The neutron spin echo technique uses the neutron spin as an indicator of the individual velocity change the neutron suffered when scattered by the sample. Due to this trick the instrument accepts a broad wavelength band and at the same time is sensitive to velocity changes down to 10^(‑ 5). However the information carried by the spins can only be retrieved as the modulo of any integer number of spin precessions as intensity modulation proportional to the cosine of a precession angle difference. The measured signal is the cosine transform S(Q, τ) of the scattering function S(Q, ω). All spin manipulations only serve to establish this special type of velocity analysis. For details see “Neutron Spin Echo”, ed. F. Mezei, Lecture Notes in Physics, Vol. 128, Springer Verlag, Heidelberg, 1980.
Due to the intrinsic Fourier transform property of the NSE instrument it is especially suited for the investigation of relaxation-type motions that contribute at least several percent to the entire scattering intensity at the momentum transfer of interest. In those cases the Fourier transform property yields the desired relaxation function directly without numerical transformation and tedious resolution deconvolution. The resolution of the NSE may be corrected by a simple division.
For a given wavelength the Fourier time range is limited to short times (about 2 ps for the FRM II-setup) by spin depolarization due to vanishing guide field and to long times by the maximum achievable field integral J. The time is proportional to J × λ³. The J-NSE may achieve a J = 0.5 Tm corresponding to τ= 48 ns at λ = 8 Å.
The instrument itself consists mainly of two large water-cooled copper solenoids that generate the precession field. The precession tracks are limited by the π/2-flippers and the π-flipper near the sample position. The embedding fields for the flippers are generated by Helmholtz-type coil pairs around the flipper locations. After leaving the last flipper the neutrons enter an analyzer containing 60 (30 × 30 cm²) CoTi supermirrors located in a solenoid set. These mirrors reflect only neutrons of one spin direction into the multidetector. By the addition of compensating loops the main coils and the analyzer coil are designed such that the mutual influence of the different spectrometer components is minimized.