Neutron Beam Conditioning for Focusing SANS Spectrometers

Abstract

Multibeam focusing offers an appealing compromise between high resolution and high incident flux configurations for SANS spectrometers. In fact so many "spectrometers" operate in parallel as the number of channels in the collimator. Each channel provides high resolution by small spot size on the detector and long sample-to-detector distance, involving significant limitation of the transmitted beam phase space volume, thus reducing the flux. The flux on the sample is increased by the large number of channels. In view of the multibeam collimation it is beneficial to increase the beam cross section and decrease the divergence at the same time. Two aspects related to the use of rotational velocity selectors are investigated. First the transmitted phase space is determined from the selector parameters. It is found that the beam azimuthal divergence with respect to the rotor axis has a significant effect on the selectivity. Neutrons flying along different paths are treated differently, leading eventually to energetic non-uniformity of the illumination of various collimator channels. Then the effect of the gap in the neutron guide at the selector location on the phase space uniformity at the collimator entrance is investigated and optimal selector location along the beam is proposed together with optimal neutron guide shape in the vicinity of the gap, which accommodates the selector.