X-RAY SPECTROMETER FOR ANOMALOUS SCATTERING EXPERIMENTS - TESTS OF A GRAPHITE MOSAIC AND A GERMANIUM MONOCRYSTAL AS ANALYZERS

Anomalous X-ray scattering provides a useful way for improving the con trast between different atoms present in multi-element samples. To tak e full advantage of this technique, the detector has to have a good en ergy resolution in the diffracted beam to separate the elastic scatter ing signal from the inelastic one due to fluorescence or resonant Rama n scattering (RRS). Solid state detectors are usually used, but the en ergy resolution is limited to about 150 eV which does not allow the se paration RRS from elastic scattering. Ice and Sparks [1] described a m osaic crystal spectrometer which resolves this problem. We have adapte d their setup on our beam line D23A at LURE-DCI but we have encountere d a loss of resolution when the Bragg planes of the sample are incline d. The shape of the diffracted beam incoming on the position-sensitive detector depends strongly on the inclination of the diffracting plane s; this explains the loss of resolution which is increased in our case by the large horizontal incoming beam size. Alternatively, we recomme nd the use of a perfect crystal as an analyzer coupled with a 0d detec tor which gives a very good resolution (approximate to 10 eV) without modification of resolution when Bragg planes of the sample are incline d in chi. Furthermore this kind of spectrometer allows a high counting rate so that it is possible to measure close to Bragg peaks (clusteri ng studies, for example) and even Bragg peaks. The loss of efficiency is nor dramatic considering the high brightness of third generation sy nchrotron radiation and is mole than compensated by the improved signa l to noise ratio.