Optimisation of a crystal design for a Bonse-Hart camera

Bonse-Hart double-crystal diffractometers (DCDs) with multi-bounce channel-cut crystals show rocking curves that depart dramatically from dynamical diffraction theory in their wings. The intrinsic background is many orders of magnitude higher than the predictions of dynamical diffraction theory. This effect was studied at the ultra-small-angle neutron scattering facility at the Atominstitut in Wien and at facilities in Grenoble, Jülich and Villigen. The scattering intensity contains Bragg reflections from the front and the back faces, and thermal diffuse scattering from the internal volume. The aim of this study was to eliminate this contamination and develop a new crystal design which provides optimal resolution. Therefore different ways were tested. In the first step the contamination was eliminated by cutting a groove in the middle of the back plate of the channel-cut crystals and inserting a cadmium absorber in this groove. With this modification an additional suppression of the wings of the rocking curve of about one order of magnitude was achieved. After this, we developed a new design for a DCD. The concept for this new crystal design was to avoid the back reflection and the thermal diffuse scattering. The different steps on the way to produce these crystals are presented in a detailed way. The crystal preparation and the different instruments where these crystals have been tested are also described.