Density and thickness determination by X and gamma ray absorption

The X and gamma ray absorption method has been in use for several decades for density and thickness determination. The method is noncontacting, so can be used for such diverse applications as measuring the thickness of fast moving steel sheet at 800°C on a hot rolling mill, or the phase distribution of two-phase steam-water mixtures through the walls of a thick walled stainless steel pipe during explosive decompression from 160 bar and 325°C. The authors describe some general characteristics of the method and also one family of radiation detectors, based on optically stabilized photomultipliers working in the `current integration´ mode. Performance is detailed, and limitations mentioned. Comparisons are made with `counting mode´ and photo-diode based systems. Brief descriptions are given of some industrial and laboratory applications