To determine the ash content of coal on a conveyor belt, the transmission of two gamma-rays with different energies is measured. As radioactive sources 241Am with 60 KeV and 137Cs with 660 KeV are used. The transmitted radiation is detected with high sensitive szintillation-counters.
The absorption ofthe low energy gamma radiation of 241Am increases strongly with the atomic number of the absorbing material.
The ash consists of elements with a higher atomic number compared to the elemental composition of the clean coal. Therefore the absorption oi the 241Am radiation depends on the ash content of the coal. ln general the absorption ofthe radiation depends on the irradiated area weight, i.e. the bulk density and the layer thickness of the material on the conveyor belt. The area weight is measured by the second gamma ray transmission line with the high energy radiation source 137Cs.
The combination of both signals called “Dual Energy Method” allows to compensate the low energy 241Am-signal for bulk density and layer thickness and to get a measure for the ash content of the coal. The dual energy method works exactly only if the elemental compensation ofthe ash is constant. Especially changes of the iron- and calcium content generate remarkable measuring errors.
To compensate these errors, |nduTech has introduced several compensation methods. Separate information sheets are available.
shielding container Americium
weight approx. 14 kg shielding container equipped with 3 . 7 GBq (100 m c i) 241 Am
szintillation-detector SZ A1 44/5
weight approx 6 kg front end radiation window OPTION: radial radiation window
Cs Measuring path
shielding container LB 7440
weight approx 31 kg
shielding container equipped
with a 137Cs source
(typ.37O Msq 10 mCi)
Szintillation-detector Sz 5 D1 40/35
weight approx 14 kg
radial radiation window
front end radiation window