Abstract: A detection system for detecting sheets of material includes a device for moving along a path a container which can harbor a sheet of material sought to be detected; an X-ray scanner having a beam for scanning across the path of the container through a predetermined angle and a device for moving the scanner to shift the origin of the scanning beam to align during at least a portion of the scan the scanning beam with the sheet for producing a high projected density contrasted with its surroundings.

Abstract: A dual beam modulator (10) includes a rotatable cylinder (12) having annular walls (14, 15) and a set of a plurality of discrete chordal passages (18) extending through the walls between opposing holes in the outer surface of cylinder walls (14, 15). Each of the passages (18) is spaced radially about each of the walls (14, 15) and longitudinally in the direction of the longitudinal axis of the cylinder (12) for producing, in response to an incident fan beam of radiation (30), a series of discrete pencil beams (28) as the cylinder (12) rotates. The cylinder (12) further has reduced radius sections (16, 17) on the outer surface of the walls (14, 15) for periodically passing the incident fan beam (30) alternately with the series of pencil beams (28) as the cylinder (12) rotates.