Investigating Phase Contrast Neutron Imaging for Mixed Phase-Amplitude Objects

Phase contrast imaging is an imaging modality that has been extensively applied in X-ray imaging and was demonstrated using neutrons over the past few years. In this case, contrast in the image, especially at edges, is enhanced due to phase shifts that take place as the neutron wave passes through regions in the sample that differ in the coherent scattering length density. Usually, a pure phase object approximation is used to formulate the problem, whereas realistic samples represent mixed phase-amplitude objects. In this work, a formulation for mixed phase-amplitude objects with moderate neutron attenuation coefficients and its effect on the neutron image is presented. A computational simulation technique has been devised to study this effect on different types of samples. Using simulations, it is observed that the pure phase object approximation results in over enhancement of edges for a phase-amplitude object, with significant variation (in the case of neutron imaging) depending upon the edge forming material characteristics. The total contrast for the mixed phase-amplitude object is less than the sum of the individual attenuation and phase contrast components. The difference depends on the scalar product of the gradient of the coherent scattering length density and the attenuation coefficient. The presented formulation can aid in predicting and optimizing the performance characteristics of neutron phase contrast imaging experiments.