A new version of a medium-resolution double-crystal diffractometer for the study of small-angle neutron scattering (SANS)

A new version of a SANS instrument based on two bent perfect Si crystals in the nondispersive (1, -1) setting is presented. Unlike an earlier proposal which employed bent crystals in symmetric diffraction geometry [Kulda & Mikula (1983). J. Appl. Cryst. 16, 498-504], use of the crystal analyser in the fully asymmetric geometry enables the angular dependence of the SANS intensity to be transformed into the positional dependence along its longest edge. This makes it possible to employ a one-dimensional position-sensitive detector and thus significantly increase the speed of collection of experimental data. Momentum-transfer resolution may be easily controlled by an adjustment of the bending radii of the crystals. Use was made of diffraction by the (111) crystal planes at a neutron wavelength of 0.2 nm to measure powder samples of Si₃N₄, poly(vinyl chloride)/poly(methyl methacrylate) and CrO₂ in order to demonstrate the practical efficiency of the proposed device.