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Choose big...eg Endress & Hauser manufactures such level sensors, and are familiar with the proper requirements. You need best vendor advice..

In almost all countries, you require a government authority/ licence to own a radioactive source.

Note that the transmitter itself is not radioactive.

Typically, there is a radiation source (isotope) for which strength/ activity you require very professional advice...A radiation sensor continuously detects radiation passing through a vessel from this source. The process medium resists/ inhibits the radiation stream, but at the point where there is no resisitance to the stream , the sensor / receiver will be exposed to a higher activity of radiation.

eg in a horizontal drum/ vessel, the process medium's resistance to the radiation stream will be highest at the widest point of the drum, if the process level is above that height .

This signal is converted (typically) into a 4-20mA signal and your 'level' value is transmitted as an electronic signal by an independent transmitter. Other configurations are possible, but this is the most common.