How does nuclear/radiometric level measurement work?

Nuclear level gauges use a gamma radiation source (typically Cs-137 or Co-60) mounted on one side of a vessel and a detector (scintillation or Geiger-Muller) on the opposite side. Radiation intensity reaching the detector decreases as level rises and absorbs more radiation. Non-contact, external mounting allows measurement through vessel walls without process penetration. Used for harsh conditions (high pressure, temperature, corrosive media), thick-walled vessels, and slurries where other methods fail. Requires radiation safety licensing.