The X11 beam line is used solely for protein crystallography and accepts of radiation from bending magnet K of DORIS III. The geometry of the beam line is shown in figure . The X-rays are focussed in the horizontal plane by a bent triangular germanium crystal which may be rotated around the vertical to select the required X-ray energy to be reflected. The monochromator is cut with its () reflecting planes at to the illuminated surface and set in beam compressing geometry. The X-rays, after reflection from the crystal, are incident on an elliptically bent -segment quartz mirror which performs focussing in the vertical plane and finally pass through a Be-window, into a long collimator consisting of two pairs of vertical and horizontal slits, and , and onto the sample. As with the X31 beam line, the collimator, sample, and detector are mounted on an optical bench whose alignment with the X-ray beam may be optimised via CAMAC interfaced to a PC.
The detector used is a MAR image plate scanner. The dynamic range of this detector is four times that of the RED image plate scanner used on the X31 beam line which is more appropriate given the higher flux produced by X11.
Figure: Schematic diagram of the X11 beam line on fan K of the DORIS III storage ring.
A change in the X-ray energy on this line is achieved by two independent actions. The monochromator must be rotated to change the Bragg angle and in turn the mirror and optical bench must be swung around to follow the reflected beam. This makes the use of this beam line for experiments in energy-scan mode impractical and introduces a delay of several hours if the energy needs to be changed. Energy calibration of the beam line is generally performed by measuring the diffraction patterns of crystals with known lattice spacing and assuming that the crystal to image plate distance is a known parameter.