Abstract: The invention relates to a method and an apparatus for analyzing a flow of material using X rays. The method comprises radiating the material with at least two energy levels and measuring the transmission of radiation through the material for each level separately, and is characterized in that a sensor is used for measuring the radiation transmission, which sensor comprises a plurality of substantially adjacent pixels, and on the basis of the transmission values measured determining the thickness and composition of the material. This may be performed in combination with one or more blank contact detection techniques, for example on the basis of infrared radiation, visible light radiation, of ultraviolet radiation.