Answer to Question #7624 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Personnel Monitoring (PM)

The following question was answered by an expert in the appropriate field:

Q
In general, for personnel working in a hospital, which personal radiation detector must they use? That is to say, regarding their working condition (radiotherapy, x ray) which detectors are suitable?

A

We should perhaps first note that the major radiation type of concern in the medical arena is photonic radiation, either x rays or gamma rays. There are some instances where it might be necessary to measure particulate radiation, such as beta particles, or neutrons (for-higher energy accelerators), but these represent a small dose component for most situations. The most pressing need then is to be able to assess photon dose to medical staff.

Perhaps the most commonly used personal dosimetry devices used in medical work environments are passive devices. The most common of these include film badges, thermoluminescent dosimeters (TLD), and optically stimulated luminescence (OSL) dosimeters. These devices are worn on the body and integrate dose over the wear period, commonly one month in duration. At the end of the wear period the accumulated dose is read out from the dosimeter; the user is usually issued a new dosimeter at the time his is taken for processing. Some facilities have also been using active devices, such as silicon diode-based electronic dosimeters, to monitor personal dose. These electronic devices, also worn on the body, have the advantage that they provide real-time measurement of dose and or dose rate. They also have the capability of providing an audible alarm when accumulated dose or dose rate has exceeded a preset level. There are also some electronic dosimeters that use small Geiger Mueller (GM) tubes instead of the silicon diode as a detector. In some environments, such as pulsed accelerators that might be used in therapeutic applications, electronic dosimeters may require special testing because of the potential for failure in the pulsed radiation fields; the pulse rate and pulse intensity are important parameters that affect the detector response. There is also the possibility of RF (radiofrequency) interference with electronic dosimeters near accelerators.

Naturally, in addition to personal dose measuring devices, there are other portable instruments that are used for survey work around medical facilities. In nuclear medicine departments GM detectors are very commonly used for routine survey work. Scintillation detectors, such as NaI(Tl), are also common, especially for monitoring low-energy photon emitters such as 125I. They are also frequently used to monitor technologists’ thyroids to assess possible uptakes of radioiodine, such as 131I that is used in therapeutic treatments of patients with thyroid disease. Surveys and special measurements that might be required around diagnostic x-ray machines and therapeutic machines often employ air ionization chambers. Ionization chambers, in general, are able to measure higher dose rates than GM detectors or scintillation detectors and are often more suited for these situations. Ionization chambers with charge integration capabilities are very useful for measuring high dose rates and are often advantageous around accelerators that produce pulsed radiation that may not be adequately assessed with a GM detector.

You can find more information about the various detection devices and instruments by consulting the literature and the Internet. The popular book by Glenn Knoll (Radiation Detection and Measurement, 3rd ed., Wiley and Sons, 2000) is very useful. You can find a number of questions and answers regarding many aspects of radiation detection on the Health Physics Society Ask the Experts Web site under Experts Answers. Simply type a word or phrase (e.g., film, TLD, OSL, etc.) in the search box and see what comes up. Good luck.

George Chabot, PHD, CHP

 
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