Phase contrast neutron imaging of mixed phase-amplitude objects

A neutron imaging facility has been set up at the North Carolina State University (NCSU) PULSTAR reactor. Currently, work has started on tailoring the design of the facility to allow the performance of phase contrast neutron radiography. This 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 formation 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, the 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 being presented. Using computational simulations, it is observed that the pure phase object approximation results in over enhancement of edges for a phase-amplitude object, with significant change 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 the performance characteristics of neutron phase imaging experiments.