Absorption and phase contrast neutron imaging

The aim of this study is to describe the experimental procedures of different imaging methods like thermal-, cold-neutron absorption radiography (NR) and phase contrast neutron radiography (PCR). In the last years, many studies have focused on the PCR, which is superior to the standard methods based on the beam attenuation, especially for the inspection of low absorber materials (phase objects). Basically, image contrast on a detector screen performed by standard methods displays neutron transmission through elements within sample. The image contrast depends strongly upon the scattering and absorption cross sections of materials. A contrast enhancement at the edges can be achieved by means of PCR, which is based on the wave properties of neutrons and arises from the neutron refraction (rather than absorption). Elements having different refractive index within a sample cause a phase shift between coherent neutron waves. The beam coherence is increased partially using a point source (pin-hole on a strong absorber foil), and the distance between source and sample is varied to observe refraction and diffraction-related fringes. Some examples of PCR and neutron absorption imaging performed at Atominstitut-Wien and NEUTRA-PSI were demonstrated to express their application areas. While the thermal and cold neutron imaging has priority over the PCR for thicker or absorber materials, the PCR is the only promising method in the investigations of thinner phase objects. (author)