Appl. Phys. A 74 [Suppl.], S191–S193 (2002) / Digital Object Identifier (DOI) 10.1007/s003390201879 Applied Physics A Materials Science & Processing Thermal neutron optical experiments with tuneable many-bounce channel-cut crystals W. Treimer 1,2, ∗ , M. Strobl 2 , A. Hilger 1 1 University of Applied Sciences (TFH), Fachbereich II, 13353 Berlin, Germany 2 Hahn-Meitner-Institut, SF1, 14109 Berlin, Germany Received: 12 July 2001/Accepted: 13 November 2001 –  Springer-Verlag 2002 Abstract. We realized the first tuneable channel-cut crystals to be used as monochromator and analyzer in a double-crystal diffractometer and demonstrated the superior operation for ultra small angle scattering. With this new system it was pos- sible to measure the ultra small angle scattering from a sin- gle 14-μ m glass fibre, the small angle scattering from an Al wedge and the diffraction pattern of a Si phase grating. The last measurement was only possible due to the reduction of the Darwin width of both crystals from 5.7 sec of arc (natural Darwin width) to 1.4 sec of arc. PACS: 61.10.Eq; 61.12.Ex; 61.12.-q In ultra small angle scattering (USAS), very narrow Darwin widths of the monochromator and analyzer crystals of a dou- ble crystal diffractometer (DCD) are necessary to determine “large" (∼ μ m) scattering objects in a sample, due to the law the larger the scattering object the smaller the momentum transfer q. The use of asymmetric Bragg reflections do re- duce the Darwin width but it is always inherently connected with a geometrical broadening of the reflected beam width. In the case of asymmetric Bragg reflection the surface is in- clined by an angle α to the the reflecting net planes which causes the change of the beam geometry. Nevertheless it was up to now the only possibility to change the Darwin width for a given wave length and [hkl ] reflection. With such asymmet- ric cut crystals as well with other systems (lamellae crystals) several neutron optical experiments could be realized. They were used to separate symmetric and asymmetric reflected rays from a crystal surface to determine bc [1], to determine harmonic wave lengths in a neutron scattering experiment [2] and to suppress them [3]. However these the long ranging wing intensity of the Darwin pattern and therefore also of the rocking curve is not suitable for USAS. Now it is shown that these disadvantages of the broadening of the beam width as well as the suppression of the wing intensity can be overcome ∗ Corresponding author. (Fax: +49-30/4504-2705, E-mail: treimer@hmi.de) if special channel cut crystals (CCC) are used. The advan- tages of the new system are a constant beam width, no wing intensity and a tuneable Darwin width and therefore a tune- able FWHM of the rocking curve of the analyzer crystal. Channel cut crystals as monochromator and analyzer in a DCD are widely used in ultra small angle X-ray and neu- tron scattering due to their nearly rectangular Darwin pro- file [4, 5]. However due to spurious reflection the wings of the rocking curve are still enhanced, larger than dynamical the- ory predicts. This can be removed by so-called blocking cuts as proposed by A. Wagh [6] and realized by M. Agamalian et al. [7, 8]. The rocking curve then is a nearly perfect tri- angle, the perfect realization and agreement with the theory of dynamical diffraction was achieved recently [9, 10]. The suppression of the enhanced wing intensity is important be- cause the angular resolution indeed principally depends on the FWHM of the rocking curve for a given wavelength λ, however it also depends on the steepness of the wing inten- sity of the rocking curve. In the best case this intensity should be zero for | y| > 2. A tuneable reduction of the FWHM without a change of the beam width means an improvement of the angular resolution of a DCD and due to a constant beam geometry an intensity gain with respect to asymmet- ric Bragg reflection for all experiments. A solely reduction of the FWHM however does not involve an improvement of resolution which will be demonstrated by ultra small angle scattering experiments. 1 Theoretical background and experiments The formula of the diffraction pattern in the case of dynamical diffraction of X-rays and neutrons for their interaction with perfect crystal lattices is given for “thick” crystals (Darwin solution) as R D = 1 if | y|≤ 1 =  | y|-  y 2 - 1  2 if | y| > 1 (1)