Radiation Detection

One of the ways to avoid widespread contamination in your laboratory or on your person is to monitor with a properly calibrated radiation survey meter. There are many types of survey meters available. Each type of survey meter has limitations, both in its' operation and the type(s) of radiation that it can most effectively detect. You, in consultation with the Office of Radiation Safety, can determine what type of survey meter is needed for your particular situation. The most common types of instruments are discussed below.

Geiger-Muller Counters - G-M detectors are the most common type of radiation detector used on campus. They are easy to use, portable, relatively inexpensive, and good for many types of lab surveys. G-Ms are most efficient for detecting high energy beta emitters, such as ³²P, but can also be used to measure low energy beta emitters, such as ¹⁴C and ³⁵S, if they have a thin end-window. However, they cannot be used to monitor for tritium (³H) because the beta particle emitted from tritium does not have sufficient energy to penetrate even the thin end-window of the G-M detector. G-M detectors can also be used to detect gamma and x-ray radiation from radioisotopes or x-ray machines, but the efficiency is much less than that for beta emitters. G-M counters usually measure counts per minute, so are often calibrated with an electronic pulser, but they can also be calibrated using a radiation source to measure mR/hr for specific x-ray/gamma energies. In addition to a display, many G-M survey meters have an audible output that produces a click for each photon or particle detected. This feature is very useful when doing a survey since the meter response is sometimes quite slow. G-M survey meters should be calibrated every year.

Liquid Scintillation Counters - LSCs are common on campus. They are not portable, and are primarily used for counting laboratory samples including contamination wipes. These counters are sensitive to low energy beta emitters, and provide one of the few methods for counting tritium and other very low energy beta emitters. In addition, LSCs can be used to detect the radiation from just about any other radioisotope used on campus. LSCs measure counts per minute, seconds, etc., and typically have excellent efficiency, so can usually be used to count the wipes taken periodically as part of your laboratory survey program. LSCs should be calibrated on a regular basis.

Scintillation Detectors - Scintillation detectors are used primarily to detect gamma radiation. They are much more sensitive to gamma and x-ray radiation than G-M detectors, but cost more. Some portable survey meters can use either a G-M detector or a scintillation detector. If the detector crystal is covered with a metal case, the detector may not be sensitive to low energy gamma and x-ray radiation, therefore, special detector crystals are available for low energy gamma emitters such as ¹²⁵I. Scintillation detectors typically measure counts per minute and may have an audible output like G-M detectors. Scintillation detectors should be calibrated every year.

Ionization Chambers - Ionization chambers are used primarily to determine the exposure rate from gamma and x-ray emitters and are particularly useful when measuring machine-produced x-rays. In general, ion chambers are not used to monitor for beta contamination. Ion chambers are expensive, but portable and most models are easy to use. Ionization chambers usually measure exposure in milliroentgens or exposure rate in milliroentgens per hour. They should be calibrated every year.

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