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Basic for Badges
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Radiation detection is important in the field
of radiography. If you work with ionizing radiation as a
radiographer, you wear a radiation badge that detects and
measures the radiation to which you are exposed. These film and
thermoluminescent dosimeter badges are used for monitoring
personnel radiation exposure. This article provides some of the
basics for these badges.
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WHY A BADGE IS WORN
overexposure to ionizing radiation is detrimental to your health. The radiation badge is the means for determining to how much radiation you have been exposed. The tolerable levels are different for everyone, depending on age and health. Averages have been established from years of data from medical, accidental and weapon exposures. Some values to keep in mind are displayed in Table 1.In addition to the immediate effects, there are also delayed effects, such as an increased risk in developing cancer. Present estimates are that 10 mSv (1 rem) of exposure over a lifetime gives a risk of 1 per 10000 of developing cancer. Compared with the natural occurrence of cancer, a 200 mSv (20 rem) lifetime occupational exposure will produce 20 instances of cancer in 10000 radiographers while 2000 will die from naturally occurring cancer.
Radiation badges are worn to keep track of workers' radiation exposure so that problems can be detected earlier and overexposure can be prevented. Of course, to do this, the badges must be worn.
Either type of badge can provide information on personnel radiation exposure.
HOW THEY WORK
There are two kinds of badges regularly used in the industry: film badges and thermoluminescent dosimeter badges. Figure 1 depicts these two types of badges.Film Badges
The film badge has been in use for a longer period of time. It is simply a small plastic case with a clip containing a packet with two films and some metal filters. The two films have different sensitivities to radiation so that a wide range of exposures, from a few hundredths of a millisievert to 20 Sv (2000 rem), can be covered. The badges are generally worn for a month and then traded in for a new badge. If an overexposure is believed to have occurred (from survey meter or pocket ion chamber readings), the badge is sent immediately to be read. For regular service, the radiographer's name, date of the badge and a numerical identity are stamped on the film packet.The plastic case has a window permitting very soft or low energy radiation to reach the film packet. The case also has small metal filters, such as lead, cadmium or aluminum, to permit differentiation between photon energy and exposure to neutrons. (The cadmium absorbs thermal or low energy neutrons very well, converting the neutrons to gamma rays which expose the film.) Low energy photons are as much as 20 times more effective at exposing the film, so the metal filters are very important.
One of the problems with film badges is that they are sensitive to heat (such as from being left on the dashboard of a vehicle), light (from the paper case being torn or cut open), moisture (if accidentally washed in the laundry) and mechanical pressure (if run over or stepped upon). Also, radiation exposure of the badge is lost with time. Therefore, one month intervals for exposure before development of the film are necessary. Records of the individual's exposures must be kept by the employer and given to the individual each year.
Thermoluminescent Dosimeters
The thermoluminescent dosimeter badge is now frequently used instead of the film badge. The thermoluminescent dosimeter badge depends on thermoluminescent phosphors, such as lithium fluoride, that trap electrons freed by radiation exposure. The trapped electrons release light when the application of heat frees the electrons, returning them to stable energy states. The process can be repeated many times. Instead of reading the blackness (optical density) of a film, the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured as a "glow curve" which is then related to the radiation exposure.The lithium fluoride in the form of powder compressed into pellets has advantages over film badges by being tissue equivalent, by having an exposure range from a few hundredths of a millisievert to over 100 Sv (10000 rem), by exhibiting little fading over time in storage at room temperature and by being free of many of the problems with film. In the badge, a pellet is placed behind each of the filters so that radiation energy information is obtained as well as consistent readings. The ability to reuse the pellets is also an advantage. If the lithium is enriched with Li-6, the lithium fluoride becomes very sensitive to thermal neutron interaction and a pair of pellets (one enriched and the other not enriched) can provide neutron exposure information.
ENSURING SAFETY
Either type of badge can provide information on personnel radiation exposure. The story told by the older radiographers to the new ones is not true: the badges do not provide safety through some process of radiation shielding. The safety comes about only if the badge is conscientiously worn by the personnel involved and turned in as required (Burnett et al., 2002; Bush, 2001).
REFERENCES
Burnett, William D., Garry L. Balestracci and Frank A. Iddings, "Radiation Safety," Nondestructive Testing Handbook, third edition: Volume 4, Radiographic Testing, P. Moore, ed., Columbus, Ohio, ASNT, 2002, pp. 114-138.Bush, Joe R., Jr., Gamma Radiation Safety Study Guide, second edition, Columbus, Ohio, ASNT, 2001.
* 1635 Rob Roy Lane, San Antonio, TX 78251; (210) 647-7717; e-mail <profiddings@satx.rr.com>.
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