X31 is one of three EMBL beam lines which accept synchrotron radiation from fan D and
was one of the first easily tunable X-ray sources made available to the protein
crystallography community.
It was first constructed in-house in the late 70's and has
been modified several times although the basic design has been preserved.
Fig. 
is a schematic representation of the lines major components and their positions.
Figure: Schematic diagram of the X31 protein crystallography beam line in HASYLAB I.
The first element in the line is
a set of vertical slits. The slits have been fitted with a photo-emission device for use in MAD experiments.
These are mounted 
before a channel-cut monochromator which is 
from the source. The monochromator
and front slits are mounted on a common bench which may be raised or lowered remotely.
Focussing in the vertical and horizontal planes is achieved by a gold-coated toroidal mirror,
metres from the source giving a 
demagnification. In the
experimental hutch the beam is collimated by two sets of horizontal and vertical slits, 
apart. Both sets of slits are fitted with small ionisation chambers, 
and 
,
which provide a measure of the X-ray flux through the rear and front ends of the
collimator respectively.
In front of the collimator, a device has been installed which allows absolute calibration and on-line monitoring of the energy of X-rays incident on the protein crystal samples. Fluorescence from these samples is measured by a detector (not shown) mounted in the horizontal at right angles to the X-ray beam. The protein crystal samples themselves are mounted on an ENRAF NONIUS rotation axis and diffraction from the sample is recorded on an image plate scanner. The image plate scanner is the prototype of the MAR scanner [82] and was constructed in-house. The detector has a dynamic range of 1:16000 (ADC units).
The calibrator, collimator, rotation
axis and detectors are all mounted and pre-aligned on a single optical bench.
The position of the optical bench with respect to the X-ray beam may be optimised
by measuring the signal output from the chambers and . The bench is first
translated horizontally and vertically so as to maximise the signal from and
then rotated about to maximise the signal in .
On X31 this is performed automatically
by a Vax-Station interfaced via CAMAC to the ionisation chambers and stepping motors.
The major components will now be described in more detail.