Scanning method and apparatus

A beam of gamma radiation from a source is emitted through a pipeline or process vessel to detect density changes. It is detected by an array of detectors which are each collimated to detect radiation over a narrow angle relative to the width of the emitted radiation beam. The scintillators have a depth away from the surface at which radiation is incident which is at least twice that of their thickness. The thickness may be 1 5 mm with other dimensions 10 - 80 mm depending on the source chosen. The source and detectors are linked by a support arm to form one unit capable of rotation around the pipeline or other structure. A collimation block is formed of steel with layers of lead or a heavy alloy placed above and below the block