Monochromator (OH1)

General Description

ID32 uses a double-crystal monochromator with two crystals on a common axis of rotation in a vertical scattering geometry.

Due to the heat load of the white beam, both crystals must be kept at liquid nitrogen temperature. They are attached to a copper block, which is connected to a closed circuit cooling system.

When the energy is changed by rotating the two crystals around a common axis, the exit beam height is kept constant by changing the gap between the two crystals. Final alignment of the lattice planes of the second crystal with respect to the ones of the first crystal is achieved with a stepper motor for the horizontal rotation axis ("roll") and a stepper motor and a piezoelectric device in the vertical axis. The latter (the fine-tuning of the diffraction angle of the second crystal) can be controlled by an intelligent digital feedback device according to the principle described by Krolzig et al.

The monochromator vessel is mounted on a linear translation stage perpendicular to the beam to allow the choice between two different setups of crystals. For the moment, a set of two silicon (111) crystals is used. A set of multilayers will be installed during the winter shutdown 2008-2009.

Reference: A.Krolzig, G. Materlik, M. Swars, and J. Zegenhagen, Nucl. Instr. and Meth. 208, 613 (1983) (doi:10.1016/0167-5087(83)91193-6)