffraction experiments using coherent X-rays.