REFSANS has been designed as a reflectometer for measuring both specular reflectivity and evanescing small-angle neutron scattering from solid samples as well as from the air-liquid interface.
These requirements are met and optimum measurement conditions are provided by its novel design:
REFSANS is located at the end position of a the tilted neutron guide NL-2b at FRM II providing a broad cold neutron beam with a width of 170 mm and a height of 12 mm.
The instrument is designed as a ToF instrument (Fig. 1) to cover a huge range of momentum transfer with only a few instrumental settings (air-liquid interface).
The three double disc choppers allow performing measurements with high and low wavelength resolution to cover the demands of high resolution reflectometry ( Dl / l ~ 1%) as well GISANS investigations ( Dl / l ~ 10%). The chopper further allows inelastic measurements and setting the wavelength range to be used to optimize the chopper transmission.
The neutron optics of REFSANS comprise neutron guide elements with different channels and special apertures to provide on the one hand slit-height smeared beams for conventional reflectometry and on the other hand point focussed beams for GISANS measurements.
Up to 13 partial beams can be focussed in a detector plane at a distance of ~ 9 m from the sample to increase the beam intensity in GISANS geometry. A 2D-positon sensitive detector with an active area of 50 cm × 50 cm, a spatial resolution of 2 mm × 3 mm, a very low g -sensitivity and a maximum global count rate up to 106/s can be set at distances between ~ 1.5 m and 12 m from the sample.
Fig. 1: Schematic view of REFSANS
The specular reflectivity from a fluid cell measured at REFSANS demonstrates that REFSANS allows high resolution measurements covering large ranges of momentum transfer and reflectivities (Fig. 2) with further room for improvement.
Fig. 2: Reflectivity and density profile of an Si-wafer with an ~ 75 nm thick oxide layer
Fig. 3: TOF GISANS Scattering Patterns
By means of GISANS-measurements lateral structures of extremely small amounts of surface material can be derived from measurements at REFSANS.
As an example Fig. 3 shows GISANS data from a surface layer with thickness of only 3.1 nm corresponding to a mass of the layer of only ~ 4 × 10-09 g. A sample goniometer is available at REFSANS which can be loaded with sample environments up to 200 kg or a large Langmuir trough. Polarized neutrons will be available at REFSANS in 2010.