Ultra-small-angle scattering with polarized neutrons
Introduction
Ultra-small-angle neutron scattering (USANS) allows to study structures with a size in the range of a few tenths of a micrometre up to a few tens of micrometres. The main components of a corresponding instrument are channel-cut perfect silicon crystals acting as monochromator and analyser with the sample placed in between. These crystals are arranged as double-crystal diffractometer in Bonse–Hart configuration. Only recently it was proposed [1] and experimentally verified [2] that the use of polarized neutrons enables to also study magnetic structures by this technique. We have installed polarization options on our USANS instruments, KWS at the Triga Reactor of the Vienna University of Technology and S18 at the Institut Laue-Langevin, and performed first experiments on both instruments. We present the method used for polarization, discuss the influence on the instrument resolution function and show measurements exemplifying the prospects of USANS with polarized neutrons (USANSPOL).
Section snippets
Instrument characteristics
Neutrons are polarized by magnetic prisms which are also used for polarized neutron interferometer experiments [3]. We place two prisms between the monochromator crystal and the sample as schematically shown in Fig. 1. The prisms have triangular-shaped front pieces with an air gap of 1 cm height where the neutrons pass without interacting with any material. The magnetic field in the centre of the prisms is homogeneous with a strength of about 0.9 T but shows a complex three-dimensional behaviour
First experiments
We have used cylindrical iron rods with 1 mm diameter and magnetized in a 0.1 T magnet at the sample position as a first sample for our tests of USANSPOL at the Atominstitut. When magnetized, these rods also act as birefringent media for neutrons with different spin orientation and the index of refractiondepends on the nuclear and magnetic scattering length densities. Neutrons are refracted at the entrance and exit surfaces and the intensity of the refracted neutron wave
Acknowledgements
This research project has been supported by the European Commission under the 6th Framework Programme through the Key Action: Strengthening the European Research Area, Research Infrastructures. Contract no: RII3-CT-2003-505925.
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