Presumably, they were measuring the level of coal (or other material) in a tank. Look up "ohmart radioactive level transmitter".

It looks as if this type of device is still sold today.

At first I thought you might be referring to a radiation detector because coal ash is radioactive.

https://www.scientificamerican.com/article/coal-ash-is-more-...

However, it was a Cesium-137 radiation source.

It may have been used as a calibration source for one of their sensors, or for the detection of non-coal materials in loads of coal, or for the measurement of coal material flow rate through a feeder. https://cbprocess.ca/products/field-instruments/nuclear/

It is a somewhat difficult process to measure the amount of rock mass passing a specific point. You can assume that a truck load has a particular volume, you can assume a specific gravity, you can measure with difficult to keep calibrated weight sensors, etc. But once the material starts to flow you will have difficulty knowing the flow rate and the amount of material remaining.

Using a radiation source and a sensors you can detect the flow of the mass of material, or the volume in a container, based on scattering, reflection, and absorption of the radiation.

Based on the images in this article, the source may have been used to determine the volume of the tank it was attached to.

https://web.archive.org/web/20230316103321/https://asianews....