The natural existing potassium is a composition of isotopes. One of it is the isotop K-40 which is radioactive. It emits 2 different kind of rays, ß-particles and gamma-radiation.
ß-particles are electrons and therefore not able to irradiate big layers of material.
Gamma-rays are physicaly electromagnetic waves and accordingly the same as X-rays. They are able to pass through walls.
Both kinds of rays have there advantages to be measured.
ß-radiation is emitted to a much larger extend and so the measurement could be more accurate in a short time. It also can be an advantage that only a small amount of material is necessary for the measurement. Unfortunatelly a small sample is not as representative as a bigger one.
Gamma-radiation irradiates even 1 m of salt. The measurement can be taken as representative for a sphere of 2 m diameter if the detector is able to measure a 4 pi geometry. If this amount is not available it helps to measure only half a sphere with a so called surface probe. A measurement of even smaller amounts would include an error in case of changes in bulk density or thickness.
Both measurements therefore have their prefered fields. For a good representativity and in cases when a steel wall has to be irradiated the measurement of the gamma-radiation will do it. In laboratories and on small layers the ß-radiation must be measured.
Both measurements require a different type of detector.
Gamma-radiation normally is measured with a scintillation counter whose sensitive part is a NaJ crystall. Depending on the place the crystall size can be chosen.
The measurement of the ß-particles can be carried out by a counter tube or again with a scintillation counter. Because the ß-rays are absorbed by very thin layers the counter tube must be made of thin and light material such as aluminium. The disadvantages are the corrosion and the mechanical sensitivity. Another way is using a plastic scintillation material. This is not as cheap as a counter tube but more sensitive and more robust.
Instruments using plastic scintillation counters for the ß-measurment are applied for laboratory analysers and for continuous measurements on filters.
Applications of the gamma -measurement are not countable. They are on or in hoppers, brine vessels whereever there is enough salt.
A special application is the measurement of gamma-radiation on a belt with not enough material to reach the saturation layer thickness. The simple measurement of the K-40 radiation is influenced by the
variation of the bulk density and the layer. Therefore the loading has to be measured too - either with a radiometric or electro mechanical belt weigher.
The weight is used to correct the error by the changing loading of the belt.
Not to forget the two different hand helt potassium meters.
The first is a surface meter to hold against the wall.
The other portable instrument is a bore hole meter, used for the measurement of the concentration after drilling a hole into the wall.