Using Your Survey Meter

Radiation survey meters must be used properly if they are to be an effective tool for detecting radioactive contamination or measuring radiation exposure. An audible output is useful when doing surveys since the meter may respond quite slowly. When monitoring for low energy beta emitters with a G-M survey meter, the detector must be passed slowly across and very close to the surface that is being monitored. Failure to do this could result in missed contamination. If the detector is covered with plastic, etc., to reduce the possibility of contamination, the sensitivity for low energy beta emitters will be greatly reduced. Remember, you will not be able to detect tritium with a portable G-M detector.

For most isotope contamination surveys, meter surveys are not sensitive enough to meet contamination limits. Refer to the Radiation Safety Manual for proper contamination monitoring techniques.

It is impossible for your survey meter to detect every disintegration that is emitted from a radioactive source. Your instrument will indicate counts per minute (cpm) while the source emits disintegrations per minute (dpm). The response of your meter in cpm is typically only a small fraction of the actual dpm. The ratio of cpm to dpm is called the efficiency of the instrument for that particular isotope or type of radiation. The intrinsic efficiency of the different types of detectors for the different types and energies of radiation can vary greatly and is also dependent on the geometry (distance of the source from the detector and source configuration). The G-M efficiency for low energy beta emitters is quite low, only about 1 to 6 percent. The G-M efficiency for high-energy beta emitters is high, about 50 percent. For gamma emitters, a G-M detector has an efficiency of less than 0.1 percent. Of course, the distance of the source from the detector can influence these factors greatly.

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