Properties of elliptical guides for neutron beam transport and applications for new instrumentation concepts

Abstract

The use of nonlinear tapered guides is becoming more common in advanced neutron scattering facilities around the world. Elliptical guides offer the promise of high performance not only as focusing devices but as an efficient way to transport neutrons over long distances. Here, the analytical expressions to determine their performance are derived and discussed. Under certain conditions, an increase in flux delivery is observed with increasing guide length, due to an increase in the angular spread of the neutrons reflected in the guide. The performance is only limited by the distance between the source and the guide entrance, the dimensions of the instrument placed after it, and the supermirror coating. As an example of the potential of elliptical geometry in instrumentation, a new small-angle neutron scattering (SANS) instrument concept is proposed, in which the neutron source is directly coupled to a half-ellipse, and the instrumental performance is evaluated by means of analytical expressions. The results show that such an instrument may provide a viable alternative to conventional pinhole SANS for high-resolution measurements and small samples, being substantially more compact and simpler to operate. The main limitation comes from the coma aberration which is inevitable on extended sources. The extent of the coma problem is also analysed.