Neutron resonance spin echo using spin echo correction coils

Abstract

We report on a recent experiment which was performed in order to examine a new experimental realization of the neutron resonance spin echo (NRSE) technique. In neutron spin echo (NSE) neutrons accumulate spin phase in long magnetic fields before and after the sample. The measured quantity is the final scattered beam polarization, which contains information on the sample dynamics (the intermediate scattering function S(Q,t)). This method is limited by the inhomogeneity of the magnetic field, leading to differences of the spin phase of individual neutrons and thus influencing the scattered beam polarization even in the absence of sample dynamics. The NRSE technique can overcome this limitation by reducing the dimensions of the field. However, all previously built NRSE spectrometers use transversal magnetic fields, making it impossible, in practice, to correct for additional limiting effects, such as the beam divergence. For the first time, we have built a NRSE setup with longitudinal field geometry, which does not have this disadvantage. In order to test this new approach, we combined a longitudinal NRSE setup in one spectrometer arm of the IN11 instrument at ILL with a (conventional) NSE setup in the other arm. This experiment demonstrates, how NRSE can be realized in longitudinal field geometry, and sheds light on the differences to previous NRSE setups. As a main result, we show that the effect of beam divergence could be corrected by means of standard Fresnel coils. The proper operation of this hybrid spectrometer is demonstrated by measurements of diffusive dynamics in a well-known micellar sample.