Polarized Neutron Beam at the SANS Diffractometer KWS2 of the JCNS

This article describes a high-efficiency transmission polarizer that has been installed at the high-intensity SANS diffractometer KWS2 of the Jülich Centre for Neutron Science. The polarizer is primarily designed to be used in the low resolution/high Q-range mode of this diffractometer for the purpose of the separation of coherent scattering on biological objects from an intrinsic background caused by incoherent scattering on their hydrogen atoms.The polarizer operates with a rather divergent incident beam and is placed at about 2m from the sample (upstream in the beam). The diffuse spin-flip scattering that would become critical for such geometry is suppressed due to the use of a strong, about 0.14T, magnetic field. The polarizer has been characterized by a 3He neutron spin filter and provides very high polarization - 93% at 4.5 Å and 99.7% for neutrons with wavelength above 6 Å – for the SANS collimation 4m. The polarizer transmission at 4.5 Å amounts to 94% of the desired spin component.The polarizer is placed in the collimation base of the instrument and can be easily put in and out of the beam thus allowing for “an instant” switch between polarized and non-polarized neutron beams.