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.
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