Wide angle NSE: The multidetector spectrometer SPAN at BENSC

The magnetic field configuration of the spectrometer SPAN allows simultaneous NSE measurements to be performed over a particularly large angular range. The NSE precession field is created by three pairs of coils with diameters 1, 3 and 4.8 m, respectively, and the set-up has a symmetry axis, which is perpendicular to the scattering plane and crosses the sample position. The resulting magnetic field integral does not change with the scattering angle and thus fulfils the condition for multidetector NSE. In order to preserve the symmetry of the magnetic field, remanent supermirrors are used as analyzers. These supermirrors perform in reflection, in the stray magnetic field of the precession coils, and are mounted on analyzer frames with an angular opening of 25.7° each. The neutron optical elements (analyzer, polarizing guide) of SPAN have a high transmission over a large wavelength band. As a consequence of this, NSE can be done for incident wavelengths ranging from 2.5 to 9 Å. At the lowest wavelengths high q values can be reached, which were not previously accessible for NSE experiments (at λ = 3 Å: 0.08 Å^-1 ≤ q ≤ 4 Å^-1).