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Geiger-Mueller (GM) Meter

The GM survey meter is the most common device used for the detection of radioactive contamination. You might hear this meter referred to as a “pancake” or “GM pancake” because of the look of the device. A GM meter is often portable, and the efficiency of a GM for detecting radiation depends on four factors:

  1. The type of radiation.
  2. The energy of the radiation.
  3. The amount of activity in the source.
  4. The direction of the source relative to the instrument detector active area.

If a beta particle or gamma ray can enter the active portion of a GM meter and interact, it can be detected. Because the detection efficiency of the GM meter is dependent upon the energy of the particle or ray, the efficiency of a GM meter is much higher for detecting beta particles than it is for detecting gamma rays. Beta particles do not travel far, so if they are able to enter the active portion of the meter, they have a high probability of interacting and being detected. Gamma rays, on the other hand, have enough energy to enter the active portion, but may exit without interacting.