Radiation level measurement techniques

The radiation/nuclear level measurement technique is used to measure the level of fluid or solid in a closed tank using Gamma rays. Gamma radiation sources are chosen for use in level detecting equipment because gamma rays have great penetrating power and cannot be deflected.

Principle:

Level measurement with radiation works on the principle of passing gamma radiation through the material to be measured. As the radiation passes through this material, the level can be determined by the amount of attenuation. The wave attenuates when it passes through materials.

Working and Construction:

The Gamma-ray is emitted from a source to the tank and propagates through the tank. There is a continuous strip detector that detects all the Gamma rays pass across the tank. If the continuous strip detects rays equal to the length and it is in maximum absorption, the tank is empty. As the level rises the absorption level decreases.

Different components of Radiation meter:

The source:

The main component of this type of measuring device is the radioactive source. The two common types of radioactive sources are Caesium 137 (Cs 137) and Cobalt 60 (Co 60). The activity of the radioactive substance decreases with time. The time taken for the activity of such a substance to halve is termed its half-life. Cobalt 60 has a half-life of 5.3 years while Caesium 137, on the other hand, has a half-life of 32 years.

There are strip source and point source:

The stripped source is more accurate as it radiates a long, narrow, uniform beam in the direction of the detector. As the level changes, the detector is covered and protected from the source and the corresponding response changes. The response is uniform and linear over the entire span, producing a linear signal that corresponds with changes in level.

The point source works in a similar way to the strip source system, in that the strip detector measures the radiation from the source. The radiation sensed by the detector is still attenuated with level, however, the point source system produces a non-linear response with level change.

The Strip Detector:

The detector for continuous measurement is a type of scintillation counter and photomultiplier. This type of sensing has the advantage of the high sensitivity of the scintillation crystals (compared to Geiger counters) coupled with the safety and economy of a point source.

The rod scintillation counter is a rod of optically pure perspex within which scintillation crystals are uniformly distributed. In the presence of gamma radiation, the scintillation crystals emit flashes of light which are then detected by a photomultiplier at the base of the rod and converted into electrical pulses.

To improve linearity and accuracy, we use multiple point sources:

Advantages:

  • Suitable for a variety of products
  • Mounted without obstruction
  • Can be mounted external to the vessel

Disadvantages:

  • Must always be mounted on the side of the vessel
  • Special safety measures are required for the use of gamma radiation
  • May also involve licensing requirements
  • Expensive

 

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