Aspherical refractive lenses for small-angle neutron scattering

Aspherical neutron lenses are discussed in detail as focusing elements for existing small-angle neutron scattering (SANS) diffractometers. The conceptual design can be obtained by analytical equations, while computer simulations provide the intensity distribution of the primary beam, which is strongly correlated with the instrumental resolution and the minimal momentum transfer Q. For large illuminated sample/lens areas of 50 mm diameter the aspherical lens design is essential, while spherical lenses are limited to ca 20 mm diameter with a 60% spherical aberration for standard SANS experiments. Scattering experiments on different samples proved the applicability and revealed the resolution limits of the two lens types. From theoretical considerations it is proposed that lens cooling can be used to reduce the thermal diffuse scattering.