1. An apparatus for monitoring in real time the movement of a plurality of substances in a mixture, having at least one fluid layer, along an axis of flow within a first tube, the apparatus comprising: an annular X-ray tube positioned radially outward from said first tube wherein said annular X-ray tube has a plurality of X-ray sources; an annular sensor array radially positioned between said annular X-ray tube and said first tube wherein said annular sensor array comprises a plurality of sensors arranged in a plurality of rings, and wherein said X-ray sources sequentially emit radiation and wherein each portion of said sensor array receives radiation from X-ray sources positioned furthest from said sensor array.