Measurements using the Si(111) monochromator.



next up previous contents index
Next: Measurements using the Up: X31 beam line Previous: Intrinsic resolution of

Measurements using the Si(111) monochromator.

The first series of experiments with the calibrator on X31 were performed using the Si(111) monochromator on the 31st September 1992 while DORIS III was operating in parasitic mode at . The instrument function was measured as a function of the width of the the first slits by scanning over the (771) reflection of the calibrator which was excited at an X-ray energy of . The (771) reflection was appropriate for the measurements for two reasons. Firstly the plane of scattering for this reflection was vertical meaning that only the vertical beam divergence contributed to broadening of the line width. Secondly the Bragg angle was further reducing the possibility of broadening due to beam divergence as can be seen from Eq. gif. The contribution of the vertical divergence to the width of the (771) reflection was per of vertically divergent beam. The vertical source size at the tangent point of fan D was estimated to be with a vertical divergence of as calculated from source parameter data provided by HASYLAB. gif. This divergence therefore makes a negligible contribution to the width of the (771) calibrator line.

The smallest observed FWHM was at a slit setting of . Cutting down the slits further had little effect on the resolution. Given the vertical source size quoted above a line width of had been expected.

During a three week shut down in October '92 the monochromator crystal was removed and inspected. The crystal was found in poor condition. A black substance covered the crystal in regions where the white beam had been incident on it. After a thorough cleaning and re-etching the crystal was re-installed.

A second measurement of the instrument function was performed after restarting on the 22nd November '92, this time while DORIS III was operating in dedicated main user mode. The vertical source size and divergence during main user time were calculated to be and . The resulting scan over the (771) reflection using front slits set at is shown in figure gif. The FWHM is () which is better than the expected width of .

  
Figure: X31 instrument function with the Si(111) monochromator in place and front slit setting of . The calibrator (771) diffraction profile has a FWHM of ().

Previous studies of the absorption edges of some 3rd transition series metals [77] and the Lanthanides [94] have shown that the typical widths of the white lines of these elements may range from up to . The measurements on the 3rd transition series metals were performed using an energy resolution of . Although use of the Si(111) crystal with a resolution of would not seriously reduce the white line height there was some concern at this stage about the reproducibility of the results given the inconsistencies in the DORIS III operating parameters around the time of these experiments.

As a result it was felt that a higher dispersion monochromator would be appropriate for obtaining the required energy resolution even when the source size was as large as . A Si(311) crystal was obtained gif and used for all experiment described hereafter.



next up previous contents index
Next: Measurements using the Up: X31 beam line Previous: Intrinsic resolution of



Gwyndaf Evans
Fri Oct 7 15:42:16 MET 1994