Focussed beam reflectometer for solid and liquid surfaces

Abstract

Neutron reflectometry from liquid surfaces implies reflection in a vertical plane at variable angles and/or variable wavelengths. The TOF wavelength-scan method conventionally used for this kind of geometry offers many technical advantages, but it requires an end position on a beam tube/guide delivering a white beam. Due to space restrictions the reflectometer V6 at BENSC had to be installed using a crystal monochromator. In order to allow variable angle reflection on a horizontal sample surface, a 10-cm high curved monochromator crystal assembly is used. The polarized monochromatic beam is focussed in the vertical plane to the sample at some 3 m distance (angular range of grazing incidence ∼ 1.9°, i.e. a q range of 0–0.09 Å −1 at 4.75 Å wavelength). The instrument can be used in different modes of operation: (1a) Fixed collimated narrow beam: the beam is defined by two 0.1–1 mm wide horizontal slits between monochromator and sample. (1b) Fixed collimated, wide beam: for small area samples the slit close to the sample is opened up, so that the beam width is determined by the projection of the sample surface. (2) Collimated beam with the beam direction scanned via moving slits: for liquid surfaces. (3) Focussed, fan-like incoming beam and simultaneous observation of the specular reflections with multidetector bank. Modes (1a), (lb) and (2) allow for the investigation of diffuse scattering effects by the use of the multidetector, too.