Neutron Scattering - JUWEL - Forschungszentrum Jülich

KWS-3 3
1 Introduction
Ultra small angle (USANS) and small angle neutron scattering (SANS) experiments are
performed by two different types of instruments to cover a combined Q-range from 10 -5 Å -1 up
to 1Å -1 . Double crystal diffractometers are used for USANS experiments, whereas the
"standard" SANS experiment is performed using a pinhole camera. In principle, the Q-range
of both instrument classes overlaps. Typical USANS instruments like S18 (ILL) or PCD
(NIST) may reach maximum Q-vectors of 5�10 -3 Å -1 . The disadvantage of these instruments is
that they do not allow taking a full area image on a 2D position sensitive detector. On the
other hand, the well-known pinhole instrument D11 at Institut Laue-Langevin (France)
reaches a minimum Q-vector of 3�10 -4 Å -1 by use of largest possible wavelength 22Å and
sample-to-detector distances (>40 m). But the required instrumental settings push both types
of instruments to their limits, mainly due to signal-to-noise level and the reduced flux at
sample position. The use of neutron lenses as additional elements of a pinhole SANS
instrument has been tested to overcome this intensity problem [1].
An alternative design is realized by the KWS-3 instrument [2]. The principle of this
instrument is a one-to-one image of an entrance aperture onto a 2D position-sensitive detector
by neutron reflection from a double-focusing elliptical mirror. It permits to perform SANS
studies with a scattering wave vector resolution between 10 -4 and 10 -3 Å -1 with considerable
intensity advantages over conventional pinhole-SANS instruments and double crystal
diffractometers. Therefore it perfectly bridges the "Q-gap" between USANS and SANS: Very
Small Angle Scattering (VSANS). The increasing need for these intermediate Q-vectors arises
from the growing interest in biological and colloidal samples, which partially deal with length
scales in the �m range. An investigation of the multilevel structures in partially crystalline
polymer solutions performed using a combination of those three above depicted types of
SANS instruments can be found in [3].
Figure 1: Focused VSANS fills space between USANS (double crystal diffractometer)
and classical SANS instruments.
The main innovation and challenge of KWS-3 was to build a large mirror having a shape
as close as possible to an ellipsoid and with a surface roughness less than 5 Å. The mirror is a