Neutron beam extraction and delivery at spallation neutron sources

We address two main issues in neutron beam extraction and delivery at spallation neutron sources: (a) the relatively small divergence of the beams impinging on samples at typical distances of 10-20 m from usual size moderators and (b) the beam losses in long neutron guides used to bridge large distances of the order of 100 m or more between moderator and sample, as required by the desired time-of-flight wavelength resolution or other instrument optimization conditions. Our Monte Carlo simulation results show, that novel neutron optical solutions can substantially enhance neutron fluxes on the sample in both cases, compared to the current practice on existing spallation source instruments. Converging supermirror-coated neutron guides with proper geometry can lead to substantial intensity gains even for wavelengths as short as 0.7 Angstrom at short moderator-sample distances typical for hot and thermal neutron inelastic spectrometers. Ballistic guides, consisting of a supermirror-coated diverging section, an Ni-coated uniform portion and a supermirror-coated converging section can deliver much higher beam fluxes over large distances than the conventional supermirror or Ni-coated guides currently in use. (C) 2000 Elsevier Science B.V. All rights reserved.