Large solid-angle polarisation analysis at thermal neutron wavelengths using a 3He spin filter

The strongly spin-dependent absorption of neutrons in nuclear spin-polarised 3He opens up the possibility of polarising neutrons from reactors and spallation sources over the full kinematical range of cold, thermal and hot neutrons. In this paper we describe the first large solid-angle polarisation analysis measurement using a 3He neutron spin filter at thermal neutron wavelengths (λ=2.5 Å). This experiment was performed on the two-axis diffractometer D1B at the Institut Laue-Langevin using a banana-shaped filter cell (530 cm3 ) filled with 3He gas with a polarisation of P=52% at a pressure of 2.7 bar. A comparison is made with a previous measurement on D7 using a cold neutron beam on the same sample, i.e. amorphous ErY6Ni3. Using uniaxial polarisation analysis both the nuclear and magnetic cross-sections could be extracted over the range of scattering-vectors [0.5⩽Q(Å−1)⩽3.5]. The results are in qualitative and quantitative agreement with the D7-data, where supermirrors were used to analyse the spin of the scattered neutrons. The paper analyses the essential parameters governing this technique and describes its experimental realisation.