What is a Gamma Counter?

Angie Bates

The device known as a gamma counter measures gamma radiation in a sample. Working on the same principle as a scintillation detector, these counters use crystals which emit light when photons from the gamma rays interact with them. Samples are usually placed into test tubes, which are then placed into the machine. Gamma counters are generally used in research labs since most are not portable.

Radiation detectors are often used to monitor areas that are known to have low levels of contamination.
Radiation detectors are often used to monitor areas that are known to have low levels of contamination.

Gamma radiation, or gamma rays, is a harmful type of high-energy radiation which consists of photons of light, or quantum particles. Photons are emitted as a radioactive substances decays, or attempts to become stable. Unlike less harmful types of radiation, gamma rays are not stopped by skin or clothing and can enter the body, damaging or killing cells. Lead is generally used to shield people from the harmful effects of gamma radiation.

Like a scintillation detector, gamma counters rely on crystals with luminescent properties to work. This means these crystals light up, or scintillate, when they come in contact with gamma radiation. The more they light up, the more radiation a sample is emitting. The light energy is usually converted into electric energy via a photomultiplier in order to give the scientists a meaningful measure.

The accuracy of the counter is dependent upon the crystal used. If the radiation is low, and therefore interacts little with the luminescent crystal, the light photons may not register. Conversely, if radiation is extremely high, it may be powerful enough to pass straight through the crystal without interacting enough to produce light as well. Thick crystals help to reduce on these non-reactions, and so are extremely important in quality gamma counters.

Modern gamma counters may have multiple detectors and channels for samples. Machines also usually have lead shielding to help protect researchers from radiation. Many gamma counters are approximately the size of cash registers or photocopiers, with a small electronic screen attached for readouts.

There are also small gamma counters, not much larger than a square foot (0.3 m2). Even the small counters, however, are not easily portable, as the lead shielding makes them extremely heavy despite their size. A square foot size counter may weigh as much as 55 pounds (25 kg).

Proper calibration is extremely important when working with gamma counters because they produce such precise measurements. Many counters have internal calibration which rely on light emitting diodes. Some older counters may need to be calibrated by hand, however. When calibrating by hand, a substance with a known radiation and approximately the same volume as the sample to be tested is used as a reference.

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