After May '93 DORIS III was running as a dedicated source of synchrotron radiation.
In November '93 with the Si(311) monochromator in place the instrument function for the beam line was this
time measured by scanning over the calibrator's (553) reflection. For the particular 
and 
angles used the (553) line appeared at an energy of 
in the upper channel of the
calibrator. The plane of scattering is almost vertical for this setting and the Bragg angle for
the reflection is 
. The expected broadening due to vertical beam divergence was still
only 
for this reflection if it was assumed that the source parameters were the same as
those for dedicated user time. A large
improvement in the energy resolution was observed with the new crystal with a measured FWHM
of 
at a front slit setting of 
.
At this time no data were available for the source parameters and an attempt was made to
measure the size by making a series of scans over the (553) line as a function of the front slit size.
The resulting series of measurements are shown in Fig. 
and their FWHM's are plotted as a function of the slit width in Fig. .
The best FWHM of is 
times larger than would
be expected given the intrinsic energy resolution for the Si(311) reflection, demonstrating that
the vertical source size dominates the energy resolution in this case. A vertical source size
of 
(FWHM) would result in a limit of for the energy resolution.
Figure: Results of eight scans over the (553) calibration line for front slit settings
of 
and 
. The
diffraction profiles are
normalised to 
and placed on a relative energy axis where 
represents the centroid
position of the fitted Gaussian profile. Calculated FWHM values were taken from the results
of a least squares Gaussian fit to the profiles.
Figure: Plot of the (553) diffraction profile FWHM against the slit height before the
monochromator for slit sizes smaller that 
.
The smallest obtainable FWHM of 
is a consequence of the finite vertical source
size of which was estimated to have a FWHM of 
.
An increase in the width of the (553) reflection of about was observed as the slit width became
less than than 
. This is believed to be due to diffraction of the white beam at the slit. The
angle through which radiation of wavelength 
is diffracted by a single slit of width 
is given
approximately by 
. Thus assuming a slit width of 
and 
Å
radiation, 
is equal to 
. The total increase in the vertical divergence due
to this effect is double this quantity. The resulting increase in the bandpass of the monochromator
crystal at this energy due to this increase in divergence is 
compared to an observed
increase of . The discrepancy may be due to the error in the measurement of the slit widths.