Correction of Optical Aberrations in Elliptic Neutron Guides

TL;DR

This paper introduces a hybrid elliptic-parabolic neutron guide design that effectively corrects optical aberrations like coma, improving beam quality without sacrificing performance, thereby enhancing neutron scattering instrument capabilities.

Contribution

The paper proposes a novel hybrid elliptic-parabolic guide geometry that reduces optical aberrations in ballistic neutron guides, improving beam quality and instrument performance.

Findings

The new guide shape significantly reduces coma aberration.

It maintains the same beam current as standard guides.

Improves resolution functions for neutron instruments.

Abstract

Modern, nonlinear ballistic neutron guides are an attractive concept in neutron beam delivery and instrumentation, because they offer increased performance over straight or linearly tapered guides. However, like other ballistic geometries they have the potential to create significantly non-trivial instrumental resolution functions. We address the source of the most prominent optical aberration, namely coma, and we show that for extended sources the off-axis rays have a different focal length from on-axis rays, leading to multiple reflections in the guide system. We illustrate how the interplay between coma, sources of finite size, and mirrors with non-perfect reflectivity can therefore conspire to produce uneven distributions in the neutron beam divergence, the source of complicated resolution functions. To solve these problems, we propose a hybrid elliptic-parabolic guide geometry. Using this new kind of neutron guide shape, it is possible to condition the neutron beam and remove almost all of the aberrations, whilst providing the same performance in beam current as a standard elliptic neutron guide. We highlight the positive implications for a number of neutron scattering instrument types that this new shape can bring.