Double monochromator with three pairs of crystals for a SANS machine: Monte Carlo simulation studies

Small-angle neutron scattering (SANS) diffraction experiments involve scattering of a monochromatic beam of neutrons and measuring the angular distribution of the scattered neutrons. Unlike conventional diffraction experiments, the scattering angles involved in a SANS experiment are small. In view of the small angles involved, the uncertainties in the angles are usually quite large (∼10%) and thus one uses a comparable wavelength resolution (Δλ/λ) for the incident neutrons to optimize the intensity and the resolution. That is, while conventional diffractometers use Δλ/λ ≃ 2%, SANS machines require Δλ/λ ≃ 10%. This paper presents the results of Monte Carlo simulations of a double-crystal monochromator system in which the wavelength resolution is spoiled by replacing the individual crystals of the double monochromator by three slightly misaligned pairs of crystals. It is seen that for a misalignment of 1° and a mosaic spread of 0.8° for all the crystals, three wavelengths selected by monochromatization are well resolved, with a width of 0.05 Å around each wavelength, thus giving a total wavelength spread of about 5%. Based on the above, a monochromator system has been built and tested. The measured wavelength distribution is in reasonable agreement with the calculated one.