A NEW ENERGY DISPERSIVE-X-RAY MONOCHROMATOR FOR SOFT-X-RAY APPLICATIONS

Energy dispersive x-ray monochromators allow time-resolved XANES and EXAFS measurements in the millisecond time scale. With this technique it is possible to study changes, during chemical reactions and phase transitions, in the structure of chemical substances within the environment of the absorbing atom. The instruments used up to now are not built for use under vacuum and are thus restricted to experiments above 4 keV. An energy dispersive monochromator is described which is currently being built at the Bonn 3.5 GeV electron stretcher and accelerator (ELSA). The purpose of this monochromator is to be able to measure downwards in energy at least to the sulfur K edge at 2.4 keV. The problems which had to be solved are (i) the construction of a windowless vacuum chamber for the Bragg crystal, the sample, and the detector, (ii) the short distance between the crystal and the sample, and (iii) the small radius of curvature of the Bragg crystal. Furthermore, the monochromator has to be rapidly tuned with ease to different energies.