Electro-magnetic waves like X-rays or gamma-rays are absorbed in matterial by different ways. Therefore it depends on the energy of the radiation and on the material itself in which way it is absorbed. High energy waves and low atomic numbers are absorbed according to the density ( or bulk density) of the material. Of course the absorption also is depending on the layer thickness. That means the absorption is depending on the weight of the material the radiaction has passed through ( The weight increases if the way becomes longer or if the density becomes higher).
. Density and thickness can be measured by means of radiations: If one is constant the other can be calculated. Lower energy radiation and higher atomic numbers are absorbed in the same way but additional according to the atomic weight. As higher the atomic weight and as lower the energy as higher the absorption. For analysis purposes a weak energy is needed to determine the atomic composition. But also the weight is influencing the absorption. So a higher energy radiation also must be applied to get a signal which is influenced by the weight only but not by the atomic composition. Both signals can be used mathematically to determine the composition.
Practical Application
A radiation source with Caesium-137 is an emitter of relatively high energy radiation. The energy is high enough to be independent on the composition and low enough to handle it in a secure way. It is able to detemine the weight of the material which it has passed through with a high precition so that for instance radiometric belt scales are using it. Another material emmiting radiation is Americium-241. This energy is quiete low: A sheet of steel of 0.8mm thickness is weakening the intensity to the half. In coal the part which is called ash absorbs the radiation of the Am-241 much more then the carbon. So a higher ash content gives an relativly strong increase of the absorption. Unfortunatelly there is no practical way that this absorption is not influenced by density or layer thickness e.g. loading on a belt. Therefore both signals are needed together to calculate mathematically the percentage of ash.
