The measurement principle
An encapsulated gamma source emits a radiation beam through the material being measured. Part of the radiation is absorbed in proportion to the mass per area or the level of the material. The detector on the opposite side measures the transmitted radiation, and the electronics convert the signal into a continuous measurement value or an alarm threshold.
This makes the technology particularly suited where:
- The material or medium is moving and physical contact is out of the question
- Dust, water mist, build-up or high temperature defeat other sensors
- The measurement must remain stable for decades without recalibration
Typical isotopes
| Isotope | Half-life | Typical application |
|---|---|---|
| Caesium-137 (Cs-137) | 30 years | Level and density measurement in process vessels |
| Americium-241 (Am-241) | 432 years | Lower radiation levels, thin webs |
| Cobalt-60 (Co-60) | 5.3 years | Density in thick materials |
System components
- The source — an encapsulated radioactive isotope in a metal housing, mounted on the outside of the process vessel
- The detector — a scintillator or ionisation chamber that measures the transmitted radiation
- The electronics — signal processing, temperature compensation and outputs (4–20 mA, alarm relays, Modbus)
- Local diagnostics for operational status and source integrity
Why gamma still beats optics
Optical, capacitive and radar-based systems have come a long way — but in hot, humid, dusty and vibration-heavy environments, gamma measurement remains the most reliable. It sees straight through dust, mist, steam and build-up that knock out other techniques.
Furthermore, the sensor sits entirely on the outside of the process vessel — no wear, no process contact, and no risk of the sensor becoming sticky or covered.