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.