Guidance for Radiation Accident Management

How to Detect Radiation

Detecting radiation is achieved through the use of a variety of instruments. The most common type of radiation detector is a Geiger-Mueller (GM) tube, also called a Geiger counter.

How to use a Geiger-Mueller (GM) counter to detect radiation

Get the GM counter and batteries from storage; prepare the instrument and determine background level.

Preparing the meter:

  • Position the Geiger counter with the meter away from you. Locate and open the battery compartment.
  • Put the batteries in the meter using proper orientation (up/down).
  • Close and latch the battery compartment.
  • Check the batteries using the "range" switch or "bat" button; the method depends on the type of instrument. The meter needle should move to area on scale marked battery, indicating the batteries are good. If the battery check fails, replace the batteries with a fresh set and repeat the battery check.
  • Turn the "F/S" switch to "S" (slow). Some instruments may have a picture of a hare and a tortoise to represent fast and slow.
  • Generally, the fast setting is used to detect radioactivity, and the slow setting is used to obtain counts. Turn the "audio" switch to "ON."

Measuring the background radiation:

  • Check that the "F/S" switch is on "S" (slow).
  • Move the range switch to the most sensitive position.
  • Remove the probe cover if one is in place.
  • Measure the background radiation for 60 seconds: write down the reading. Since background radiation varies with time, it may be desirable to make several counts and average the results. Record the reading.
  • Readings may vary, but a range of 20-100 counts/min, or a reading of approximately 0.02 mR/hr (i.e. 0.2 on the 0.1 range setting), or 0.2 micro Sv/hr, is reasonably expected in normal circumstances.
  • Record background reading.

How to survey a person

Using the instrument:

  • Move the "F/S" switch to "F" (fast response).
  • Set the instrument selector switch to the most sensitive range of the instrument.
  • Holding the probe approximately 1/2 to 1 inch from the person’s skin, systematically move the probe across the entire body from head to toe on all sides. Try to maintain a consistent distance.
  • Move the probe slowly (about 1 inch per second).
  • To avoid contaminating the probe, do not let it touch the skin or clothing.
  • Locate the point that produces the most clicks. (Turn the "F/S" switch to "S" to take a reading at this location. Remember to reset it to "F" before continuing survey.)
  • If the meter reads off-scale, adjust the range of the instrument by moving the range selector switch and pushing the “reset” button.
  • Document time and radiation measurements.
  • In general, areas that register more than twice the previously determined background level are considered contaminated. If the accident circumstances indicate that an alpha emitter (such as plutonium) or low energy beta emitter could be a contaminant, a health physicist should always be consulted.
  • Note that some GM instruments cannot detect alpha radiation and some low-energy beta radiation. Because alpha radiation is non-penetrating, it cannot be detected through even a thin film of water, blood, dirt, clothing, or through probe cover.

Ending the radiation survey:

  • Switch off the meter.
  • Replace the cap on the meter probe.
  • Take the batteries out.
  • Put the Geiger counter back in its case.

The following procedures are recommended for personnel monitoring:

  • Have the person stand on a clean pad.
  • Instruct the person to stand straight, feet spread slightly, arms extended with palms up and fingers straight out.
  • Starting at the top of the head, cover the entire body, including the soles of the feet.
  • Have the subject turn around, and repeat the survey on the back of the body.


This probe is used for the detection of alpha radiation.