For most of the diffraction work performed on the X31 line a channel-cut Si(111) monochromator is used. The limited energy resolution obtainable with this cut of crystal meant however that it was necessary to use a Si(311) crystal for some of the MAD experiments. The motivation for this change will be discussed later.
The two crystals are essentially identical, distinguished only by their plane orientation.
The crystal in use is mounted on a goniometer which allows rotation
around three directions. Two of the axes are normally fixed and for most
experiments the crystal lies such that the plane formed by the incident and diffracted
beam directions is vertical. Incident X-rays are diffracted twice from the
two parallel faces ensuring that the exit beam travels parallel to the input
beam, only shifted vertically by a small distance. The basic arrangement is shown in Fig. 
.
Figure: Schematic diagram of the silicon channel-cut monochromator used on
the X31 beam line.
The photon energy of the monochromated beam is selected by rotating this set-up
around a horizontal axis perpendicular to the beam direction thus changing the angle
between the incident X-rays and the diffracting planes of the crystal. This
rotation is controlled
remotely and performed by a 500 steps/rotation stepping motor attached to the goniometer
via a 
gear and worm-drive. One full rotation of the worm-drive is thus equivalent
to 
steps of the motor and produces a 
change in the Bragg angle.
The vertical shift in beam height produced by reflection from the two crystal faces does not remain constant with changing Bragg angle and may be calculated using the equation
where 
is the separation of the two reflecting surfaces, 
is the Bragg
angle for the reflection and 
is the shift in the exit beam height given a
change in Bragg angle, 
.
The crystal rests on a water-cooled copper block and is held lightly in place by three copper-beryllium leaf springs. To reduce the distortion of the silicon lattice at the reflecting surfaces arising from strain fields originating at the points of contact to the cooling block there is a second groove cut into the crystal parallel to the first which acts to relieve this strain.
Originally the crystal was not cemented to the cooling block and it was suspected that
the thermal contact between the two surfaces was poor since neither were highly
polished. An attempt was made to counter this situation by introducing a thin layer
of silicone grease between the two surfaces but no significant improvement has been
observed. The effects of the thermal load on the monochromator's performance are discussed
further in Sec. .