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. 
. 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. 
. 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 . 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 and used for
all experiment described hereafter.