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

Small-angle neutron scattering (SANS) diffraction experiments involve scatt ering of a monochromatic beam of neutrons and measuring the angular distrib ution of the scattered neutrons. Unlike conventional diffraction experiment s, the scattering angles involved in a SANS experiment are small. In view o f the small angles involved, the uncertainties in the angles are usually qu ite large (similar to 10%) and thus one uses a comparable wavelength resolu tion (Delta lambda/lambda) for the incident neutrons to optimize the intens ity and the resolution. That is, while conventional diffractometers use Del ta lambda/lambda similar or equal to 2%, SANS machines require Delta lambda /lambda similar or equal to 10%. This paper presents the results of Monte C arlo simulations of a double-crystal monochromator system in which the wave length resolution is spoiled by replacing the individual crystals of the do uble monochromator by three slightly misaligned pairs of crystals. It is se en that for a misalignment of 1 degrees and a mosaic spread of 0.8 degrees for all the crystals, three wavelengths selected by monochromatization are well resolved, with a width of 0.05 Angstrom around each wavelength, thus g iving a total wavelength spread of about 5%. Based on the above, a monochro mator system has been built and tested. The measured wavelength distributio n is in reasonable agreement with the calculated one.