Neutron imaging using zone plates
The high penetration power of neutrons makes them ideal for non-destructive study of materials. However, the neutron beams used in the process must be efficiently focused onto the target if the application is to be successful. A new optics device has been developed which makes use of the Fresnel lens concept and improves neutron focusing. Zone plates, sometimes called Fresnel zone plates are used for focusing light. Unlike lenses however, zone plates use diffraction instead of refraction. A zone plate consists of a set of radially symmetric rings, known as Fresnel zones, which alternate between opaque and transparent. Light hitting the zone plate will diffract around the opaque zones. The zones can be spaced so that the diffracted light constructively interferes at the desired focus, creating an image. Improvements in the efficiency and resolution of zone plates have been made possible by advances in nano-lithography, which are used in their manufacture. Exploitation of these nano-lithographic techniques has opened up a new era in the field of neutron optics. The improved neutron zone plates can be used successfully in the optimisation of spectrometers, which need low divergence neutron beams with a small spatial size. Simple systems of neutron focusing can be achieved by assembling several zone plates with different properties. Complex designs like matrices can also be produced for applications in high-resolution neutron imaging and radiography.
