Choosing the Right Detector by Angela Ran Meng

If your laboratory uses radioactive material (RAM), it will be necessary to conduct periodic radiation surveys under various circumstances. Surveys can be used to detect RAM contamination, evaluate radiation levels, to evaluate concentrations or quantities of radioactive material, or locate and define other potential radiological hazards that may be present. Surveys are performed during and after use of RAM, for monthly documentation of work area cleanliness, upon receipt and prior to shipping of RAM, and when disposing of RAM waste. Since the human body cannot sense radiation, we must rely on proper use of various types of radiation detectors for these surveys.

Using the right type of radiation detector is similar to having the right prescription glasses: readings can be skewed or completely missed when a wrong detector is used. The choice of radiation detector depends on the purpose of the survey and the radioisotope of concern. Liquid scintillation counters (LSC) are commonly used to detect contamination from alpha, beta or gamma emitters, such as 3H, 14C, 18F, 32P, 33P, 35S, 51Cr, 125I or 131I; however they require time for sample preparation and processing. If a quick reading of radiation exposure and estimated dose from gamma emitters and high energy beta emitters is needed, an ion chamber is the best survey meter choice. In the laboratory, sodium iodide (NaI) detectors and Geiger Müller (GM) detectors can be used to search for contamination; the NaI detector is particularly good for detection of low energy gamma emitters such as 55Fe and 125I, while a GM detector is better suited for medium- to high-energy beta and gamma emitters, such as 18F, 22Na, 57Co, 32P and 33P. Neither a GM detector nor a NaI detector will detect 3H, 14C or 35S efficiently; it is best to use a liquid scintillation counter for these isotopes.

A simple way to tell if your meter is right for the type of survey to be performed is to check the calibration sticker on the handheld detector. If the isotope of interest is listed there, your detector is calibrated to measure that isotope and can be used for your radiation survey. If the isotope is not listed you may still be able to use the detector for that isotope, if the purpose and isotope of the survey fit the properties of your detector. For more information see the Radiation Monitoring Equipment policy @ http://www.ehs.columbia.edu/RadiationMonitoringEquipment.pdf or contact rsostaffcumc@columbia.edu at the Medical Center, or rso-ehrs@columbia.edu at Morningside for more information.

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