Exploring performance of neutron guide systems using pinhole beam extraction

We perform an optimization of pinhole neutron guide systems under systematically varying conditions. It is investigated how neutron guide systems consisting of a parabolic feeder inside the biological shielding followed by a pinhole and an elliptical guide perform with different pinhole sizes and divergence requirements. We have clarified in which situations such a guide system is a viable choice and when the parabolic feeder is necessary in terms of neutron transport. The advantage of this design is the reduction of background from fast thermal neutrons compared to a system without a pinhole, hence the smallest possible pinhole is of interest. It is found that instruments with divergence requirements of ±1.0° will have excellent neutron transport with a 3×3 cm2 pinhole, while lower divergence requirements of ±0.5° can do with a smaller pinhole of 2×2 cm2. The feeder effectively reduces the necessary pinhole size, and is especially beneficial for short instruments. In addition to these qualities, a feeder will often smoothen the divergence profile, mostly for longer instruments.